Disclaimer
Let's get it very clear from the outset - I do not dislike Naim equipment. Further, I have no quibbles with the company or any of it's employees. Quite the opposite is in fact true. I consider the marque to be one of the great success stories of our time and during meetings with the late Julian Vereker, we exchanged pleasantries with no pique from either side. I have nothing but admiration for the Naim product.
However, Thomas Edison once stated: "Anything that can be manufactured, can be improved". This is a principle on which I rely and which has proved to be true on many occasions. Naim Audio is no exception to this rule and therefore produces very tangible bones onto which may be added some extremely sophisticated flesh.
Should you be fortunate enough to be totally satisfied and besotted with your Naim system then read no further. Conversely, if you happen to be one of those less than satisfied owners who happen to contact us weekly, this is for you.
Read on with hope............
Legal Bits
The use of trading names, titles, styles and model designations throughout this publication are intended solely for reference and identification purposes only.
No claim is made by the author or publisher to imply or infer any connection with or endorsement by the original manufacturer of any product contained herein nor does the author claim or warrant that any advice, theory, procedure or practice contained herein is fit or suitable for any purpose other than academic or theoretical interest only.
Readers follow any advice, procedure and practice described herein, entirely at their own risk.
Neither the author nor any recommended supplier can be held responsible for any damage, loss or accident caused by employing or following any advice, theory, procedure or practice contained in this publication.
Introduction
Naim Audio Ltd are now firmly established as one of this country’s leading makers of audio equipment. Their range extends to loudspeakers, tuners, CD players and a unipivot tonearm designed specifically for the Linn LP12.
Life began for Naim Audio in the 1970s and among their first products were the NAC12 pre-amplifier and the NAP120 power amp. The company has not looked back since their beginnings and winning the Queens Award for Export in the 1980s has put their business on a firm footing both in the UK and abroad.
In common with all commercially produced products, certain cost constraints have to be employed at the design stage and the products from Naim Audio are no exception to this universal rule.
Simply by changing certain key components and re-configuring circuits on these otherwise fine amplifiers, they may easily be upgraded to perform as well as the original design layout suggests they might.
Although the subject of this book is the well regarded range of amplifiers and pre-amplifiers from the Salisbury based company, the techniques used throughout these pages may well be used in upgrading equipment from other manufacturers.
The work is well within the scope of the average constructor and may be approached with confidence once the mysterious art of desoldering/soldering is mastered.
Carefully consider each step before proceeding and check all stages at least once afterwards. In particular, inspect all soldered joints for ‘dryness’ and for solder bridges. The book is intended to help all owners of Naim equipment gain the best from their hi-fi system but the more advanced mods should only be undertaken by those with the necessary practical skills and equipment.
A word or two of caution to those who are not conversant with safety procedures: Before any work begins, the mains supply lead must be disconnected and the equipment be allowed to stand for an hour or so to discharge any latent potential in the capacitors.
Contents
Chapter 1......................... The NAC range of pre-amplifiers
Chapter 2.......................... The NAP range of power amplifiers
Chapter 3.......................... NAXO crossovers
Chapter 4.......................... Power Supplies
Chapter 5........................ Simple Tuning Tips
Chapter 6......................... The smaller amplifiers
Chapter 7........................ The larger amplifiers
Chapter 8......................... Pre-amps - more advanced mods
Chapter 9......................... Power amps - more advanced mods
Chapter 10....................... Power supply alternatives
Chapter 11....................... Build your own
Chapter 12....................... PCB layouts
Removal of Components:
The trick here is to remove virtually all traces of the original solder without damaging the tracks on the PCB. Tracks consist of a copper overlay bonded onto a fibreglass substrate some of which is etched away during the board making process. The remaining copper left comprises the PCB track. Although the adhesive used is relatively resistant to heat, there is a finite limit before the whole thing lifts off the fibreglass with disastrous results.
It is therefore crucial to work as quickly as possible to remove the chosen component.
A word of caution is prudent at this stage, molten solder is a very dangerous substance and eye protection should be worn during all de-soldering operations.
Method 1.
Solder Sucker - this involves the use of one of those devices which literally sucks up the molten solder, hopefully not only from the component leg but from inside the hole as well. The iron tip should be as hot as is practical, 410°C - 430°C and as soon as the solder appears to melt, the nozzle of the solder sucker is placed onto the joint at the opposite side to the bit and the plunger released. With a bit of practice, the joint will then be completely clear of solder and the component may be pulled from its locating holes. Inspect the board for damage or shorted tracks using a magnifying glass after which, the new component may be inserted and soldered in place.
Method 2.
Once the component value, location and orientation have been carefully noted, the leads of the resistor or transistor may be cut using a sharp pair of flush cutters. This leaves the remnants of the leads soldered in the board. Using either method 1 as above or the following method, the lead spills may be safely removed from the holes in the PCB. Holding the board in one hand, carefully melt each joint in turn. When the solder appears to be liquid, immediately tap the board, track side down, onto a fairly hard surface. The solder should be thrown downwards and the joint left completely cleaned by this process. As always, the advice is to take appropriate safety precautions against being splashed by molten solder.
Method 3.
For such components as radial capacitors where the legs are concealed under the case, another method is to melt each joint very quickly in turn and 'wangle' the leads from the board. This requires practice and it is strongly suggested that you experiment on a scrap PCB before attempting any of the above procedures.
Should the unthinkable happen and the track is lifted from the board, carefully apply a spot of superglue and stick it back down again. Use a tool for this purpose, fingers stuck to PCBs are very difficult to remove..!!
Soldering is not merely 'gluing' metals together as some would have us believe, it is an amalgamation by both heat and chemical action of the work piece and the solder and if carried out properly, will form a very low resistance joint.
There is an art in soldering metals together and it is not difficult to learn by experience as long as the basic rules are observed:
Cleaning of the work should take place just prior to soldering either by scraping with a blade or sanding with abrasive cloth.
A thorough cleaning with Isopropyl Alcohol will remove any invisible grease, which may be present. Examine the work using a strong magnifier if there is any doubt.
The iron should have a considerable reserve of heat, the so-called 'instant heat' solder guns are worse than useless for our purposes. A decent, temperature controlled iron with a tip in good condition is best. Clean the tip on a damp sponge made for the purpose before each joint is addressed and tin the tip just before applying it to the joint.
Recommendations given by solder manufacturers should be adhered to when choosing the tip temperature. As the tip cools in contact with the work, more heat is instantly supplied by the element.
Opinion seems to be that the track can be damaged by the application of too much heat but in most cases the damage is caused by too little heat for too long.
A 60 Watt iron with a high heat reserve and equipped with various sized tips will be best for most purposes including even the odd loudspeaker connection!
Clean the tip on the damp sponge, (do not scrape iron coated tips, they wear very quickly afterwards), then tin the tip with the solder. Apply the tip to the work and run in the cored solder until the joint is filled. Do not over apply solder, a case of just sufficient is the right technique and do not carry solder to the work on the tip!
Do not overwipe the joint with the tip to see if it has taken! If the joint has been correctly made, the colour should be bright silver with a distinct wicking up the component leg.
A dull appearance means 'dry joints' - remove the solder from the joint with solder wick or a solder sucker, scrape the joint until clean and repeat the exercise.
After finishing the work, ascertain whether all the components used are solvent proof. If the answer is yes, a scrub down with Isopropyl Alcohol using an old toothbrush followed by an aqueous solution made by Electrolube and called Safewash 2000. Give the board a thorough rinse in warm water with one drop of washing up liquid, (yes, you read right - warm water). When allowed to dry completely, the board will gleam like new and can be more easily inspected using a magnifying glass. I must repeat, please allow the board to dry most thoroughly before re-installing otherwise disaster may ensue.
Solders costing serious money are appearing on the market and although the claims are made that they are made of some exotic materials, they contain more or less the same ratio of tin/lead as ordinary multicore solder but with the addition of small amounts of Silver or Antimony.
The common 60/40 tin/lead solder has a resistivity of about 15 micro /cm whereas tin/silver is about 12.3 micro /cm, an improvement of about 10%. In actual fact, the resistance of the joint is dependent on the cross sectional area, the volume of the solder applied. The gap size between the two surfaces also has a significant influence.
By comparison, copper has a resistivity of about 1.3 micro /cm so don't try improving the resistance of tracks by overlaying solder, you're wasting your time!
From the above, it is obvious that to have as little gap as possible is more important than the actual type of solder used. For instance, valve amplifier builders probably get better performance by twisting component legs tightly together before soldering thereby decreasing the area filled by the solder.
Wire wrapping, although common in densely packed equipment for making low resistance joints has not enjoyed universal popularity in the audio field.
The Golden Rules of Soldering Revisited
1. DO clean the workpiece and scrape each component leg before
inserting into the board.
2. DO use enough heat to melt the joint within 2 seconds, any longer
then the iron just isn't up to the job.
3. DO clean the board with Isopropyl Alcohol before inspecting.
4. DO NOT overwipe the joint - 'Just in case I didn't quite get it'. (suck it dry and begin again).
5. DO NOT carry solder to the work on the tip of the iron.
The Working Environment.
Before beginning the project, it is a good idea to organise a safe and well lit working surface free from dirt and dust with a cover which will not damage the sensitive finishes found on the casework. It is a distinct advantage to be able to leave a job in progress and return to it some time later without the disruption caused by having to pack away each time. Arrange for the bench to be at a comfortable height without having to stoop or stretch. Here, an adjustable typists chair comes into its own. Fit some kind of backstop and side strips to the bench to eliminate the chance of components rolling onto the floor. One of the many bench lamps available will pay dividends, more especially when combined with a magnifying lens.
A few aerosol caps will find a use as component storage pots and when labelled with a felt tipped pen, will ensure that no part is lost in the process. Use a notebook to record the details of the work, loose papers tend to fall down the black hole, present in every workshop, solely intended for such purposes..!!
Order as many components as required before the work begins and if accurate selection is required, order extra quantities just in case some are ‘out of spec’.
A comprehensive tool kit consisting of pliers, wire cutters, screwdrivers etc, most people will, usually have built up over the years. Failing that, there are some quite cheap tool kits to be had and a trawl through virtually any stockists catalogue will reveal just the right combination of bits and pieces. Add a few non standard items such as dental picks, small pin vices along with a selection of small diameter drill bits, a miniature bench vice and something known as ‘helping hands’ and the road ahead becomes clearer. A couple of cans of isopropyl alcohol and switch cleaner will be about all that is required.
Spare soldering iron bits in a variety of sizes will ensure that you have just the right sized bit for the different jobs. A good non-recoil desoldering tool is worth its weight in gold. A tip here is to stretch a silicone rubber sleeve over the PTFE nozzle to lengthen the nozzle life and to 'wrap around' the joint just prior to sucking the molten solder.
The choice of test instruments is vast. Beginning with a good quality digital multimeter, the range of options goes on and on. Buy the best multimeter the budget will allow, the investment will be repaid many times over and the instrument can be expected to last a lifetime of use. Some models incorporate transistor test modes and although not to laboratory standards, will ensure that any dud devices are quickly eliminated before inserting into a board.
Today, some very sophisticated test gear is coming on to on the surplus market. Signal generators, oscilloscopes and distortion analysers may be bought from a variety of sources. For instance, one of the many amateur radio rallies across the country will yield many items at a fraction of the original cost.
Chapter 1
The NAC range of Pre-Amplifiers
I'm given to understand that NAC stands for Naim Audio - Control Amplifier of which the little 12 and the 12S were the first models to appear. Consisting of a sheet metal box fabricated from aluminium, these amplifiers rocked the hi-fi world when partnered with the then new Linn LP12. The tenuous marriage of the two companies, Linn Products and Naim Audio was set to dominate the industry for at least the next decade and the two manufacturers’ products became an industry standard for magazine reviewers against which all newcomers were judged.
On examining the early products from Naim, it is obvious that they all share substantially the same circuits, albeit differently laid out, as the later models. Only certain components have been changed in most instances. Consisting basically of a unity gain (X1) input buffer stage followed by tape switching after which there comes the volume control and the X12 gain stage proper. All the NAC range circuits are instantly recognisable as being similar if not absolutely identical. This makes the job of finding your way around so much easier when working on these amplifiers. The early pre-amps were provided with only one channel (+24Volts) of power which was shared between all the various stages.
The pre-amp supply was usually provided from a separate circuit within the main power amplifier and the use of an external dedicated supply such as the SNAPS gave the necessary boost to the sonic performance.
Series pass resistors and shunt capacitors in an RC network (described later) are the only means of preventing intermodulation between sections and channels.
Phono cards are equipped with their own Zener controlled pass regulator systems to achieve the desired voltage stability for these critical stages.
The early 12S has a rather badly designed phono stage occupying a lot of space within the casework and as such needs to be changed for the later 323 board if not already done so.
The balance and volume controls on the 12 are really crude, these particularly need to be replaced by more sophisticated variants.
