Zero Feedback Impedance Amplifiers

[405man]

Susan
I was rereading your description of the amplifier and noticed that you comment about the direct electrical connection between input and output of the amplifier. Does this path represent a form of feedback

Stuart

[Susan-Parker]

Hi Stuart,

Thanks for your post.

Originally posted by 405man
Susan
I was rereading your description of the amplifier and noticed that you comment about the direct electrical connection between input and output of the amplifier. Does this path represent a form of feedback

Stuart

No, not as far as I am aware. The mosfet gate (or tube grid) is a high impedance input (unlike a bipolar device) when powered.

However when the mosfet is not powered there is enough electrical energy for some of it to get through the device and make a noise (it needs a reasonable signal and fades in and out a bit, is weak and VERY distorted - but it does happen).

This is shown in the picture at the bottom of this page:

http://www.susan-parker.co.uk/zeus-se-amp.htm

N.B. Because this pic is for a SE amp the transformer is storing some current and one sees a portion of "real" signal in the bottom half of the trace.

BTW This direct connection effect also occurs with the 6C33C triode tube version once the heaters are switched on but before the HT is applied.

It is a characteristic of my transformer coupled follower mode and would not be seen in a regular amplifier with active gain stages.

Hope this helps.

Best wishes,
Susan.

[henrik04]

I am in the process of getting output transformer made locally, but for the input transformer.. Not enough info.. (Or I probably missed the thread related to it).


I quote Susan:
The input transformer was made for me by E A Sowter Ltd. It is a simple split bobbin with each half having one secondary (one ends up with two primaries, one on each half which are then wires in parrallel). The bobbin is wound as a (S/2)P)S/2) (i.e. split secondary wound either side of the primary) with a 1:10 turns ratio.

Sowter designated it as part # 8160 and they recently quoted me UKP 38.25 (plus VAT) each ex-works for some more (they list specials they design for people, but they don’t charge a big up front design fee).
End of quote.

Beside being just on the other side of the world, their search engine doesn't know about part 8160.
The local transformer manufacturer requires more details..

Second question:
What is the best way (exept an external cd player) to drive this innovative creation? (And no I do not wish 350volts to drive a triode plate).

There are plenty of driver of all kind around, but..

Thank you in advance.
Sorry about my English (Not my first langage)..

Cheers
Henri

[Susan-Parker]

Hi John,

Originally posted by jkeny
Hi all

I wonder if there could be a sonic comparison posted between this unusual design amp and some recognised reference amps, AKSA etc

I presume some people have now built a variant of this design and feedback on sonics would be helpful for others thinking of embarking on this path.

John

Further to your question I have now had this reply from the recent audition of my system (note references to negative feedback, i.e. NFB, are referring to external loop negative feedback, not the follower internal negative feedback):

========= Review ============

Recognised reference amps you’re kidding I presume?

If you manage to identify a reliable example of what constitutes the perfect amplifier, let alone any typical group of listeners who would agree with your choice when applied to a real-world listening environment, I’d like to hear about it?

If somehow you were able to make the amplifier ‘disappear’ you would still hear anomalies attributable to the rest of the system, and probably more clearly than if the amplifier was adding its own signature to the performance.

Susan’s design which she has based on the concept of “wire with gain” no matter how well carried out, will still leave room for an argument along the lines of “what sort of wire?”

From a (relatively brief) listening experience, and from some preliminary experiments of my own using the Zeus output configuration, it is my opinion that her design has some interesting features which set it apart from the usual configurations.

1: It uses transformer coupling to the speaker and hence gives an extra degree of autonomy to both components. When transformer coupling is also applied to the input side (as in conjunction with the line stage) this gives some additional freedom from ground loop generated noise and distortion and can also remove the need for an input capacitor.

2: By use of a very low impedance solid state output devices on the primary of the transformer, back EMF from the voice coil can be absorbed and rendered relatively harmless to the integrity of the sonic picture.

3: By optimising conditions to achieve very low levels of generated distortion in a circuit with adequately wide frequency response there is no need to introduce negative feedback into the equation. NFB can be a highly effective destroyer of imaging and soundstage accuracy, due to its unavoidable effect on phase response when applied to even well designed circuits.
If you are rash enough to attempt to apply NFB across ANY coupling transformer you will further compound the problems. Fortunately for the NFB brigade most composite systems are already so deficient in the phase domain that gross NFB generated phase errors are able to slip by largely unnoticed.

As predicted by theory, initial auditioning of the Zeus power amplifier reveals a smooth surefooted presentation that is free of grain and artifacts of the type that induce listener fatigue.

It was not possible to focus on anything in particular that would identify the presence of the amplifier which suggests wide frequency response and good transient ability. This makes comparison with other amplifiers in other systems somewhat difficult. Even very highly rated amplifiers can and do interact with the connected source and load and hence sound ‘different’ in different systems. It is likely from what I have observed so far that the Zeus will be largely free of this variable but its presence no doubt will reveal the strengths and weaknesses of other components in the system, making it somewhat unwise to predict instant magic.

If you are looking for a competent reliable and easy to construct power amplifier, and one that will readily reveal any shortcomings in other parts of the system, thus allowing you to make and assess intelligent upgrades, a Zeus offers a lot of performance for relatively little money and effort. Being such an elegantly straightforward ‘easy to get on with’ circuit, it would take a high degree of constructor incompetence to damage. It also has the major advantage that even under the most catastrophic of DIY failures, your input stage and speakers are protected.

One thing you can be pretty sure it will not do is colour (rose or otherwise) the sound to complement less able components in the system. Some complex speaker systems might prefer a higher damping factor but most should be ok. I’d guess that this promises to be an amplifier eminently suited to single driver configurations where its excellent phase coherence can be fully appreciated.

