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AKSA

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AKSONICS

POWER SUPPLIES

 

 

 

POWER SUPPLIES


What sort of power supply is recommended for the AKSA?

Why do you recommend UG10DCT, BYQ28E or SBYV28-200 diodes for the power supply?


I will have separate rectifiers for each channel, but can I use just one transformer?

Will my 28-0-28 transformer produce too high a rail voltage for the AKSA?

Could you help me understand the trade-offs regarding transformers?

I am going to use two 300VA Plitron transformers.  Are these too large?

How would improving power supply regulation by using say 300VA toroidal transformers and larger filtering capacitors suit the AKSA?

What brands of power supply filter capacitors do you recommend?

I'm trying to decide between two approaches to the power supply. Which is best?

In your power supply instructions, you state that C1 must be "X2".  What does "X2" mean?  What brand do you recommend for that?

The 100 Ohm resistor in series with C1 in the power supply, should have what Wattage rating, and what tolerance?

I have some air-cored chokes that could be used in a pi filter. Could they be used with perhaps a couple of high quality 1,000u F capacitors to any benefit?

Do you have any hum issues with the AKSA? 

Could I use circuit breakers instead of fuses for the power supply?

Can the power supply be built in the same box as the two amps?

 


What sort of power supply is recommended for the AKSA?


The design of power supply we recommend can be found on the web pages and in the accompanying kitset documentation. We can supply some but not all of the components because of cost, weight, shipping and space considerations, particularly when in most countries it is very easy to obtain a suitable transformer.
The AKSA power supply is a full dual mono. Put another way, this means two transformers, two rectifiers, two sets of filter capacitors (four for the 55W and eight for the 100W AKSA), all giving two independent positive and negative supplies for the two separate amplifier modules. This is electrically the same as monoblock construction, and can either be constructed in two quite separate enclosures, or the one enclosure.

Excellent dynamics and realistic, subjective 'slam' from a good solid-state amplifier largely relate to the ability of the power supply to resist voltage sag whilst delivering high, sustained current to a highly reactive load. Voltage ‘sag' is minimised by using a high current transformer with good regulation, quiet rectification, and an adequate, low impedance reservoir capacitor with low self-inductance and thus low ESR at high frequencies.

We therefore recommend toroidal transformers with good regulation - 10% or better. Filter capacitors can be almost any good quality Japanese, Taiwanese, German, Swedish, US or French 50VW types. 10,000uF is the maximum recommended for each rail of a single channel, no more; 4,700uF is adequate. A single stereo supply may use 15,000uF per rail. We have used Elna, Cerafines, Fischer-Tausch, Sprague, Rifa, and Nichicons. There are some sonic differences, but most of the sonic characteristics of the supply derive from the transformer and particularly the rectifiers, which should be General Semiconductor/Philips, types as discussed elsewhere in this FAQ, for the best sound.
We have found that overly large toroidal transformers do not actually improve this amplifier, rather, they appear to slow it down. We have found that two completely independent supplies prevent one channel from interfering with another, and this has far-reaching implications sonically, especially for imaging. Once you have heard the difference, you'd never use a single, larger transformer for both channels. Mounting all components in the one box is fine, and appears to cause no problems.

Transformer regulation should not be better than about 5%; more regulation sharply increases primary and secondary inductance, which adversely affects the sonics of the amp. For similar reasons, the capacitance should not exceed 10,000uF per rail per channel. We realise this is in surprising disagreement with many commercial solid-state amps. They seem to benefit from very large power supplies, but the reasons are related to the nature of the feedback overload function and the operation of the voltage amplifier, which in the AKSA amps is super-fast; we use 100MHz transistors in this role. We emphasise that the amp was not merely developed along the usual engineering grounds, with CROs and meters. The basic AKSA design was set up and studied on the bench, then tweaked iteratively and carefully for sonic effect over hundreds of hours of listening.
The engineering is mandatory, of course, but there is much, much more to competent amplifier design; and ceasing development with good specs alone is a false dawn. In auditioning with the AKSA we have ranked alongside Leak tube amp’s, Marantz PM66 Signature Series, Sugden A21 Class A amp’s, and Audiom Silver Knight SE 300B amps. It is utterly astonishing how good its sonics really are; these are all exceptionally good amplifiers.

It is worth noting that all transformers do not sound the same, and a smaller transformer, while conferring slightly less bass control, offers better midrange and top end. We attribute this to the lower charge currents, which produce charge currents in the filter capacitors, particularly with ordinary, regular quality bridge rectifiers. It is evident from our work in this area that switching noise, although at RF frequencies, is somehow perceived by the ear - particularly when the associated hash is eradicated altogether by the use of ultra-fast, soft-recovery diodes.