Turning to the NAC42, this is housed in the now familiar extruded aluminium sleeve and comes in two different lengths according to vintage. Circuits are again the standard layout and vary in detail only. The 42 as with the 12, will only accept one channel of power and as such is ripe for improvement. The release of the 42-5 meant that the HICAP power unit with its twin channel power circuits could be utilised.
The 42 is laid out in a much larger format and has only the phono cards fitted using the pin and receptacle method used throughout the earlier models and by the 32 and the later 72. Layout of the 42 varied according to vintage and several variants of the same model have been identified over the years of production. The 42-5 was a ‘fill in’ model just in advance of the 62 in which the only significant improvement over the 42 was the provision of one extra input and a mother board mod to take the twin channel HICAP power supply.
Soon after the introduction of the 42-5, there came the 62 with a tape operated selector switch and provision to take the HICAP or the SNAPS power inputs. The circuits, yes you've guessed, were exactly as before. The use of non-polarised electrolytic capacitors in the power rail decoupling circuits was about the only radical modification to the 62.
We now come to the 32 in it's various guises and the first of these will be found to share the same style of casework as the original 12. Considerably more complex in terms of circuit 'links' and switching, it nevertheless shared the circuits of its less complicated stablemates. Having inputs for two tape machines, two phono stages and an extra line input meant that this model is a little cramped inside. Luckily, there are few about, the model having been quickly superseded by the later extruded case variant.
The later versions of the 32 and 32-5 in the extruded case are relatively simple to perform major surgery upon. The only drawback is the number or wire connections to the sockets on the back panel, which all have to be carefully removed and resoldered once the work is complete. The plug in boards however, lend themselves ideally to all stages of conversion as removal and replacement takes only seconds.
So to the 72 which is simply, you've guessed right again, a variant on the 32. So-called 'Time Aligned' boards are fitted to this model as standard. This circuit takes a small portion of the left channel, inverts its phase and then feeds the resultant into the right channel. The same thing happens to a portion of the right channel which is fed to the left output. The result is an artificially widened stereo image albeit at the expense of increased harmonic distortion. You will be shown later how to remove these boards and simply install links for a clearer and less distorted performance.
Lately, the introduction of the NACs 82, 92, 102 and 52 in the full width cases complete the line-up. These units again share the same basic circuit architecture but with the refinement of relay switching for the inputs and some have remote control facilities.
That just about concludes this brief tour of the NAC series and puts some widely held beliefs into perspective. In reading through this book, the road will become clearer as to how these amplifiers have evolved and more importantly, how they may be improved.
Chapter 2
The NAP series of Power Amplifiers
The first out of the stable was the NAP120, a small fabricated cabinet containing a twin channel amplifier and although all the later amps shared the same circuit, the case size here is the limiting factor and therefore we shall not dwell upon this model. There are of course several modifications that will take the NAP120 into quite another league and referral to the later model data will acquaint the owner with the general principles.
Taking the earlier fabricated cased models first, the 160 and the 250 set new standards for power amps within the industry. Virtually all reviewers at the time used a piece of Naim equipment in some form or another. Several notable persons had entire systems by which all others were judged. This grip on the industry was to last for many years until the emergence of some of the very large amps from America.
Identified by their having screws to hold the case together and by having the output transistors fitted to the rear panel which acts as a heatsink. Inside, the two amplifiers share much in common, which the exception that the 250 has an extra pair of boards which act as voltage and current regulators supplying the main output boards. The 160 output boards share a common transformer and one pair of output capacitors of rather limited specification but in spite of this, gives a respectable performance. The 250, with the added complication of the regulator system, performs very well until driven outside its power envelope.
The first model to emerge from the Naim stable in the extruded case was the NAP 110. As a matter of interest, the description NAP is understood to denote Naim Audio Power (Amplifier) and the numerical component of the name gives the output power from BOTH channels into 4 Ohms.
For example: 110 divided by 2 = 55 Watts per channel into 4 Ohms.
The little 110 was quite simply a little gem, its performance was well within the boundaries set by the cases heat sinking capabilities.
Sharing one transformer and a single pair of reservoir capacitors, this amplifier set the scene for others to follow. The circuit boards are forerunners of the larger models and are virtually identical but for slightly less robust output transistors
Contained within a half width case, the 110 is capable of a very refined performance especially when converted to mono format. Although the factory 'party line' is that to configure the smaller amps for monoblock use is not possible, you will find this conversion is both easy and very cost effective.
Next in line comes the NAP 160. With amplifier boards virtually the same as the 110, the 160 has a slightly larger capacity power supply. A higher VA rated toroidal (circa 220VA), transformer and a slightly larger value pair of main capacitors constitute the main differences between the 110 and the 160.
The NAP 160 in the larger size case lends itself to many stages of improvement.
What is possible with these amplifiers for instance; judicious placing of components will enable the construction of a twin channel power supply with two toroidal transformers and two pairs of computer grade capacitors to effectively convert this amplifier into a true dual mono configuration.
The later 250 in the extruded sleeve is perhaps one of the best regarded amplifiers from the Naim stable. Very professionally presented, this amplifier set the standard for others to follow. Until the release of the 135 monoblocks, these were the most powerful of the range at 75 Watts per channel into 8 Ohms.
A substantially rated power transformer feeds a pair of computer grade capacitors to form the reservoir. A regulator board for each channel, limit the amount of current each amplifier board may draw under transient conditions and thereby protect the output devices from damage.
The voltage on the supply rails is maintained at a constant level, again in an effort to improve performance. Should any fault appear at the output of the amplifier or if the case temperature rises beyond certain limits, the power supplies will 'shut down' preventing any damage and reset once the fault condition is cleared.
Until the launch of the NAP500, the ultimate Naim power amp was the NAP135 with its dedicated power supply for the single channel board, the automatically triggered fan cooling ensuring that temperature rise is kept under control. Not a lot can be said about this amplifier except to equate the cost with the more complex NAP250 which is currently listed at the same price.
Several problems are common to the entire range; the most important being that the stability of the amplifiers is particularly suspect when driving low impedance loads or when using exotic loudspeaker cables. The Naim user manual gives warnings in this area and they should be heeded.
The NAIT 1 is the first integrated amplifier from this manufacturer and with a lowish output capability from the NAIT 1, is ripe for improvement although the size of the case does impose certain limits. The later NAIT, in a larger case (interestingly back to fabrication) has more space for additional larger components.
Although well regarded by a faithful band of followers, the entire range of pre-amplifiers and power amplifiers contain nothing of an exotic or audiophile quality and the stark reality is that the fundamental designs should have been revised long ago.
Chapter 3
NAXO Crossovers
An electronic means of splitting the various frequencies for use by dedicated amplifiers capable of driving the loudspeaker cones direct. These units are visually the same as the pre-amplifiers and are designed to be powered by the HICAP power units from this manufacturer. Comprising a series of buffer blocks made from discrete transistors followed by a filter network to filter out unwanted frequencies whilst passing those for use by that section. The complexity of the circuits can lose a good deal of the information fed into the unit and a great many modifications are possible to improve the NAXO in this area. Layout of the main circuit board includes three mute relays for the various sections, which eliminate switch on thumps as the circuits stabilise during power up.
Circuits of the NAXO range tend to follow on from the first incarnation with its 'plug in' modules. Gain stages in all models share the same unity gain (X1) buffer, which is not dissimilar to that found in the NAC pre-amps. Shown in figure ( ), the buffer stage is seen to be a two transistor common collector stage fed by a constant current source sometimes called a 'ring of two'. As discussed, this simple circuit has only a gain of one and is introduced to convert the incoming signal high impedance into a low output impedance to drive the filter networks in the crossover.
Begin the work by replacing all the resistors in the circuit by high definition components such as Welwyn RC55, they will bring a considerable improvement to the higher frequencies. If the budget will allow, Welwyn or Vishay Bulk Foils in the feedback locations will speed up these areas to good advantage. As with the NAC series of pre-amps, fitting the gyrator circuit or an LM317 regulator in place of the series resistor networks will prove to have a very noticeable effect. Don't forget the crucial 'slugging' of this particular module: they can 'go off' at some ultrasonic frequency if the power supply proves to be fast enough. The capacitors used to give the frequency roll off in the modules should be replaced by the best polystyrene variants that money could buy. Sadly, Suflex is no longer in business but newer materials are finding their way onto the market, the details will be found in the directory at the end of the book. Obviously, size constraints have to be considered but within reason, many products from different manufacturers will be found to be suitable.
The work is simplified if the NAXO is one of the older models utilising the 'plug in' board construction. Each module can be individually rebuilt and then tested to ascertain if all is well. If the model is one of the newer types, the entire board will have to be lifted from the casework before the work can begin. Make a detailed note of all the connections before carefully unsoldering all the connections from the board; (store the notes in a safe place!).
As with the NAC series of pre-amplifiers, the NAXO benefits from being supplied with as many autonomous channels of power as possible. The NAXO will accept eight totally separate power channels and the performance does benefit as a consequence. The chapter on power supplies gives a variety of power supplies for the home builder and it is advised that only the best components are considered when constructing any of the alternatives.
Chapter 4
Power Supplies
Dedicated power units for the range of pre-amps and crossovers began with the SNAPS in its earliest guise contained in one of the fabricated cases and later in one of this companys aluminium sleeves. Although containing a fairly large (circa 120VA) toroidal transformer, the output from the SNAPS is mainly limited by the simply configured regulator assembly.
The SNAPS power unit can easily be modified to comprehensively outperform the considerably more expensive HICAP.
All Naim pre-amps and crossovers accept only +24 Volt, single rail supplies and as such, do not have the rejection of supply aberrations of a 'split rail' design. This means the quality of the supplied power has to be of the highest order.
The HICAP is a standard requirement for the later equipment and consists of the following arrangement:
A large (circa 450VA) toroidal transformer feeds a pair of rectifiers each followed by a computer grade capacitor. The regulator assemblies consist of a 3 Amp rated TO3 regulator from the LM317 family decoupled by a couple of Tantalum capacitors. It must be said that this is a rudimentary circuit to say the least and as such, this basic level of regulation is in no way difficult to improve upon. Noise levels from this rather crude regulator system are attenuated by the RC networks present on the supply rails of the various stages. Unfortunately, this strategy, whilst removing most of the generated noise also raises the supply line impedance at certain spot frequencies.
For some strange reason, explained by technicians at the Naim factory as essential for 'correct' earthing practice, the signals from the various modules are routed through the power supplies before arriving at their final destination. As will be discussed later, this means extra connections having to be made plus, the close proximity of a signal line to a potential source of mains interference is very undesirable and best avoided if at all possible.
The heart of any amplifier system is the power supply and the importance of this component cannot be over emphasised.
The following section suggests certain modifications, which have been proved to work in a very satisfactory way and may be undertaken with confidence.
Power Supply Modifications - SNAPS:
The SNAPS power unit is tackled first and a circuit diagram is given below. This is a conventional transformer, bridge rectifier, and capacitor arrangement followed by a single LM317 regulator capable of supplying around one ampere of current. Later versions, sometimes called Snaps 2, already have twin regulators installed on the single PCB. It's then a simple rewiring job to bring the unused regulator into play after fitting the necessary five pin Din socket on the rear panel. High frequency stability has been given scant consideration and a pair of Tantalum capacitors around the I.C. (Integrated Circuit) regulator is all that prevents oscillation. Much may be achieved with this unit if the planned modifications are carried out and in this form, this little power supply will even out-perform the very much more expensive factory standard HICAP.
Looking at figure ( ), the rather simple architecture of the LM 317 and its decoupling components may be seen. This is as rudimentary as it gets folks and is not difficult to improve upon. Figure ( ) shows just one way of improving line regulation and reducing noise by a considerable margin. By installing a further regulator in the configuration shown, the performance of the humble 317 is taken to new heights. Follow the circuit exactly and don't forget to insulate each 317 from the chassis when mounting them. This is the simplest way to upgrade the Snaps and repays the small effort many times over.
If you need to fully upgrade the Snaps power supply, follow this route:
Begin by removing the circuit board and its wiring, the main capacitor, the input mains socket and the rectifier from the case. If intending to purely use the SNAPS as a power supply, removal of all the signal wiring and the sockets is recommended leaving a clean case into which the new supply is to be built.
Begin by installing a new capacitor, Philips 154 Series - 15,000uF @ 40 Volts. Fix the new bridge rectifier into the case floor in a convenient position and wire as shown. Better still; replace the standard rectifier by fitting a fast recovery or Schottky module for lower harmonic distortion. Wire the rectifier to the capacitor using a good grade of 6A cable or silver plated PTFE cable if the budget runs to it. Fix the new circuit board to the case ensuring that there is a smear of heat transfer paste between the case and the module. Fit the supplied heatsink to the rear of the case in the position indicated and don't forget the heat transfer paste on the rear face. The module operates in pure Class A and therefore a good amount of heat is generated, this is quite normal. Wire the module to the capacitors and then to a new five pin output socket as indicated.
Check ALL wiring at least twice against the diagram then install the mains filter IEC socket and carefully wire it up, not forgetting the case earth.
Powering up should result in a precise +25 Volts appearing between the ground connection and the test points indicated. Factory adjustment of the two outputs to +25 Volts on each channel ensures reliable operation of the mute relay within the pre-amps.
Power Supply Modifications - SNAPS - (Alternative Strategy):
In the PCB overlay section of this book, a more sophisticated layout (the APX) is provided to enable the more experienced constructor to etch a printed circuit board for the SNAPS power supply. Consisting of an advanced LM317 configuration as described by Neil McBride, this supply totally outperforms the standard Naim offering. Ripple is less than a tenth of the original regulator assembly and the series of decoupling components ensure the fastest delivery of current at all frequencies. Follow the work schedule in the preceding paragraphs but instead of installing the custom made module from Avondale Audio, construct and install the DIY regulator assembly following the circuit assembly details very carefully.
Power Supply Modifications - HICAP:
Before commencing any work, it is important to decide whether the HICAP is to be used as a pure supply or wired as intended by Naim. There are a couple of interim stages for modding this unit but the full modification to the HICAP involves removal of the regulator module and substituting with a much more sophisticated version.
Start by replacing the two main capacitors by the Philips 154 - 33,000 @ 63 Volts. These new capacitors will provide a smooth and vibrant source of power right across the audio spectrum and have a reserve capacity over twice that of the originals.
The wiring may be renewed for silver plated PTFE insulated cable for better resolution over the audio spectrum and a mains input filter of three or four amps rating may be installed which attenuates any mains clicks and noise by a factor of ten.
One of the new generation of audio grade toroidal transformers now available will guarantee low mechanical noise, low external fields and the lowest harmonic generation yet measured.
Replace the two bridge rectifiers by Schottky or Ultra Fast, Soft Recovery diode equivalents taking care with the wiring. In addition, a 1n (1000pF) and a 100 Ohm resistor/capacitor network in series placed across each fast recovery diode lowers even further the generation of harmonic distortion.
You are then left with alternatives: Either construct the APX regulator modules as shown in the accompanying details or, replace the existing regulators by one of the Class A modules available from Avondale Audio.
If the HICAP is to be used purely as a power unit, the signal wiring and associated sockets may be removed and discarded. The one remaining socket for the low voltage power output may be changed for an XLR type having gold plated pins for better contact.
Power Supply Modifications - FLATCAP:
Basically a re-vamp of the SNAPS contained in the full width casework, this very simple power supply has loads of space into which many and various goodies may be installed. Consisting of one 120VA toroidal transformer followed by a single storage capacitor the twin LM317 regulators then take control of voltage regulation. Again, the simplest and the cheapest supply it is possible to devise to provide owners with an external power source for their pre-amplifiers. The options for upgrade are many but the transformer is the most obvious candidate for replacement. Twin, high-grade toroidals of a more substantial current rating are recommended. Around 220VA audio grade versions should be just about right. A pair of computer grade capacitors of 22,000uF followed by a choice of the DIY regulator module from the circuits section of this book, the APX module or the class A module from Avondale Audio, is left to your particular choice. Any of these alternatives will totally overshadow the original regulators performance.
Chapter 5
Simple Tuning Tips
Earthing:
There is no better way to extract more information from the Naim pre-amps than to re-arrange the rather strange earth arrangements as recommended by the factory.
This peculiar system is explained by Naim technicians as being the best way to avoid earth problems.
If an earth problem does exist in any equipment, hum and noise will be the result. In explanation of the system used by Naim, they try to encourage the 'clean' earth point which is at the junction of all the earth tracks on the pre-amps circuit board, to appear to be within the power supply which is at the other end of the interconnect lead.
Current wisdom is that the 'clean' earth and the so-called 'dirty' earth should be quite distinct and separate. If we accept that the 'dirty' earth is best confined within the 'noisiest' item of equipment, i.e.. The power supply, then it is undesirable to attempt to re-configure that point as the 'clean' earth.
Quite candidly, combining signal and power supply in a single lead is a highly suspect arrangement and should be avoided at all costs.
Although the SNAIC lead is screened overall, the signal and power conductors lie side by side with no shielding screen between them.
The answer to these problems lies with making up a pre-amp to power amp signal lead using some high definition cables and new plugs then removing the signal cables from the existing SNAIC lead by simply snipping off the redundant conductors within both Din plugs. The modified SNAIC lead is then used only to carry power into the pre-amp and the new signal lead is free to convey a noise free signal into the power amplifier. If desired, make up a replacement SNAIC lead just to carry power into the pre-amp using silver plated PTFE insulated cables available from a number of suppliers. This improves definition and perceived speed.
It's perhaps best at this stage to install a pair of high quality phono sockets on both the power amp and the pre-amp to carry the signals. Naim defend their selection of Din connectors as being able to match impedance more accurately. The truth is that Din connectors do not have a characteristic impedance at any frequency which makes a nonsense of the statement from the factory. In any event, Din plugs will only accept certain types of cable and even then, are an absolute pain to solder properly.
The above procedures will enable the delicate details within the music to be heard as the noise floor drops away.
Casework:
The extrusion housing most of the range is a little 'bell like'....it rings horribly. As all semiconductor devices are microphonic to some degree, arrangements should be made to damp the walls of the pre-amp sleeve by using a proprietary brand of damping pads which are thin enough to enable the inner tray to slide in without binding.
A handy tip, try playing the pre-amp without the sleeve to investigate how much quieter the performance is (try not to spill your tea into the pre-amp at this time and don't attempt this trick with the power amp - there are high voltages inside which can kill you...!!)
Later models have a channel shaped piece of soft plastic over the lip of the chassis tray at the rear. This is obviously intended to prevent the case ringing by adding a damping effect.
Removal of the motherboard fixing screws and replacing them with the self-adhesive plastic stand-off pillars will also pay small but noticeable dividends in sound improvement.
Standing the pre-amp on isolating cones, particularly the ones sold by RMS and Stands Unique as their Carbon Fibre Isolators, proves a very worthwhile exercise.
Cleaning the rather undesirable Din sockets using a proprietary brand of contact cleaner will reap benefits as over the long term; terminals oxidise and begin to do a fair imitation of diodes, degrading the signal. Even better, as there is usually sufficient space on the rear panel of all the units, high quality phono input and output sockets may be installed enabling use of more exotic signal cables.
By connect the earthing terminal of the pre-amp to a metal stake or, better still, a series of stakes at 300mm spacing driven into damp earth, a quite noticeable reduction in background noise should be the result. Electrical supply houses stock copper earth stakes at about one metre in length and if a suitable spot can be found in the garden or even under the floor into which to drive the stake, so
much the better. Saturate the staked area periodically with a solution of rock salt or potassium permanganate and water to ensure a low impedance path to earth. Connect to the pre-amp using a fairly thick (2.5mm), copper cable.
Periodic removal of the cases of the power amps and removing the push on connectors from the boards one by one then replacing them, will clean the contacts of any oxidation products accumulated with time. A thin smear of a synthetic oil (Mobil One) or grease will ensure gas tight connections over long periods.
Phono and other plug in cards sound cleaner when supported using a thin fillet of silicone sealer (RTV) between the track side of the card and the motherboard carefully avoiding getting any sealant on the connectors. Do not be tempted to solder in the cards, no advantage will be gained, treatment of the connector pins with a good brand of contact enhancer is all that's needed.
In the more advanced section, removal of the balance control for a better stereo image is recommended and should be viewed as essential work for the serious listener.
On all equipment, periodically replace the fuses for new ones as the corrosion found on the end caps causes degradation of the sound. Better still; replace the fuses with a thermal trip of appropriate current rating. The lower series resistance of the thermal trip does help transient delivery and noise performance.
The volume control is usually an Alps 'Blue' or ‘Black’ on later units whilst early units used AB controls. A Noble or Panasonic variants with multi-finger wipers may replace the Alps for an increase in resolving power. The rather cheap and cheerful plastic selector switch found on the earlier 42 and 42-5 models is in dire need of being replaced for a cleaner sound and greater reliability. Several major manufacturers offer very reliable switches with a combination of contact arrangements. It is a relatively simple task to add extra line inputs at this time, thereby expanding the features of these pre-amplifiers.
Multiple power supplies:
As discussed earlier, the gain blocks used within Naim equipment may be split into single units for power supply purposes.
Two or more HICAP type units may be used to supply one pre-amp by splitting the track under the boards and installing a new connector socket to take the additional cable. See chapters 10 and 8 on advanced NAC mods for details.
Obviously, special leads to operate multiple power supplies will have to be made, either by the DIY enthusiast or by one of the specialists listed at the end of the book. It is well to remember that having a good quality power lead is money well spent and this can have a serious effect on the sound of a pre-amplifier.
Some of the very latest pre-amps from Naim now use multiple power supplies, the factory now having taken the view that there is some benefit in this configuration after many years of denying there would be any advantage in using more than one supply.
The way forward is as follows: after modifying the track by splitting the various sections off into as many parts as the number of power supplies will support, add another socket at a convenient spot on the rear panel along with the relevant connections to the track side of the pre-amp mother board. As far as is known, the only minor drawback, which may be encountered, is to defeat the anti-thump mute relay on switch on. This is easy to avoid by switching on the power amplifier after the pre-amp circuits have had chance to stabilise.
Should the owner require more channels of power than the budget for Naim power supplies will stand, it's perhaps best to consider building the kind of supply shown below. The 42 will take as many as six channels of power, the 32 and 72 models will accept up to eight channels. At this level, these pre-amps can be expected to easily outperform the larger and much more expensive 82 and even the flagship 52 at a fraction of the manufacturers list price.
It cannot be emphasised too strongly that the heart of any amplifying system is the power source and any supply line aberrations produced will ultimately find their way on to the reproduced signal.
In introducing the power supply architecture in this book, the reader may be surprised to discover that only relatively small transformers are specified. Today, the use of large toroidal transformers is widespread. It would seem that the manufacturers think it quite sufficient to throw in these 'brute force' transformers as a substitute for well designed and highly stable electronics. It has been found that a well designed and manufactured laminated transformer of moderate size, will outperform even some of the larger toroidal transformers. The minus effect of using toroids is to increase the bandwidth open to the incoming signals, this enables mains borne interference to pass through the transformer and into the circuits of the amplifier.
A 'haze' or 'smog' is the result and is largely eliminated by simply removing the toroidal and replacing it with a good quality ‘EI’ laminated or 'C' core variant.
The circuit described below has been tested over many years and in many different systems all over the world. Beginning with the mains transformer there follows the rectifier which is a dual Schottky device. The harmonics generated by the Schottky rectifier are many times less that those produced by conventional rectifiers. This means a cleaner, more stable direct current supply to the main reservoir capacitor. In this design, series main capacitors are used decoupled from each other by small value inductors.
The inductors serve two purposes:
any high frequency or radio frequency line interference remaining on the DC rails will be attenuated and
any mutual reactance caused by using two similar capacitors in parallel is nullified.
There follows a primary stage of regulation, which provides a reservoir of very clean and stable DC power from which the main regulator may draw upon. Surrounded by a network of carefully selected bypass components, the delivery of current at any frequency is assured and the benefits in the improvement to the sound quality of the amplifier will be immediately obvious. The main regulator may be either one of the LM317 family from suppliers such as Linear Technology. This LM317 version has an output impedance and noise level much lower than the standard LM317. The output from the module is decoupled again by a selected network of components and a soft start relay ensures that the supply is only presented to the pre-amp circuits once the voltage has reached a certain level.
The advantages of constructing a power unit such as that described rather than converting an existing unit are obvious. Firstly, even early second-hand examples of SNAPS and HICAP are expensive and sometimes difficult to find. The configuration of a home constructed power unit, once chosen, may be upgraded at some later time once budget allows. There is no need of course, to build all the modules at once, more may be added later as confidence in the work builds and as the budget allows. Although the earlier versions of the NAC series will run from one channel of power, it is recommended that at least a twin channel be constructed to begin with. Adding additional modules will show an improvement in sound quality at each stage.
Begin the work by first deciding how many modules will be required and prepare the circuit board of the pre-amp as described in the following chapter. You may use your favourite make of cable to internally wire the pre-amp and the power supply modules.
Chapter 8
Pre- Amplifier Power Supply Construction.
The section containing the Printed Circuit Board overlays has a full sized layout for the APX power supply module. In order to produce a PCB from this artwork, an acetate photocopy of the layout will have to be made. This service is available at most copy shops for a modest sum. Taking care to orientate the artwork the correct way up and following the manufacturers instructions, a board may be made using pre-sensitised single sided PCB material. There are several books available through the major electronics magazines and retail outlets, which give details of how to manufacture PCBs as DIY projects. Alternatively, professionally finished boards, roller tinned, protected with a solder resist layer and silk screened with all component locations are available from one or more suppliers at the end of the book. The constructor may specify one, two or three section modules either as unpopulated PCBs or fully built and tested modules when ordering.