The physics of transformer dimensioning in audio is not particularly well understood as it relates to sonics. Suffice to say that the transformer, rectifiers and filter capacitors comprise a system, together with the amplifier, whose primary function is to modulate a current through a low impedance voice coil in exact replica of a tiny alternating voltage applied at the input of the amplifier. We have found that a larger transformer slows the sound, while a smaller transformer lacks bass punch and impact. A middle course is recommended; two 160VA transformers and 4 x 4,700uF for the 55W stereo AKSA, and two 300VA transformers and 8 x 4,700uF for the 100W AKSA. You may use other transformer sizes and capacitor values, but in our experience the sonics will not be optimised.
We do not recommend a shunt or series regulated supply.

The reason is that the speed of the system then depends as much upon the active circuitry in the supply as in the amplifier, and it really is prudent to avoid introducing more active elements into the system. A high quality, unregulated system is therefore cheaper and sounds best. We auditioned a good quality series regulated supply, but while it sounded different, it did not necessarily sound superior and was considered unworthy of the extra time and expense.

In any event, the performance of the AKSA is so good it rivals most tube systems anyway, and one of the strong drawcards of solid-state is the low cost, low maintenance feature, which a regulated supply partly offsets.
You can, of course, choose to build your AKSA stereo amplifier with just one supply. But if you do this, you will destroy the marvellous imaging and depth of the AKSA, depriving yourself of one of life’s little pleasures!

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You recommend using UG10DCT, BYQ28E or SBYV28-200 diodes for the power supply. Why is this, and how is the TO-220 device connected?


These diodes are ultra fast, soft recovery diodes, with two diodes incorporated into each TO220AB package. They are difficult and expensive to source, but their rectification produces DC which is almost as clean as batteries. This absence of switching hash is crucial to the perceived detail and clarity of the AKSA amplifiers, and has only been possible at affordable cost in the last decade. Most rectifier hash is subliminal, since it is manifest at very high frequencies, but it finds its way through the output devices and to the speaker, creating a low level excitation of the voice coil which raises the subliminal noise floor and militates against high resolution at audio frequencies.
The diodes are strongly recommended, and are sold as an option.

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I will have separate rectifiers for each channel. Can I use one transformer (Avel Lindberg D4051) for three channels, if the amp is used at moderate levels?


You may implement the power supply this way, but the result will be a partially collapsed sound stage.

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Will my 28-0-28Vac transformer produce too high a rail voltage for the AKSA?


Mains voltages vary throughout the world, and if, when used with your local mains supply, this gives you a DC voltage at the amplifier rails of 41V or more, it may NOT be suitable. The 55W AKSA may not be operated over 41V rails since there is a danger, particularly with 4R loads, that the output devices will be driven into failure.

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Could you help me understand the trade-offs regarding transformers. 

Is there any reason why I wouldn't just maximise the transformers and use two 330VA Avel Lindberg’s or 300VA Plitron’s?  Is this overkill when running totally separate power supplies?


There seems to be an optimum size transformer, and for the 55W AKSA this is 160VA per channel.  The larger 100W amplifier is best with 225VA to 300VA transformers.  A larger transformer, and a larger filter capacitor than 4,700µF, (2 x 4,700uF per rail on the 100W AKSA) seems to perceptively slow the amplifier;  it becomes a little slower off the mark, and lacks the attack for which it has become renowned.  We don't recommend a single power supply for both channels;  we strongly recommend a pair of separate, 160VA transformers, and the regulation of these transformers is not really the big issue.  It seems to me that a smaller transformer seems better able to respond quicker to transient demands than a larger transformer;  of course, the steady state situation is certainly different and does depend on the regulation, almost by definition.

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I am going to use two Plitron transformers (077316211--300VA). Are these too large? Will my midrange suffer? If so, which Plitron’s do you recommend for the sweetest mids and highs?


We haven't ourselves trialled the Plitron; in Australia we have our own manufacturers, and a lot of imported toroidals as well. Actually, the larger toroids are not quite as good as the smaller, 160VA (or thereabouts).  The power supply considerations are essentially conflicting;  you want a transformer with good regulation, so the voltage sag on heavy signals will be quickly restored.  This means a larger transformer, like a 300VA.  However, many of the current demands are rather transient, very quick;  so the inductance of the primary and secondary is important, since it prevents current flowing rapidly.  A smaller transformer might have worse regulation, but this is essentially a constant current parameter; the transient voltage regulation is arguably something better.  We are using 160VA transformers with amorphous cores, and they perform outrageously well. The bass and image depth is really so much more remarkable than ONE 300VA quality toroid.  I am inclined to think the best sound doesn't really matter too much in terms of transformer;  what is important is the fact that the supplies are separate.  The filter capacitors do make a difference, but the onboard, PCB capacitors make a difference too, and you'd probably be better playing with them; (they are much cheaper than altering the main filter capacitors).

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How would improving power supply regulation by using say 300VA toroidal transformers and larger filtering capacitors suit the AKSA?


We have found that overly large toroidal transformers do not actually improve this amplifier, rather, they appear to slow it down.  Two transformers give much superior stereo image, particularly depth, and once you have heard the difference, you'd never use a single power supply for both channels.  Mounting in the one box is fine, and causes no problems.
 
Transformer regulation should not be better than about 5%;  more regulation sharply increases primary and secondary inductance, which adversely affects the sonics of the amp.  For similar reasons, the capacitance should not exceed 10,000u F per rail per channel.  We realise this is in surprising disagreement with many commercial solid-state amps.  They seem to benefit from very large power supplies, but the reasons are related to the nature of the feedback overload function and the operation of the voltage amplifier, which in the AKSA amp’s is super-fast;  we use 100MHz transistors in this role.  We emphasise that the amp was not merely developed along the usual engineering grounds, with CRO’s and meters.  The basic AKSA design was set up and studied on the bench, then tweaked iteratively and very carefully for sonic effect over hundreds of hours of listening.  The engineering is mandatory, of course, but there is much, much more;  and ceasing development with good specs alone is a false dawn.  In auditioning with the AKSA we have ranked with Leak tube amp’s, Marantz PM66 Signature Series, Sugden A21 Class A amp’s, and Audiom Silver Knight SE 300B amps.  These are all very good amplifiers; it is utterly astonishing how good the sonics of the AKSA really are.

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What brands of power supply filter capacitors do you recommend?


Every reservoir capacitor sounds just a little different, with the chief differences appearing in the bass/midrange and in what I term 'engagement' (holding the listener's emotional attention). We have trialed;
* Elna Cerafine’s.  Very, very good with ethereal, detailed mid and top end but a little thin on bass which slowly improves over time and gets to be very impressive eventually - the Cerafines are marvellous, but they do take some weeks to come up to speed.  At first they are thin and lack bass, then after some weeks they are very good, with very nice but subdued, musical bass.  Very expensive, however, and not cost-effective compared to other, less esoteric capacitors.
* Nichicon KMHs.  A good all rounder with very strong bass and good midrange and top end.
* EVOX-RIFA.  Excellent all rounder with strong bass, very good mids and high and slightly warm).
* Sprague.  Fantastic bass, moderate mid and slightly indistinct treble.
This is not too many samples, but shows clearly that there are differences.  It also demonstrates that bypassing is important, and to some extent you can tailor the sound with different capacitors on the PCB at the supply rails. Use 100u F Black Gate or Hatano EXR or Cerafine’s here (for C10 and C12), and you should alter the sound again with just any old capacitor in the reservoir.
For dual mono supplies, it is not advisable to go higher than 4,700u F per channel per rail.  Too much seems to 'slow' the amp, and the same is true for very large toroidal transformers.  Best size is 160VA per channel.  Too little reservoir capacitance seems to speed it up to the point where it becomes skittish and etched, but generally hum is not an issue.

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I'm trying to decide between two approaches to the power supply.

I could build the power supply into each block, keeping the power "local" and leads fairly short.  Alternatively, I could build a central power supply, (with two toroidal transformers and rectifier bridges), and run umbilicals (five feet long) to the amp’s.  This approach keeps the power supply noise and heat far away from the amp’s, but I wonder whether the umbilical approach might significantly slow the AKSA’s.  Or will the added inductance further smooth the rectified DC?  If I built the central power supply, I would probably put the filter capacitors in the amp enclosures.  Is this correct?  How about the rectifiers?  I'd probably put those in the power supply box.  What's a good structure for the umbilical?  10 or 12 gauge stranded copper twisted together with a good shield?


Both methods work well.  I am more inclined to recommend using the short wire approach;  combining the power supply in the same box as the single channel it supplies.  Joining the two should be a 15A auto cable of high purity, stranded copper;  or your speaker cable of choice.  I have used wires between power supply and board of up to five feet with no problem.  This places very heavy demands on the 100µF bypass capacitors on the board, but that's fine and these capacitors are of suitably high quality.  The free passage of current between the power supply system, (transformer, rectifier bridges and capacitors), to the amplifiers greatly affects the bass and to some extend the midrange of the sonics, so the bypass capacitors are very important with long wires, and the potential bass with short wires is marginally superior, but scarcely audible.  What is audible is the use of dual power supplies, without interaction of any sort, and to this end even using separate power cords to the primaries of the transformers is a good idea.  There is an audible difference there.
If you choose toroidal transformers there is no hum issue with the AKSA, but this is not so with standard E-I or double-C core transformers, which have a substantial EM field surrounding them which finds its way by induction into the low level input circuitry of the AKSA.  Nonetheless, put a metal barrier between the toroid and the PCB, mu-metal is best, but not necessary;  two pieces of tin plated iron, about 0.5mm thick, separated by 5mm, is recommended.  This scotches any EMI field, even when the toroid and the PCB are very close together.