Begin the construction by using the bare PCB as a template to mark the mounting holes in the chosen enclosure. Drill the holes and again offer up the PCB to check the accuracy of the work. Once the mountings have been checked, the board may be populated. Insert the smallest components first after thoroughly scraping the leads with a sharp Stanley blade. Ensure that polarity of electrolytic capacitors is strictly observed. Solder each joint in turn and check for solder bridges, which if left, will undoubtedly destroy some of the components.
The final components to be fitted to the board are the main capacitors and again, care should be taken to observe polarity. If they are installed the wrong way round, a loud bang will result as the electrolyte overheats and blows off the protective canister with catastrophic results for the cap and a high laxative factor for the owner.
After cleaning the finished board with Isopropyl Alcohol and a toothbrush to remove the resin around the soldered joints, inspect yet again with a magnifier for poor joints and solder whiskers across tracks.
Connect the mains transformer ensuring that the wiring protocol is observed and with a voltmeter connected between test point one (TP1) and 0v, the meter should read 35 Volts DC 1Volt. Should this voltage be outside this specification or there being no voltage at all, shut down the supply and check for short or open circuit tracks and in particular, incorrectly orientated polarity conscious components.
If the voltage test proves satisfactory, apply the test leads between 0v and test point two - (TP2) . There should be a voltage present of between 18 and 27Volts DC. By adjusting the preset P1, the output voltage may be adjusted to read 25Volts after being allowed to stabilise for a minute or so. After five minutes, re-check the voltage, which should not vary by more than 1/10Volt and if all is satisfactory, the time has come to connect the supply to the pre-amp.
A word or two about mains transformers is called for at this stage: In choosing the type of transformer for use in pre-amplifier power supplies, much thought has been given to the bandwidth, external flux densities and that old problem, radiated noise. As mentioned earlier in different sections of this manual, toroidal transformers, although efficient for their size and weight, transmit far too much spurious mains borne grunge into the rectifier circuits to be considered 'audiophile quality'. Unless size is absolutely crucial, the strongest recommendation is to avoid toroidal transformers at all costs. By carefully siting an 'EI' laminated core transformer or, better still, a 'C' core variant, a valuable improvement in performance both in terms of noise and linearity will ensue. For comparison purposes, figure ( ) shows the size relationship between the various types of transformer.
As the pre-amplifier power supply does not generate untoward heat within the casework, the constructor may consider using a non-metallic enclosure. Several smart ABS cases are now available which are suitable for this job. MDF enclosures are easily home made by having the material cut to size at one of the major DIY super stores and using a proprietary adhesive, a suitable case may be cheaply made. It is possible by using this method, to reproduce the Naim sleeve and by fashioning an aluminium tray, make something that looks as if has come out of the factory at Salisbury. In order to reproduce the textured finish on the casework, buy an aerosol can of black Hammerite SMOOTHRITE. This product is unique in that it contains tiny glass beads and by applying several very thin, almost dry MIST coats to a clean surface, the classic textured effect, not unlike the Naim factory finish, will eventually appear. Care should be taken not to apply too much paint at any one time and allow a couple of hours between coats for drying.
Steel casework has inherent shielding properties against electro-magnetic radiation and of course, RF (Radio Frequency) radiation. The drawback is that the case may resonate or 'ring', imparting a certain sound aberration onto the music. This problem is easy to overcome by applying damping pads sold by the car spares emporia and known as dead sheets. Aluminium cases are useless for magnetic shielding but are very good for RF resistance. MDF or plastic cases are virtually transparent to RF and magnetic radiation but hardly ring at all. The constructor may consider an MDF or plastic case lined with aluminium foil, which is solidly glued onto the insides of the material and carefully bonded electrically to the mains earth. There is a metallising spray on the electronics market, which enables a plastic enclosure to be screened simply by applying a coat or two and bonding the dried layer to an earth connection. The only drawback is the cost, these aerosol cans tend to be very expensive. Siting such a supply unit must be carefully considered particularly if it utilises a frame or laminated transformer.
Finally, an important word about safety: ALL metal parts of casework, transformer metal cores and switches, MUST BE EARTHED. Please have your work checked by a qualified electrician if you have any doubt about what to do.
Chapter 9
Advanced Pre-Amplifier Mods
The Basics:
The NAC series pre-amps from Naim as discussed earlier, share the same basic circuit. After initial signal switching by the front panel rotary switch, a unity gain (X1) buffer amplifier is followed by the volume control and the final amplifier stage with a total gain of around X12.
The main problem with the design of these circuits is that the final and therefore most critical system of regulation of the incoming power is solely dependent on one resistor and a capacitor. As this RC (Resistance - Capacitance) network is frequency dependent, the performance of the circuit is nowhere near as good as it could be. The impedance of this rather rudimentary regulator varies according to the frequency at which power is required. Certain frequencies are favoured above others so the performance and timing of the connected circuits suffer adversely as a consequence.
The signal coupling capacitors are invariably 10uF @ 35V Tantalum beads. These were once hailed as the best thing since sliced bread in audio equipment. Sadly, they've failed to live up to the forecast promises and may be replaced by some rather special components from Japan with immediately noticeable results, more on this later. Although the Naim modules are constructed from fairly good specification components, as mentioned earlier, nothing of an audiophile quality has been incorporated into the construction.
The typical basic architecture of the NAC series is shown in figure ( ) and the progression of the signal, as it passes through the stages, may be easily followed. To those who believe that the original signal actually passes through one stage to the next, it may come as a surprise to discover that this is actually a myth. Think of the progressing this way: take a small photograph to your local copy shop and make an enlarged copy. From the enlargement, make another then another and so on until the desired size is reached. At each stage of enlargement, there is a loss of detail and quality. So it is with amplifier stages: each stage in an amplifier, passes its output on to the next stage for amplification (magnification if you prefer) for subsequent further increase(s) in signal level. Each stage adds or subtracts something from the signal and even the finest amplifiers in the world cannot recover any information thus lost. It is therefore important to ensure that each stage of amplification processes or influences the signal as little as possible.
The factory standard architecture is shown for those not fully conversant with circuit diagrams, in block form so as to be more easily understood. The triangular objects represent the active gain stages in the pre-amp and the rectangular objects, the resistors.
From the diagram, the progression of the signal through the pre-amp can be clearly seen. Beginning with the phono stages, the lowest level of signal, after which the rotary selector switch chooses whether to select phono or one of the line level inputs. There then follows the source - mute – tape toggle switch located at the top of the front panel. The output signal for tape recording is merely ‘tapped off’ the signal line using a ballast resistor of 68 Ohms. This is not ideal as the tape machine may easily be ‘seen’ by the circuits and a degradation of the signal is quite audible as soon as a tape recorder is connected. The unity gain buffer is next in the equation and this is intended to equalise the impedance of any connected equipment before the signal reaches the balance and volume controls. As most modern day kit has a low impedance output configuration, this stage becomes superfluous and we are able to use it instead as a tape output buffer thereby eliminating one obvious cause of signal degradation. The balance control is a potential cause of signal pollution and crosstalk. The factory standard volume control is an Alps black and is not difficult to improve upon by a simple replacement. At the end of the amplifier chain, the X12 output stage takes the received signal from the volume control and magnifies it before passing it to the output relay section. The relay is connected so that the signal has to pass through the contacts causing a small but noticeable reduction in perceived quality.
The Unity Gain Buffer Stage:
In designing these amplifiers, attention was obviously given to the source impedance of various components such as cassette decks and tuners. The unity gain stage is introduced as an easy to drive input to the pre-amp proper to ensure linearity of the received input signal. The fairly low output impedance of the stage serves to 'drive' the volume control with a consistent, frequency independent signal. Today’s source equipment generally poses no such problems and in particular, virtually all CD players are capable of driving power amplifiers direct.
All this serves to illustrate that the unity gain stage has become redundant and may be removed from the signal path with a subsequent improvement in resolution. The stage may now be put to a more useful purpose, that of acting as a high impedance input, low output impedance tape buffer. In standard NAC pre-amps, the tape output signal is simply tapped off the main signal path and only buffered by a couple of 68 Ohm resistors. Using this method, the pre-amp signal is able to 'read' any connected tape machine and as a consequence, the sound quality of the pre-amp is somewhat degraded. Using the unity gain section as a tape buffer fed by a fairly high value series resistor, ensures that the pre-amp signal 'sees' much less of the tape machine than in the standard pre-amps.
Unity gain simply means that the stage act merely as an impedance converter and has no voltage gain at all. The stage may not be configured to have gain and happily for our purposes, there is no reason to do so.
Beginning with the 470R input resistor and the 470pF capacitor forming a low pass input filter, the signal then passes into a compound pair of transistors TR1 and TR2. This configuration is fed by a constant current from a ‘ring of two’ formed by transistors TR3 and TR4. As the circuit does not use overall feedback, the question of replacing the associated resistors does not arise. Modification of this circuit requires replacement of the input and output capacitors by either Philips 128/129 series solid aluminium or the Oscon from Sanyo with its organic electrolyte. Replace the 470pF polystyrene at the signal input with a lower value to improve the high frequency response – a 150pF or 220pF should do the trick here. At the same time, high resolution resistors from the Welwyn RC55 range replace the input 4K7 and 2K2 metal film versions. The stage can be a little unstable in standard form and the drill is to connect a small value (47pF) silver mica capacitor across base and collector of TR2 to keep things on an even keel. The factory now employs this technique on current models.
It is feasible to replace the transistors by low noise variants such as the 2SB737(PNP) and the 2SD786(NPN) with a small improvement in overall noise figures. Take care to orientate these replacement transistors correctly as their pin details vary from the standard components. As this stage will be in future used only as a tape output buffer, there is little point in spending too much time and effort in going further than the scheme outlined above. The 68R resistors previously used to buffer the signal output to the tape machine are discarded and the revised tape buffer circuits are fed by series resistors of 10K value. This ensures that the signal paths are as lightly loaded as possible while still guaranteeing correct operation of the tape buffer circuits. It is of course possible to introduce a toggle switch at this point to disconnect the tape buffers from the circuits altogether. There are a number of locations available including one just above the LED on the front panel.
The Balance and Volume Controls:
Referring to figure ( ), the modified architecture of the pre-amplifier is shown clearly and you will note that the balance control has been removed and replaced by superior quality fixed resistors. This procedure alone reaps large benefits in the transparency stakes and consists of replacing the control by a pair of Welwyn of Vishay bulk foil resistors to ‘pad out’ the impedance of the input to the volume control. The choice of resistor value is best determined by experimentation and around 2K7 is a good starting point.
If it is intended to incorporate the shunt connected volume control, the balance control position may be filled by the two bulk foil series resistors. This provides a neat solution of where to break into the circuit.
Replacing the standard Alps volume control by a Noble or one of the new Panasonic variants produces a worthwhile upgrade for a modest cost. These controls, being slightly smaller than the original, will require short solid wire links to be soldered on to their pins and by carefully adjusting these, the component may be fitted quite successfully so that the control shaft exits at the same position as the original. A small conversion PCB is available from Avondale Audio to carry out this mod.
It is possible to fit a stepped attenuator constructed from an Elma Swiss precision 23 way, 2 pole rotary switch. In some models, this involves removal of part of the printed circuit board and subsequent repair of the removed track by installing links of stiff tinned copper wire. As there are a good few variations between models, this conversion should only be attempted by the more experienced constructor or be entrusted to a specialist engineer.
Signal Switching:
Standard NAC series pre-amps have a duplicated tape input selection both on the rotary selector and on the Tape/Source toggle on the front panel. It makes sense to configure the toggle switch as the sole tape machine selector thereby freeing the tape position on the rotary switch for one extra line input. At the same time, the toggle switch may be replaced by a version with either silver or gold plated contacts available from any one of several suppliers.
Prudent re-siting of the rear input Din sockets will mean that the number of inputs may be increased to around five or six. One or more suppliers at the end of the book are able to offer a pre-drilled black anodised chassis for the NAC range along with a self adhesive front panel with six positions marked. A rotary switch with a corresponding number of contacts should be selected for the task. The NAC62 with its tape operated switch, is not suitable to be converted in this way unless the selector switch is replaced by a rotary version.
The rather rudimentary rotary switch fitted to most models is best renewed fro example, by one of the excellent Elma range, which has a solid action along with hard gold plated contacts for a long trouble free life. At this point, a rewire of the signal input leads with your favourite exotic cable will further improve things.
The X12 Gain Output Stage:
Apart from the phono stage if used, the most crucial stage in terms of overall sound quality is the output gain stage. Similar in architecture to the Unity Gain Buffer Stage, this stage multiplies the incoming signal by a factor of twelve. From the circuit diagram shown in figure ( ), the power supply decoupling resistor/capacitor may be clearly indentified. This rudimentary use of a low pass resistor/capacitor (RC) element is in no way appropriate in equipment at this price level. The constructor is left with a number of choices: 1. Leave the existing arrangement alone. 2. Replace the RC network by a LM317 or similar or, 3. Fit a VBE multiplier (gyrator) in place of the 27R resistor. I prefer the sound of the gyrator circuit, which is very smooth, and without emphasis on any part of the spectrum. Others may like to leave in some of the 'bite' to the sound and here the LM317 scores in terms of 'Naimness'. There is a recent LT317T (T for tested I believe), replacement from Linear Technology which, when correctly networked and configured in tracking format, figure ( ), should provide an excellent alternative to the RC network. In considering which components to replace, the 1K and 12K resistors along with the 47uF capacitor in the feedback loop must be deemed to be the most crucial. Bulk foil resistors with at least a 47uF Oscon capacitor should be the basic minimum here.
A note of caution here: just because the circuits performance benefits from replacing certain resistors, this formula does not necessarily work when applied to the rest of the circuit. Many an amplifier has been ruined because the owners went 'overboard' with new component technology and the response of the circuits was taken beyond what is considered prudent - the motto here is - 'use sparingly' and although already mentioned, go one step at a time in case you have to back track.
The Phono Stage:
Consisting of the now familiar ZTX384 NPN transistors banked in parallel to reduce thermal noise, each device has a ballast resistor to ensure current sharing. The rather ordinary commercial grade components are as usual, not in the least esoteric and as such are ripe for renewal. The voltage regulator system varies from the line stages in that a zener diode is used for reference purposes, the ZTX384 series pass transistor being decoupled by a resistor/capacitor network. The entire phono section along with the moving coil amplifier shares the same single regulator and the performance suffers as a result. The following section will show how to introduce an extra series regulator into the circuit enabling the full performance of the phono stage to be realised.
Let's start at the front end of the board by replacing the ZTX384 transistors by a rather more sophisticated and quieter device - the 2SD786. The pinout of the 2SD786 varies from the ZTX384 and the correct orientation is shown in figure ( ).
Carefully unsolder the original components using one of the methods described in the soldering section of this book and install the bank of 2SD786 transistors in a neat fashion. Solder with a good grade of silver loaded solder for a low resistance joint. Replace the surrounding resistors by Welwyn RC55 series equivalents or the rather more expensive Welwyn/Vishay VSRJ series for a flatter response at all levels. This is best done one by one and to avoid mistakes, measure each resistor coming out of the board against the resistor being installed. The remaining ZTX384 transistors may be replaced by the same 2SD786 variants and the PNP ZTX214 transistors by a 2SB737 equivalent for a more lucid and quieter performance again ensuring that all the transistors are correctly orientated.
The watchword is check then recheck all the work and a good tip is to work on one phono card at a time to allow any mistakes to be easily identified.
The 10uF Tantalum input and output capacitors may be renewed by either the Philips 128 solid aluminium version or the new organic Oscon components from Sanyo of Japan. Both these capacitors sound somewhat different and the listener may prefer one against the other once they have had several days to 'burn in' but in any event, the sound will be far superior to the original Tantalums. The 47uF standard electrolytic decoupling capacitor is best replaced by an Oscon from the SG series. This reaps dividends in sound quality and is immediately apparent as an improvement.
The capacitors originally installed by Naim for the crucial RIAA network are close tolerance Polyester components. In this sensitive area, Polyester capacitors are not considered to be appropriate as their internal losses are far too high to be regarded as high fidelity units. A minimum of a Polypropylene or better still, a Polystyrene component is what is needed here. The difficulty comes when trying to find suppliers of these rather rare components and of such a size that will fit onto the Naim boards without too many hassles. Arrangements have been made with the suppliers in the directory at the end of this book to stock suitable components for this important modification. In particular, the 22nF and the 68nF in small format will be made available to the constructor.
On one side of the phono card will be found a 470R, ¼W resistor, which will invariably be discoloured by excess heat. A seeming miscalculation by Naim designers here as this resistor should at the very least be rated at ½W. Substituting a 470R 1W metal film here ensures reliable working from now on.
A ZTX384 series pass transistor takes care of the local voltage regulation whose base and therefore emitter voltage is governed by a zener diode decoupled by a series resistor. Zener diodes are well known to radiate wide band noise and are best avoided if at all possible. In this application, the actual voltage fed to the circuits is less important than the noise of the supply so removal of the zener diode is a practical proposition. Referral to figure ( ) reveals the before and after picture for the revised power supply circuit and this re-arrangement of the regulator reaps a small but significant improvement in noise levels.
The pass regulator supplies both the critical moving coil or moving magnet stage along with the equalisation section of the phono card. Received wisdom suggests that these sections are best served by separate stages of regulation. A glance at figure ( ) will show how this is possible. A small VBE multiplier is constructed on proprietary PCB or a miniature piece of Vero board as shown in figure ( ) . The completed module, once tested, is 'piggy backed' onto the circuit board using a spot of RTV or silicone sealant and joined by short wire links to the points indicated on the diagram. This arrangement allows a cascaded regulator system to supply the 'front end' of the phono card thus ensuring there are less intermodulation products to interfere with this vitally sensitive stage. Don't forget the decoupling capacitors from the positive rail to earth after the revised regulator stages to keep the whole circuit stable and free from spurious oscillation.
Naim’s Prefix, a several hundred pound remotely located phono stage is merely two revamped 323 stages on one relaid PCB. It stands to reason that a pair of 323 cards can be made to perform the same job of the Prefix and at a fraction of the cost. The choice of enclosure is left to the constructor and there now is a variety of combinations of power supplies which can range from a simple single supply to as many as four channels of power from a dedicated PSU in a separate enclosure. If the 323 cards are modified as in the previous section, the points at which power supplies can be connected is quite obvious. Care should be taken in mounting the 323 cards into an enclosure and the opportunity exists to incorporate some sort of compliant device to eliminate vibration problems introduced from the outside world. These special PCB mounts are available from a number of component distributors.
Mechanical Considerations:
Certain models, notably the NAC62, used a foam pad bonded to the floor of the chassis tray, presumably to offset the effects of case vibration. Unfortunately, the surface of the pad is in contact with the track side of the motherboard. The resistance of the pad will vary according to the level of humidity in the listening environment and will act as a gigantic resistor which is connected across most of the tracks with sonically disastrous results. Remove this pad using a scalpel or a very sharp carpenters chisel. The bulk of the pad may be removed this way and soaking in a proprietary label remover or white spirit may see off the remaining adhesive layer. Ensure that the PCB is well out of reach of any errant liquid during this procedure.
Chapter 10
Power Amplifier Mods
Section 1: The NAP110:
Elementary but DO DISCONNECT from the mains supply first..!!
Remove the case by undoing the four feet and the two countersunk hex head screws. Store the sleeve safely to avoid marking the rather soft finish. Begin by removing the auxiliary power supply components, which are intended to supply the connected pre-amplifier. These may be discarded as it is envisaged that any serious audiophile will use a dedicated power unit for the pre-amp.
Ensure sleeves are fitted to prevent any short circuits on the cables that were previously used to supply the rectifier.
Unplug the main boards not forgetting to support them from undue flexing and remove. The white thermal compound may be wiped off the heatsink bracket as this will be renewed when the board is re-installed. Store the boards safely away from possible damage. As the figure ( ) shows, the common Zero Volt lines are all connected to a stiff wire link soldered between the two main capacitors. This link is best removed with all the lines still connected - it will save much work later. This is achieved by sucking the solder away from the link ends (not forgetting to get them hot first..!!). By bending the ends of the wire link to a ninety-degree angle, the link may be wangled out of the capacitor solder tags whilst the solder is still in a molten state. De-solder the positive (red) leads in one go, do the same with the negative (black) bunch of leads. Remove the old capacitors and discard.
At this point it is a good idea to give the entire chassis a thorough clean with a proprietary foam cleaner. Also recommended is to check the tightness of the transformer securing screw. As the original capacitors have a non-standard diameter, the smaller replacements will have to be held in by simply packing out the clamp with a little foam rubber such as a draught strip. Re-solder the wire link with the lines still connected, to strong solder tags through which the capacitors terminal screws will pass. At this point, cut out the two green cables, which connect, to the two loudspeaker negative terminals at the rear of the amplifier.
Replace the six-amp rectifier bridge by a twenty five amp version to cater for the increased inrush current required by the new capacitors. The owner may like to consider replacing the original rectifier by Ultra Fast, Soft Recovery diodes now available. These may be assembled in bridge form on the small PCB layout to be found in the PCB section in the book. Some constructors prefer the sound of Schottky diodes and these may be used on the same PCB providing their reverse voltage rating is appropriate for the supply in question. The 200 Volt - 20 Amp Schottky devices now available from Motorola should prove reliable in most cases.
Replace the loudspeaker output terminals by binding posts from the Michell range. Either gold or rhodium plated versions will provide a much better contact than the original budget terminals. Rewire the negative loudspeaker lines by some silver plated PTFE insulated 19.025mm stranded cable and connect to a common centre point screw using well-soldered tags. Using a similar cable, rewire the positive loudspeaker lines and remove the original wiring usually coloured blue and white. If desired, a pair of high quality phono sockets may be installed over the original DIN socket and insulated from the chassis. These enable the use of larger diameter exotic signal cables than can be accommodated into the original DIN plugs. If desired, the entire amplifier may be rewired by PTFE insulated, silver plated wire of an appropriate current rating, rewards will be reaped in the transparency stakes. An image of the completed chassis ready to accept the circuit boards is here, figure ( ).
Turning now to the boards, begin by removing one component at a time after making a detailed drawing of the position and more importantly, the polarity of the component about to be removed. Using the maximum temperature that your iron will produce, melt the solder whilst at the same time, introducing the nozzle of the solder sucker. Releasing the piston will literally slurp up the molten solder into the barrel. Another method of component removal is to quickly melt each leg whilst at the same time, pulling on the body of the component with locking pliers. Re-melting each leg in turn will ensure that the component will eventually be extracted from the board. This all takes practice and patience and it is recommended that you develop your skills on a scrap PCB, say from an old TV before you take on the real thing.
Circuit Description:
The circuit common to all the NAP range is shown in figure ( ). A 'long tailed pair' input is provided with a constant current supply of around 1mA by TR3 and its associated components. The next stage TR4, fed by a constant current of around 9mA by TR6 and associated components, amplifies the resultant signal to a level usable by the two drivers TR9 and TR10. This second stage, along with TR5, is also used to control the bias current setting although for some strange reason, TR5 is not mounted on the heatsink as is common practice today. The result is that there is no automatic bias current limiting when the amplifier temperature rises. The output transistors are in what is commonly known as a 'quasi complimentary' configuration. This enables the use of NPN power transistors instead of the more usual NPN/PNP pair. NPN transistors are generally cheaper, more easily available in high current devices and are more reliable than PNPs.
The output stage is protected against over current by measuring the voltages across the 0R22 emitter resistors. Once the average current exceeds about 5 Amps, the voltage at the bases of TR7 and TR8 rises above 0.6V turning them on. The bases of the driver transistors, TR9, TR10 are then 'clamped' preventing any further increase in output current. The 10uF Tantalum capacitors average the signal to a mean level so the board is able to supply much larger short-term transient currents. On paper, the system works well but, as not all 'technically correct' designs have the desired result, this kind of output protection has a deleterious effect on sound quality and is best avoided if at all possible.
Extensive use of bypassing capacitors around the circuit board is evidence that the design is inherently unstable and therefore needs this level of compensation to prevent oscillation.
Whilst on the subject of instability, virtually all audio products exhibit some form of unwanted oscillation at HF (High Frequencies) and once the problem area has been identified, neutralising is relatively straightforward.
The NAP series of amplifiers use several pole filters in an effort to maintain stability under actual working conditions. This practice has achieved limited success yet some notable aberrations appear on the performance under a variety of loading conditions. The good news is that a solution is at hand and is quick and easy to apply.
Following the diagram, install a ¼ Watt, 47 Ohm resistor SMD (Surface Mount Devices are ideal here, mounted on the track side of the board), into the emitter leg of each of the front end transistors TR1 and TR2 – figure ( ) .
Carefully cutting the PCB track using a small power tool such as the Dremel. This procedure should be practised on a scrap PCB before diving in at the deep end.
The pair of emitter resistors introduce a small amount of local feedback into the long tail pair and ensures that they stay stable even under arduous operating conditions and enables the removal of the rather undesirable shunt capacitor in the feedback loop.
If the amplifier is to be used considerately that is to say, within the safe operating area of the output transistors and the capability of the heatsink to remove excess heat, then the protection circuit may be removed. Remove TR7 and TR6 along with the two 10uF Tantalums from the boards. If you wish, you may remove the two 1N4148 diodes, the 47R and the 100R resistors also.
The next stage is to renew the signal path input capacitors and these may be replaced by either a Philips 128 Series solid aluminium, 10uF @ 20 volts is the nearest equivalent. The second variant is one of the new Oscon products from Sanyo. Use an SG series, these are audio grade and have linear crystal copper leads and Mu-metal cans. For the feedback capacitor, an Oscon of 47uF @ 20 volts fits the bill nicely as will the same value as the bypass capacitor across TR5. The feedback resistors of 27K and 1K should be replaced by the best components money can buy. These are the hyper-critical parts of the system and bulk foil resistors are the very least that are acceptable here. Highly recommended, the signal path resistors may be changed for more revealing components such as the RC55 range from Welwyn and these do seem to work well in this application.