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In your power supply instructions, you state that C1 must be "X2". What does "X2" mean? What brand do you recommend for that?


”X2” simply means 275Vac mains rated;  any brand will do the job and has virtually no effect on the sonics.

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The 100 Ohm resistor in series with C1 in the mains primary circuit should have what Wattage rating, and what tolerance?


0.5 Watt is fine, 5%. Quality here is not specifically important.

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I have some air cored chokes that could be used in a pi filter. Could they be used with perhaps a couple of high quality 1,000u F capacitors to any benefit?

Use of chokes here is not recommended.  Due to Lenz's law, any change in current through an inductor is resisted by back EMF, which then detracts from the supply voltage.  While an inductor works fine on a Class A tube or transistor amp, it is not at all satisfactory on a Class AB amp because of the widely varying current draw.  In operation, the amp cuts out on heavy passages, the music dies, then a second later, it comes back when the amp again recovers.  It sounds quite spooky, and just plain doesn't work.

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Do you have any hum issues with the AKSA?

Hum is always possible when building audio amplifiers, and can arise from many sources. It could be an earth loop on the input, an electrostatic field around the transformer, (particularly if it is a C-core or E-I device), or a poor connection in the signal wiring. That said, the AKSA is very quiet indeed provided suitable care is taken to shield the transformer and to set up the earthing correctly. Inaudible levels of hum with even 100dB/Watt/metre loudspeakers are readily achievable.

The first issue is star earthing. The power transformer centre tap, the chassis earth, both filter capacitor ground connections and the PCB earth terminal should all come together at one point – the star point (earth). Dual, separate supplies also use one star point, connected both to chassis and to mains earth. To this end, using the optional power supply pcb offered from this website ensures you get it right, and we do recommend it strongly.

With no signal applied, and if the hum is only apparent when the transformer is on, switch off, leaving the amp running on capacitor reserve for a few seconds. If the hum disappears, the problem is the transformer field, and it may be necessary to isolate the transformer electrically from the case, separate it by some distance (say at least 6”, or 15cm) and hook it to earth via one wire to star earth. In extreme cases, many designers choose to mount the power supply remotely from the amplifier, with connection via an umbilical cord, although this measure is rare with the AKSA, and in our experience never happens if toroidal transformers are used.

If the star earthing is correct, but the hum disappears when you disconnect one of the input RCA’s to a source, the problem is almost certainly induced hum from a troublesome transformer field. Once again, the solution is to isolate the transformer electrically from the case, separate it by some distance, (say at least 6”, or 15cm), and hook it to star earth via one wire. These problems generally do not occur with toroidal transformers, since they have no 'frame' connection to chassis and all magnetic circuits are contained within the body of the transformer. Serious hum cases may call for a metal shield around the transformer – the best material is mu-metal. However, the toroidal transformer recommended for the AKSA power supply is intrinsically quiet at low current, where hum problems are most apparent. In practice, AKSA constructors who use the recommended toroidal transformers have few problems with hum.

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Could I use circuit breakers instead of fuses for the power supply?

Jon Risch recommends these vs. fuses for his AC line filters. There is a 3 Amp breaker in the Potter & Brumfield W28 series.
A Potter & Brumfield W28 Series thermal circuit breaker rather than a fuse is perfectly acceptable. A breaker offers less total resistance at high current draws, and the ability to be reset after a surge. The W28 Series is compact and fits in the same hole as a panel-mount fuse holder. They are available from Newark Electronics as follows:
5 Amps Newark part# 75F010 Type W28XQ1A-5 $1.69
10 Amps Newark part# 75F013 Type W28XQ1A-5 $1.69
15 Amps Newark part# 75F015 Type W28XQ1A-5 $1.69

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Can the power supply be built in the same box as the two amps?


Yes, and this is recommended. The amp is utterly quiet built this way, with the Star Earth configuration implemented on the custom printed circuit board we supply as an option. The design of power supply we recommend can be found on the web pages and in the accompanying kitset documentation. Physical layout is uncritical. We can supply diodes, filter capacitors and a custom printed circuit board; but not the transformers, because of cost, weight, shipping and space considerations. In most countries it is very easy to obtain a suitable transformer, and extensive recommendations of suitable toroidal transformers are made in the documentation.