Locate a convenient position on the positive and negative tracks and using a sharp scalpel, score a couple of lines across and right through the tracks about 5mm apart. Apply a hot soldering iron to this area whilst rubbing back and forth. The small piece of track should now be removed leaving a neat edge. If in doubt, as always practice on a redundant board before proceeding. Introduce some 1A – 200V silicon diodes in series with a 270-300 Ohm resistor across the formed gaps observing correct polarity, see figure ( ). Decouple the tracks after the diodes by 100uF low impedance electrolytics at a convenient point, again ensuring polarity is as it should be. This ensures a decoupled front end supply and a much cleaner and more stable sound will result.
Remove the output transistors TR11, TR12 from the board then, using new mica washers and thermal compound, re-mount them ensuring that no shorts to the metal sink bracket occurs. The use of brass machine screws here reap small but significant improvements in sound quality. Better still, replace the output transistors by a matched pair of BUV20 devices using new aluminium oxide mounting pads and the usual thermal compound. The result is a cleaner sound as the transistors are more effectively isolated from the effects of chassis vibration. Replace the original M4 steel screws by M4 x 16 brass screws for a slightly better performance figure ( ). Just a word about output transistors is needed at this point. In order to keep the amplifier stable the open loop gain is deliberately restricted by the use of relatively low gain output devices. Replacing these original devices with high gain components may upset the balance of the amplifier and cause oscillation. All this is good news however as the supply of high current, low gain transistors is plentiful and reasonably cheap.
Renew the 1N 4004 *catcher* diodes by ultra fast soft recovery diodes with an appropriate voltage rating (Vrrm) say, 400 Volts. There will be a small but significant improvement in recovery times after transients.
Whilst on the subject of vibration, I've noticed the new NAP 500 power amplifier from Naim utilises copper heat spreaders for mounting the output transistors. Copper is a very good conductor of heat and resonates at lower frequencies than does aluminium. In modifying these amplifiers, replacing the original aluminium heat spreaders by a copper alternative, available from the suppliers at the end of the book, can have significant sonic benefits.
The 0R22 ballast resistors may be renewed by the new, non-inductive TO220 packages from Caddock or Meggit. Having a rating of 2 Watts without heatsinking and up to 20 Watts with a fairly large sink, a slightly cleaner reproduction will be noticed.
The NAP range of amplifiers do not incorporate an output inductor and use one more of the 0R22 resistors instead. Before contemplating renewing this component by a non-inductive replacement, remember that this particular part of the circuit is not quite as simple as it all seems. The series resistance in the positive loudspeaker lead has a twofold purpose: firstly, it isolates the output transistors from the effects of a large reactive inductance (the loudspeaker coils) and secondly, its natural inductance prevents to some degree the effects of some external high frequencies or radio frequencies from reaching the feedback network of the amplifier and interfering with the stability. These effects are perhaps the least known areas of the performance of any amplifier and it is suggested that the more experienced constructor experiments with the effects of adding a small amount (say 6-10uH) of inductance in parallel with a 10R resistor. To make up a suitable inductor, try ten turns of 0.75mm enamelled copper wire close wound onto a 10R, 2W carbon resistor which is then soldered at both ends before being fitted into the board.
The small spade terminals should be scraped clean, a minute amount of synthetic grease applied and the board is ready to be re-installed.
Apply a modest amount of thermal transfer compound to the heat transfer bracket and install into the case. Avoid overtightening the M4 countersunk fixing screws as they can be difficult to remove at a later time.
Connect all the leads to the boards except the positive ones. These should be connected with a 10R 2W resistor in series with the supply in case of any faults being present on the board. These resistors will act as fuses under fault conditions and prevent major damage to the PCB and critical components. Using a digital multimeter set to the 300mA range, insert the leads into the positive feed to one amplifier board. After settling down, the standing current should be adjusted to read no more than 40mA over a period of ten minutes or so. Repeat with the other channel noting that the control is turned anti-clockwise to increase the current. After checking that there is no more than 50mV of DC offset at the loudspeaker terminals (meter set to lowest DC volts range), the amplifier is ready for service.
It is perfectly possible to uprate the power output of this little amp using a variety of methods, some of which will be described here:
The most basic uprating involves, as do all in this section, in removing the mains transformer from the existing casework and siting a larger variant outside in some kind of enclosure. Merely removing the mains supply from the casework is a desirable move by itself as the energies are kept very much under control and out of reach of the electronics. Careful measurement of the secondary voltages of the factory fitted transformer reveals around 24-0-24Volts AC which, when rectified and smoothed gives 35-0-35Volts of DC to the rails. It is not prudent to exceed this by any large degree as the power dissipation in the output devices will operate outside their safe operating area. It’s best to stay at around 25-0-25 Volts AC but at the same time, employ a better grade transformer such as an audio grade toroid or better still, a ‘C’ core variant. The opportunity now presented is to use a transformer with a split secondary using one dedicated transformer winding, rectifier and a pair of main capacitors per channel. Better still, using two separate transformers, build a state of the art supply with even less harmonic interaction between channels. The ultimate strategy of course is to build a totally autonomous regulated supply using the details given in the power supply section of this book. This can take many forms and is only governed by the wallet, the wifes tolerance of black boxes and the owners imagination. In this last form, the little NAP110 really does take on its larger and vastly more expensive stable mates.
Finally: a word about interconnecting cables and connectors. Use the best available cables to connect the power supply to the circuits of the 110 and gold plated connectors of around fifteen amps capability should ensure lasting sound quality and reliability.
Carefully consider the modification you intend to carry out and don’t try to go too far in one step. For example, any external power units can be built quite separately from the main unit and tested for stability and reliability before beginning work on the amplifier proper.
Section 2. The NAP 160:
Elementary but DO DISCONNECT from the mains supply first..!!
This amplifier is virtually a NAP 110 in a full width case and as such, represents a very wide scope for the constructor. As the space within the case is much greater than the 110, there are exciting possibilities for this amplifier such as: a true dual mono power supply with twin toroidal transformers, two pairs of reservoir capacitors of substantial size and a full rewire job in some quite exotic cables. In this form, the 160 totally outperforms a pair of the very expensive 135s from the factory.
Begin by removing all the standard components from the case and discarding the original capacitors, rectifiers and transformer. Once the decision is made as to which of the power supply options is to be used, the new components may be installed according to one of the plans in figure ( ). Take time in measuring the mountings for the transformers so as to avoid the possibility of having to move the holes once drilled. Any mounting holes drilled in the chassis tray of the 160 will need to be carefully countersunk to allow the tray to slide back into its sleeve after the work is completed.
At this point, decide whether or not to retain the quirky four pin Din input socket or change this for a more appropriate connector. A decent pair of phono sockets will allow the use of more exotic cables and are now accepted industry standard. At the same time, replace the budget loudspeaker sockets found in all Naim kit by installing a set of gold plated 4mm binding posts.
Very favourable results have been obtained when using Ultra Fast, Soft Recovery diodes in place of the originally fitted bridge rectifier packs. The printed circuit layout in the PCB section of the book will enable a compact module to be built and which may be tucked away in a convenient corner of the casework. Some constructors may favour the sound of the Schottky diode and these may be built onto the same PCB. A word of warning here: do take heed of the manufacturers Vrrm (Mean Reverse Voltage rating) when choosing a Schottky diode, use only the 200 Volt rated versions (Motorola MBR20-200CT). The more commonly available 100/150 Volt components have been known to fail catastrophically when too much voltage is applied.
Having built the chosen power supply into the chassis, testing to ensure that all is well is strongly recommended. At this time, remember that there is a substantial amount of current stored in the reservoir capacitors once charged and caution must be exercised when using test equipment. After all the wiring has been checked for errors, apply mains voltage for a brief moment then switch off. Remove the mains lead from the amp and test the voltages present across each capacitor - they should be roughly equal indicating all is well. Again apply mains voltage this time for a minute or two. All voltages should now be equal within a fraction of a volt indicating that all is indeed well and the supply is ready for service. After removing the mains lead, discharge each capacitor using a 10K 2W resistor until the voltage across each reads just a couple of volts or so.
Just a word or two about the heat dissipation properties of the standard Naim casework: once the strip down has begun, you will notice a small device bolted to the floor of the case. This is a heat sensitive trip, which senses when the case temperature rises above a certain level. At this point, the mains supply is interrupted until the case cools below a set figure. It is recommended that this arrangement is left in situ as the heat dissipation of the Naim casework is somewhat limited. Following the diagram, rewire the original thermal trip device so the mains fuse protects it and not as the factory first produced the amplifier.
Using the modification details below, rework the boards in the following manner:
The circuit of the NAP 160 is shown in figure ( ). A 'long tailed pair' input is provided with a constant current supply of around 1mA by TR3 and its associated components. The next stage TR4, fed by a constant current of around 9mA by TR6 and associated components, amplifies the resultant signal to a level usable by the two drivers TR9 and TR10. This second stage, along with TR5, is also used to control the bias current setting although for some strange reason, TR5 is not mounted on the heatsink as is common practice today. The result is that there is no automatic bias current reduction when the amplifier temperature rises. The output transistors are in what is commonly known as a 'quasi complimentary' configuration. This enables the use of NPN power transistors instead of the more usual NPN/PNP pair. NPN transistors are generally cheaper, more easily available in high current devices and are more reliable than the PNP version.
The output stage is protected against over current by measuring the voltages across the 0R22 emitter resistors. Once the average current exceeds 10 Amps, the voltage at the bases of TR7 and TR8 rises above 0.6V turning them on. The bases of the driver transistors, TR9, TR10 are then 'clamped' preventing any further increase in output current. The 10uF Tantalum capacitors average the signal to a mean level so the board is able to supply much larger short-term transient currents. On paper, the system works well but, as not all 'technically correct' designs have the desired result, this kind of output protection has a deleterious effect on sound quality and is best avoided if at all possible.
Extensive use of bypassing capacitors around the circuit board is evidence that the design is inherently unstable and therefore needs this level of compensation to prevent oscillation.
Whilst on the subject of instability, virtually all audio products exhibit some form of unwanted oscillation at HF (High Frequencies) and once the problem area has been identified, neutralising is relatively straightforward.
The NAP series of amplifiers use several pole filters in an effort to maintain stability under actual working conditions. This practice has a limited success yet some notable aberrations appear on the performance under a variety of loading conditions. The good news is that a solution is at hand and is quick and easy to apply.
Following the diagram, install a ¼ Watt, 47 Ohm resistor (Surface Mount Devices are ideal here, mounted on the track side of the board), into the emitter leg of each of the front end transistors TR1 and TR2. Carefully cut the PCB track using a small power tool or new scalpel blade, this procedure should be practised on a scrap PCB before diving in at the deep end.
The pair of emitter resistors introduce a small amount of local feedback into the long tail pair and ensures that they stay stable even under arduous operating conditions and enables the removal of the rather undesirable shunt capacitor in the feedback loop.
If the amplifier is to be used considerately that is to say, within the safe operating area of the output transistors and the capability of the heatsink to remove excess heat, then the protection circuit may be removed. Remove TR7 and TR6 along with the two 10uF Tantalums from the boards. If you wish, you may remove the two 1N4148 diodes, along with the 47R and the 100R resistors.
The next stage is to renew the signal path input capacitors and these may be replaced by a Philips 128 Series solid aluminium, 10uF @ 25 volts being the nearest equivalent. The second variant is one of the new Oscon products from Sanyo. Use an SG series, these are audio grade and have linear crystal copper leads and Mu-metal cans. For the feedback capacitor, an Oscon of 47uF @ 20 volts fits the bill nicely as will the same value as the bypass capacitor across TR5. The feedback resistors of 27K and 1K should be replaced by the best components money can buy. These are the hyper-critical parts of the system and bulk foil resistors are the very least that are acceptable here. Highly recommended, the signal path resistors may be changed for more revealing components such as the RC55 range from Welwyn and these do seem to work well in this application.
Remove the output transistors TR11 and TR12 from the board then, using new mica washers and thermal compound, re-mount them ensuring that no shorts to the metal sink bracket occurs. The use of M4 X 16 plated brass instead of steel machine screws here reap small but significant improvements in sound quality. Better still, replace the output transistors by a matched pair of BUV20 devices using aluminium oxide mounting pads and the usual thermal compound. At this juncture, take the opportunity to ‘stand off’ the PCB from the heatsink bracket using stepped nylon bushes – figure ( ). This results in a cleaner sound as the transistors are more effectively isolated from the effects of chassis vibration plus the PCB is better isolated from excess heat generated by the output devices. Installing new M4 x 16 brass machine screws in place of the original steel fitment ensures a lower resistance connection between collector and the PCB.
Just a word about output transistors is needed at this point. In order to keep the amplifier stable the open loop gain is deliberately restricted by the use of relatively low gain output devices. Replacing these original devices with high gain components may upset the balance of the amplifier and cause oscillation. All this is good news however as the supply of high current, low gain transistors is plentiful and they are reasonably cheap.
Replace the 1N 4004 *catcher* diodes by ultra fast, soft recovery with a Vrrm rating above 400 Volts. There will be a small but significant improvement in recovery times after transients.
Whilst mentioning vibration, I've noted that the new NAP 500 power amplifier from Naim utilises copper heat spreaders for mounting the output transistors. Copper is a very good conductor of heat and resonates at lower frequencies than does aluminium. In modifying these amplifiers, replacing the original aluminium heat spreaders by a copper alternative can have significant sonic benefits.
The 0R22 ballast resistors may be renewed by the new, non-inductive TO220 packages from either Caddock or Meggit. Having a rating of 2 Watts without heatsinking and up to 20 Watts with a suitable heatsink, a slightly cleaner reproduction will be noticed.
The NAP range of amplifiers does not incorporate an output inductor and uses one more of the 0R22 resistors instead. Before contemplating renewing this component by a non-inductive replacement, remember that this particular part of the circuit is not quite as simple as it all seems. The series resistance in the positive loudspeaker lead has a twofold purpose: firstly, it isolates the output transistors from the effects of a large reactive inductance (the loudspeaker coils) and secondly, its small but significant natural inductance prevents to some degree the effects of some external high frequencies or radio frequencies from reaching the feedback network of the amplifier and interfering with the stability. These effects are perhaps the least investigated areas of the performance of any amplifier and it is suggested that the more experienced constructor experiments with the effects of adding a small amount (say 6-10uH) of inductance in parallel with a 10R resistor. For example, try ten turns of 0.75mm varnished copper wire close wound onto a 10R - 2W carbon resistor which is then soldered at both ends before being inserted into the board in place of the original 0R22 resistor. Do perform oscilloscope tests to ensure that all is well before hooking up your precious loudspeakers.
The small spade terminals should be scraped clean, a minute amount of synthetic grease applied and the board is ready to be re-installed.
If it is intended to run the amplifier at higher than normal power for long periods, the thermal dissipation limits of the sleeve case must be considered. In this circumstance, a strip of appropriate heatsink extrusion bolted to the rear of the chassis tray and the board mounting screws re-positioned to contact the rear face of the tray, will ensure that the extra heat is dissipated safely. Suitable finned heatsinks, anodised in black, are available from the suppliers listed at the end of the book. Ensure that there is a good contact between the transistor bracket on the amplifier module and the chassis tray by applying a small amount of heat transfer compound to both the bottom and the front surfaces. Do likewise with the contacting face of the finned heatsink when mounting. If necessary, introduce a thin copper shim between the bracket and the chassis to take up any clearance. For details of this arrangement see figure ( ).
Apply a modest amount of thermal transfer compound to the heat transfer bracket and install into the case. Do not overtighten the M4 countersunk fixing screws as they can be difficult to remove at a later time.
Connect all the leads to the boards except the positive ones. The negative and positive rails should be connected with a 10R 2W resistor in series with the supply in case of any faults being present on the board. These resistors will act as fuses under fault conditions and prevent major damage to the PCB and critical components. Using a digital multimeter set to the 300mA range, insert the leads into the positive feed to one amplifier board. After settling down, the standing current should be adjusted to read no more than 40mA over a period of ten minutes or so. Under no circumstances exceed 40mA as the design has no thermal feedback arrangement and as a consequence, the current will build up over time and do serious damage to the circuits. Repeat with the other channel noting that the control is turned anti-clockwise to increase the current. After checking that there is no more than 50mV of DC offset at the loudspeaker terminals, (meter set to the lowest DC Volts range), remove the protection resistors. After connecting up all the terminals, the amplifier is ready for service.
Section 3. The NAP 250:
Yes, boring I know..!! but do disconnect the supply before removing the case.
The NAP 250 differs in that the supply voltages are regulated to their final levels by separate modules contained within the case. The rectified voltage of around 55-0-55 DC is regulated to its working level of 45-0-45 Volts DC. The regulator modules have a secondary role, that of limiting the output current to a mean eight amps or so under transient conditions. Should there be any malfunction of the amplifier modules such as that when driving a short circuited output, the trips will operate and only a small foldback current flows until the fault is removed and the amplifier reset by switching off the mains supply for a short period.
Referring to figure ( ), the regulator circuit may be viewed in detail.
Following the architecture of the NAP series of power amplifiers, the output pass transistors are quasi complimentary 'triples' made of discrete bipolar transistors. NPN power transistors are the series pass elements of the triples and are arranged so that the pass transistor on the negative rail is reverse connected just as in the power amplifiers. This design is seriously flawed in that one pass transistor (on the negative rail) operates under totally different conditions from its counterpart and therefore the gain of each regulator varies considerably.
The long tailed pair, TR103,TR104 and TR203,TR204 serve to stabilise the output from the module by acting as comparators. TR103 and TR203 are referenced above zero volts by the zener diodes ZD101 and ZD201. The voltage output of the two rails is constantly monitored though the 5K6/1K8 resistors plus the 1K variable resistor and any variation of the output level is countered by a corresponding adjustment of the output from the long tailed pair. This process however, is not instantaneous and leaves somewhat large holes in the rail supplies to be compensated for supposedly, by the miniscule 10uF capacitor at the output of the modules. Dominant pole compensation is catered for by the two RC (resistor/capacitor) networks 470pF + 680R and 470pF + 560R and it is this arrangement which reduces any chance of oscillation. At the same time however, the compensation network introduces serious phase shift and thereby slows the response of the whole regulator assembly.
The peculiar arrangement at the left hand (input) side of the module consisting of transistors TR101,TR102,TR201,TR202, ensure that current monitoring of both rails is applied to both positive and negative rails. Should the current trips operate (at around 8 Amps), both rails will be shut down and will go into a low current output mode known as 'foldback' current limiting. The amplifier must then be powered down for a short period and then the trips are reset by switching on the mains supply.
Whilst this is all well and good in protecting the amplifier circuits from abuse and misuse, the current limiting devices begin to operate much earlier than is first thought. This leads to the usual aberrations on the performance and a certain loss of tempo and critical timing.
One simple answer to a reluctance to supply even short term high current is to introduce a large reservoir of capacitance after the regulators.
This entails connecting fairly substantial electrolytic capacitors across the voltage rails after the regulators.
Although a feasible idea in theory, the ESR (Equivalent Series Resistance), of even modest value capacitors is so low when discharged as to operate the regulator current trips at switch on. What is needed here is a system whereby the auxiliary reservoir capacitors are connected via a ballast resistor and once charged to a pre-determined level, are switched on line by a timed relay. Reference to figure ( ) reveals just such an arrangement and for the constructor, a printed circuit layout for the S-Cap is included in the PCB section of the book.
Once made and tested, the S-Cap modules can be fitted into a small moulded plastic case and connected to the amplifier via miniature multi pin plugs and sockets.
Another strategy, albeit risky, to ensure there is no current limiting is to remove the low value wirewound resistors (0R07 or 0R1) at the front end of the regulator boards and replace them with stiff wire links. This procedure should be carried out only if the amplifier is to be used with some restraint otherwise, the possible overdriving of the output devices will spell doom and gloom both for the amp and for any connected loudspeakers.
On inspecting the regulator modules, it will be noticed that there are a pair of well cooked resistors just below the heatsink bracket. These are to provide a small load to the regulators under idling conditions and here is another example of a seeming miscalculation by Naim - the resistors are under rated for the voltage across them. The original value, by now well concealed under a thick layer of carbon, was 6K8 @ 0.3W and replacing them with a shiny new pair of 6K8 @ 1W metal film will put matters right from now on. Stand these new resistors 5mm clear of the circuit board to ensure that air will flow around them without hindrance.
A third option is to replace the regulator boards by the version described below:
A so called VBE multiplier has a use in stabilising the supply rails without the heavy feedback requirements of a foldback regulator system as installed by Naim. A computer model of the standard Naim foldback regulator reveals a non – linear response and undesirable phase shifts across the frequency spectrum. Power related noise generation is also another undesirable feature of the Naim system and a consequence of using an NPN pass transistor for both the positive and the negative rail in quite separate gain configurations, means the current response and more importantly, settling time, leaves much to be desired.
By comparison, the VBE circuit shown in figure ( ) is very simple indeed and the design has been optimised for a flat response between 10Hz – 500KHz.
The design is completely symmetrical ensuring that both positive and negative rails have the same response across the audio spectrum.
A printed circuit board layout is included in the PCB section of the book.
Only the positive rail regulator is shown, the negative rail being a mirror image but with the component changes shown on the shopping list.
The circuit has no global feedback whatever therefore distortion, predominantly second order harmonic, is improved by an order of magnitude. Phase linearity between 10Hz - 200KHz is very nearly perfect and noise levels are much lower than in the standard Naim circuit. Products are so low in fact, that this circuit may be used to supply pre-amplifiers and other low level circuits. Modelled in PSpice for instance, a 2 Volt AC ripple content on the input, records hum and noise at the output at less than 5 microvolts – far better than most integrated circuit regulators.
The main pass transistor is in a Darlington configuration whose output is determined by the voltage at the base of the first driver element. This voltage is supplied by a cascaded pair of VBE multiplier sections, the first of which may incorporate a zener diode to regulate the final output voltage. The use of this configuration ensures a low impedance, low noise drive to the base of the Darlington pair. The VBE regulator assembly drops approximately 4 Volts across the main transistor which at around 8 Amps continuous draw, means a dissipation in each pass transistor of around 32 Watts. In practice, this will rarely be achieved except when hard driving loudspeakers with a viciously low impedance. Provision has been made on the PCB artwork to incorporate two surface mount zener diodes in series to achieve the desired output voltage. You may also add a small value series resistor or inductor to isolate the HF effects of the zener. If a zener diode is used to bring the rail voltage even lower, the power dissipation in the pass transistors will have to be calculated very carefully. For example; a forward drop of 10 Volts will mean that the two rail transistors will have to dissipate over 100 Watts between them when driving low impedance loads, clearly out of the question for the limited heat sinking capabilities of the Naim casework.
Although the PCB pattern will fit on to the standard Naim aluminium heatsink bracket, the use of a copper bracket is recommended if high current operation is envisaged.
Modifications of the NAP 250 power supply may begin with the replacement of the main reservoir capacitors by something in the order of the Philips 154 or the Avondale Kendeil which are both available as 22000uF @ 63Volt in the same size cans as the original Naim fitment. The standard rectifiers packs may be swapped for the now familiar Schottky diodes with a corresponding reduction in HF hash meaning a quieter background.
A strange arrangement in the NAP 250 is that the centre connection (0V) between the main caps is not made until the wiring is reaches the negative loudspeaker terminals. This means that the rather crucial zero volts termination is some way off where it should actually be and under dynamic conditions will result in there being a small but noticeable distortion. A considerable amount of current flows along the zero volts line between the capacitors under drive conditions. Providing a stiff wire link between the appropriate terminals of the two main capacitors very easily rectifies this problem.
The owner may decide to dispense with the regulators altogether in which case, a new transformer with a lower secondary voltage will be required. An audio grade toroid with two secondary windings of 30-0-30 VAC on load will do just fine giving a DC rail voltage of around 45-0-45. This means that the originally specified output power into 4 and 8 Ohms is not exceeded.
Bridging Outputs.
Having been asked many times if the NAP series may be bridged, it was decided to write this section in answer to the many questions which have arisen on this subject.
What is bridging?
In the simplest of terms, two amplifier boards are made to operate out of phase with each other. This means that when one board is producing a positive going waveform, the other is delivering a corresponding negative signal. Combine the two and you have a voltage swing equal to twice that of just one board, figure ( ). Using the formula - Watts = V² ÷ R, if the voltage swing is doubled, the output into the same impedance load is quadrupled. Given that one board can swing 20 volts at its output (equal to 50 Watts into 8 Ohms), then a pair of boards in bridged format will swing 40 Volts which means an output capability of 200 Watts into the same load.
Drawbacks.
A bridged output amplifier will ‘see’ a nominal 8 Ohm loudspeaker as a current hungry 2 Ohm loudspeaker. This means a massive current hike for the output transistors and a consequent increase in generated heat. If a standard NAP250 for instance were converted to bridged form, apart from the fact that the regulators would shut down under the increased current draw, the rather poor heat dissipation of the casework would have little chance of keeping the output devices within their safe operating conditions. Theoretically, the combined voltage swing of two NAP series boards into a real load, will try to produce over 400 Watts..!! What is required in such circumstances is a much reduced voltage on the amplifier supply rails combined with some very special arrangements to get the heat away quickly into the outside air. These problems are not insurmountable but should only be tackled by only the most experienced engineers with an appreciation of the electrical and mechanical requirements.
This is the basic principle of operation of the latest NAP500 and takes the original Naim amplifier design (not much new here folks..!!) into fresh territory. The NAP500 is reputed to have custom made output transistors capable of over 250 Watts dissipation and around 70 Amps current capability. Given that the published specification of the transistors used in the NAP250 is similar in virtually all respects to those used in the ‘500, it does seem quite feasible to bridge the NAP250 modules to gain a performance level somewhere not too far removed from the capabilities of the NAP500 – ie. around 150 Watts into 8 Ohms.
Where to start.
It remains the choice of the constructor whether to strip out his 250 or to start with a clean sheet as it were. For the home constructor, cases rank as the most difficult aspect of any project and with that in mind, arrangements for supplies of essential quality components is crucial to ensure success. Several suppliers listed at the back of this book are able to supply an anodised aluminium tray, ready punched to take input and output sockets. The tray is identical in dimensions to the original Naim tray and so will slide into the 160/250 size sleeve. Also available is a self adhesive black vinyl strip to cover the front face of the tray as a front panel. As an optional extra, two sizes of finned heatsink, ready drilled and tapped with mounting holes, will be made available finished in anodised black to match the tray. Specially designed solid copper heat spreaders must replace the aluminium versions used on the NAP series for a better thermal transfer to the finned heatsinks and these are readily available from the listed suppliers. The smaller heatsink is intended for use when upgrading the 250/160 and the larger is calculated to be suitable when bridging the larger Naim amplifiers.
Whatever the choice of casework, the importance of heat dissipation cannot be emphasised too strongly and failure to observe this warning will result in disaster for the circuit boards.
Given that the choice of case has been made, the route forward lies open. Referral to figures ( & ) shows a typical physical layout for the bridged amplifier. Note that the power supply circuits are well removed from the electronics area. There are two version of the layout, one convection cooled using the large comb section heatsink whilst the alternative design uses an internal heatsink extrusion cooled by a thermostatically switched fan.
Power Supply Considerations.
As discussed previously, the DC rails of the standard NAP 250 measure around 45-0-45, which level will enable a swing at the amplifier output of around 25 Volts. Using the formula Watts = V² ÷ R, equates to an output of 78 Watts into 8 Ohms. If we bridge the same two boards we then end up with a swing of 50 Volts and using the same formula, an output of around 400 Watts into 8 Ohms becomes theoretically possible, albeit for a very short length of time.
In order to make this project viable, we have to ensure the bridged amplifier will swing no more than 36 Volts in total meaning the individual voltage rails must allow each board to deliver no more than 18 Volts each before clipping. By reverse calculation, the rails must be no more than 26-0-26 Volts DC on load. By replacing a few components on the original Naim regulator assemblies, the output voltage may be adjusted within very fine limits to achieve this output specification. Alternatively, the VBE power supply module may be installed the output voltage first being set by careful choice of zener diodes in the first VBE stage.
I won't go into any more detail here in the assumption that only experienced engineers will attempt this rather complicated project.
Previous sections on NAP series amplifiers will guide the constructor through the finer points of amplifier module modifications so the details will not be repeated here. It is left to the owner to decide whether to employ the strategic modifications outlined in previous chapters or to begin again with a clean sheet as it were. As the NAP250 has a respectable second-hand value, the choice to remove most of the major components merely to discard them merely to provide a case does seem at best, rather extravagant.
Chapter 11
The prospect of beginning with a ‘clean sheet of paper’ can sometimes be more appealing than taking an existing design and making the appropriate modifications.
Some enthusiasts prefer to construct from first principles than to ‘interfere’ with another manufacturers product. It is at this last group that this section is primarily aimed although a mixture of ‘home brew’ and manufactured products will still find its way into many systems. It is helpful to appreciate that any amplifier in its simplest form is merely a modulated power supply, the quality of which largely determines the overall performance of the amplifier.
We begin with a basic power supply for pre-amplifiers:
As mentioned in previous chapters, the use of toroidal transformers is now widespread. Simply replacing any toroid by a properly designed and constructed laminated or 'C' core transformer will have a noticeable effect on sound quality.
I'll now discuss the basic building blocks of a simple but very effective power supply for Naim pre-amps:
Beginning with the aforementioned transformer to suit your pocket and taste. Follow this by a rectifier assembly preferably constructed from appropriately rated Schottky diodes. These diodes are constructed quite differently from the common silicon PN junction diodes normally used by every manufacturer. Schottky diodes have a semiconductor/metal configuration and as such, have no minority carrier 'spike' effects to impinge on the supply rails.
Select a well known grade of capacitor from a wide range of products and ensure it is mounted with some thought to vibration resistance.
Option 1:
The humble LM317 regulator as used by Naim Audio is arranged as figure ( )
shows. If an LT317T is used, the performance betters the Naim Audio made unit as the regulator chip is tested for noise before leaving the Linear Technology factory. Use of the 10uF tantalum beads capacitors will ensure stable performance under most conditions.
Option 2:
By arranging two 317s as figure ( ) proposes, the advantages of a pre-regulation system will be found to be substantially reduced background noise and hiss.
Use low impedance105°C capacitors from such as Panasonic for decoupling.
The 'Supercap' power supply from Naim uses 317 regulators in this configuration to good effect.
You may expect to find a much darker background using this configuration.
Option 3:
The APX board figure ( ), has the advantage of being ready built and tested. To put this board into use it's simply a case of mounting the module into a suitable enclosure and wiring the transformer as described in the service information.
The regulation system is cascaded in that a bank of frequency compensation capacitors follows a pre-regulator transistor after which, the final regulation takes place. Further frequency compensation components ensure the final supply is extremely linear before powering the connected pre-amplifier.
Option 4:
A Class 'A' dual regulator module designed at Avondale Audio to overshadow virtually all comers. This power regulator system draws power from the transformer/capacitor at a rate much higher than anything demanded by the pre-amplifier. Running all the time, transient delivery is instantaneous and absolute linearity both measured and perceived, has not been bettered by any of the configurations above.
As there are some proprietary bits of technology in this design, I regret that I can't include a circuit for others to copy - sorry.
Next comes a very high tech power supply for power amplifiers:
Beginning with the transformer which may be from any one of a dozen or so different manufacturers and can be a toroidal, ‘EI’ core or ‘C’ cored variant. Even transformers salvaged from redundant equipment, once tested for safety, may be pressed into service. Transformers are inherently reliable components unless abused and should last almost indefinitely. Toroidal transformers do exhibit a high ‘inrush’ current on start up due to their lack of mass in the core and do require switches with a substantial rating to avoid contact ‘welding’. This is especially true when large values of capacitors have to be charged. For very large transformers, some form of ‘soft start’ device may be necessary and an example is shown in Fig. ( ). The soft star module operates by first relay switching a bank of high power resistors in series with the load then, after a preset time, the second relay 'shorts' out the resistor bank thus applying full power. Operation is fully automatic and the mains switch need only be a small current device as it carries only milliamps of relay current.
Mounting of Transformers.
There is much evidence to suggest that the major contributor to amplifier vibration effects is the transformer. The vibration produced by this major component is such that there is a markedly audible difference when using different transformers or when removing this component altogether from the amplifier enclosure. It must be understood that the energy produced by the cyclic rise and decay of the magnetic field must have a means of escape via a fast, high energy path otherwise, the effects will build up and appear as altogether different frequencies. There are many strategies to be employed and these vary with the size and the construction of the amplifier casework. Properly designed audio transformers with an epoxy encapsulation seem to be the best place to start as consideration of the vibration problem has been addressed during the manufacture. For standard mounting toroidal transformers, the best solution seems to be to avoid the soft rubber washer provided by the manufacturer and instead, employ a hard rubber interface as thin as is practicable under the transformer. Using a non-magnetic mounting bolt such as brass or stainless steel can reap small but worthwhile rewards as spurious eddy currents within the centre of the core are much reduced. Installing a hard mounting foot on the amplifier case just under the toroid ensures that energies are channelled away before much harm can be done. Site any transformer well away from any sensitive electronics otherwise a hum problem will be the result. Toroids do have a very low external magnetic field and for close packed cases, these are probably the best solution.
Laminated or stacked transformers do seem on the whole to be mechanically quieter by an order of magnitude than their equivalent toroidals. Direct current (DC) content on the incoming AC mains supply does not saturate the core of the stack as quickly as it does the toroid. The characteristic ‘grunt’ when DC appears on the supply is by and large, absent in the stacked transformer. By nature of the increased mass of the core of the stack, high frequency mains aberrations are attenuated to a greater effect than with the toroid.
Transformer Rating.
In calculating the VA rating of a transformer for a particular amplifier, around 250VA per mono 100 Watts should be just fine. Any further increase in rating will merely make your consumption of electricity higher than normal and will not make the amplifier sound ‘more powerful’, but the choice is yours. So then, for a stereo amplifier of 100 Watts per channel, a 500 VA transformer will be more than adequate.
To assist in the choice of transformer, below is a list of DC voltages, what power these ratings will produce and a listing of an appropriate transformer.
|
Rail Voltages DC ( On Load ) |
Max Power into 8 Ohms |
Secondary Voltages |
VA Rating for 2 Channels |
|
27-0-27 |
40 Watts |
20-0-20 |
160 |
|
35-0-35 |
50 Watts |
25-0-25 |
250 |
|
44-0-44 |
80 Watts |
32-0-32 |
350 |
|
50-0-50 |
100 Watts |
35-0-35 |
500 |
|
55-0-55 |
145 Watts |
40-0-40 |
625 |
If it is planned to use a regulator of some variety, use the next higher voltage rating on the list to give the regulator some ‘headroom’ and thereby prevent what is known as ‘drop out’ under load conditions. For instance, if the final voltage presented to the amplifier boards is to be 43-0-43 Volts DC then the supply to the regulator modules should be around 7 Volts higher ie. 50-0-50 Volts.
Once the transformer has been mounted in its appointed position and wired to the mains inlet via whatever protection devices and switch are chosen, then connecting the secondary winding to the diode bank is next on the list of duties. Don’t become too obsessed with the shortness of cabling at this point. A certain length is necessary to act as a series resistance/inductance on start up and sometimes can have the effect of removing HF from the secondary supply especially if the leads are lightly twisted together for most of their run. Obviously, this has to be treated with a degree of common sense and secondary leads should be trimmed to reasonable lengths before installation.
The use of thermal trips instead of fuses has become quite widespread in many installations and there are a couple of points to be made about these devices. The series resistance of the thermal trip is generally higher than the equivalent fuse. This is not to be taken as a criticism or a minus point and indeed, in some cases, can have slightly beneficial effects. Choose a manual reset thermal trip in preference to one of the automatic variety. In this way, any fault causing a trip to operate can be investigated before restoring the supply and could well prevent damage to connected loudspeakers whilst you are out of the room.
Next comes the choice of rectifier. The 'common or garden' silicon diode bridge is available in 25 Amp variants and is fairly cheap. Ultra fast, soft recovery diodes are next in the pecking order and have the advantages of being readily available and come with high Vrrm ratings. Schottky diodes are available in Vrrm ratings of usually no more than 200 Volts. These are fine for DC rails of up to 55-0-55 Volts or so but for higher voltages, ultra fast diodes will have to be employed. The PCB section includes a suitable board for constructing a full wave bridge rectifier using either fast recovery or Schottky TO220 packaged diodes.
Capacitors and Bypass.
There are capacitors and there are ‘capacitors’ is the caveat here and each manufacturers product has a distinct influence on the sound quality of the finished amplifier. Capacity wise, 10,000 to 15,000 microfarads per 100 Watts is just fine. Larger values will tend to slow the perceived performance ie. the tempo of the music whilst smaller values, vice versa. Choose a brand with a well regarded reputation for sound quality and by specifying the so called ‘computer grade’ components with screw terminals, connections are so much easier.
Bypass capacitors used to be heralded as the answer to the evils contained within the main capacitors but my current experience is that they do tend to stratify or fragment the performance in some systems. I've known more than one amplifier whose sound was totally ruined merely by adding too much bypass in all the wrong places. The caveat here is to use bypass sparingly and ensure there is scope for ‘backtracking’ by removing such components before listening.
Capacitor mounting should be treated with the same degree of care as with the mounting of the transformer. Capacitors are seriously microphonic devices and are best mounted using a compliant device such as a plastic clip rather than the usually supplied metal mounting clips. If the metal clip is all that you have, mount using nylon screws and rubber bushes. Isolation from the effects of heat within the case will also reap benefits in the areas of vibration control and the service life of the capacitor.
Wiring.
Apart from the obvious current rating, cable comes in all forms of conductor and insulation formats. Silver plated cable with a PTFE insulation is a particular favourite amongst audiophiles and should be soldered using silver loaded solder to prevent leaching of the precious metal. Using compression crimps to ensure what is known as a ‘gas tight’ connection is preferable to soldering in most cases but this may be beyond the scope of the home constructor. Buying a good quality crimping tool for compression crimps should prove economical if a large number of construction projects are planned. Keep heavy current wiring quite separate from the signal paths and in particular, earths or more correctly, zero volt cables should also be kept as short as practically possible to avoid the undesirable effects of circulating earth currents. Avoid laying cables together in side by side a harness as the interaction between different signals and power rails can cause harmonic distortion.