Jan 2006

======== END =========

Any questions?

Best wishes,
Susan.

[jkeny]

Susan,

Thanks for getting back - the review certainly is high praise for the Zeus and unlike a lot of reviews doesn't compare it to other amplifiers or attempt to deconstruct the overall sound into macro, micro, etc, etc.

This review approach,although highly praiseworthy, may or may not be useful to people looking to venture down this path. It talks about the Zeus in absolute terms of wire with gain and uncoloured presentation but the familiarity of the yardstick of comparison to other amplifiers is missing. I suppose, as in most things, the best approach is to try the Zeus and see for yourself.

In this regard, you have thoughtfully tested a starter & cheaper version as a proof of concept based on toroid transformers. What percentage of the final sound quality is achieved by this transformer version, given good electrics & construction?

Apologies if this ground has been covered before but I am just trying to decide on whether to commit to the time & money required to go down this route. I know it's not much money but time is a precious commodity.

John

[Susan-Parker]

Hi John,

Originally posted by jkeny
Susan,

Thanks for getting back - the review certainly is high praise for the Zeus and unlike a lot of reviews doesn't compare it to other amplifiers or attempt to deconstruct the overall sound into macro, micro, etc, etc.

The issue comes to the point where the sound variation in the quality of the source recordings becomes the dominant sound quality that one hears.

Speakers, room acoustics and the overall amplification chain obviously do play their parts, but they are secondary effects.

This review approach,although highly praiseworthy, may or may not be useful to people looking to venture down this path. It talks about the Zeus in absolute terms of wire with gain and uncoloured presentation but the familiarity of the yardstick of comparison to other amplifiers is missing. I suppose, as in most things, the best approach is to try the Zeus and see for yourself.

It is always difficult to make quantifiable comparisons between systems unless one is set up for direct A/B/C comparison "on the spot".

Personally I would put my Zeus preamp / power amp up against an ONGAKU or any other hi-end amp and expect it to be as good as if not better (within the power range).

Of course audio is a very personal experience and I fully accept that other people may prefer other systems.

In this regard, you have thoughtfully tested a starter & cheaper version as a proof of concept based on toroid transformers. What percentage of the final sound quality is achieved by this transformer version, given good electrics & construction?

The starter system, with a modicum of care in the selection of the toroids, can give at least 70% of the Push-Pull system and 90% plus in basic sound quality.

The main issue is the bandwidth limitation and there can be some slight mismatches between pairs of windings which affects the distortion characteristics. Given that these toroids are wound for mains power supplies, and so are not optimized for audio use, all things considered they do surprisingly well.

Some versions work better than others, so a little experimentation may be needed.

The output stage benefits from matched mosfets (but is not obligatory for first tests) and for good operation in 2:1 step down biasing should be a minimum of 500 mA per device with a 34 volt power rail. If you have a big enough heatsink it can be higher which will push down the distortion figures.

I really do recommend the L200 plus third same type mosfet for the bias supply.

Although it has been done, I do not recommend using a 1:1 output for driving loudspeakers unless you really do need the raw power.

If you have good sensitive speakers and a moderate size room then up to 4:1 step down will give better distortion figures.

Output transformer size of 300 to 500 VA depending on power required. Input transformer between 20 to 50VA, here differences in manufacturing technique are likely to have noticeable effects.

Apologies if this ground has been covered before but I am just trying to decide on whether to commit to the time & money required to go down this route. I know it's not much money but time is a precious commodity.

John

Not a problem. Money is money and the engineer in me doesn't like to be profligate. And I agree about time, there is never enough of it.

Best wishes,
Susan.

[henrik04]

Susan answered me and was really very helpful regarding the input transformer. So I got a quote from sowter and I did read it twice.. If you are living in Australia you'll get two input transformers for AUS$ 384.07 (163.18 GBP)

I'm still waiting for a local alternative. I was just wondering if any Aussie guy (or gal) would have started this project and how did they solved the input transformer issue

Please let me know.. Thank you in advance.

Cheers

Henri

[EWorkshop1708]

I had some 12V 6x9 car speakers with onboard amps (on the magnet) that were like your amp, but it used darlingtons and the speaker as the heatsink. Loud as hell with only a 6V lantern battery, and real loud with 12V

Transformer amps go really loud for the amount of volts they are fed with. Nice amp BTW.

[jkeny]

Susan,

Hope all is well with you.

I'm working my way to gathering parts for this amp - I currently have two toriodal trafos - Nuvotem 160VA 230V primary dual secondaries 2X25V which I'm hoping will be suitable as output trafos.

Can I use these as output inductors and ignore secondaries?

Is STW34NB20 the best reasonably priced Mosfet to use and do you offer any matched pairs for sale?

What trafo would you recommend for input use (preferably available from RS components)

Thanks for all your kind help and information in these threads
John

[AKSA]

Hi Henrik,

I buy from a local Melbourne business called Voller Transformers, on (03) 9553 3222, ask for Paul. Address is 39 Advantage Road, Highett, 3190. The quality is exceptional and the price would be less than one third of the Sowter.

Paul recently wound me a 2.5:1 step down, designed to handle 250Vpp/35mA on the primary and a 1K8 resistive load on the secondary. This trafo has 1dB points at 16Hz and 24.5KHz under load with no ringing; it's quite big, made from GOSS. It's a fully sectioned, interleaved design, and he will in fact design it for you at no charge. He's done numerous OPTs for tubes, a lot of chokes, and a huge diversity of power trafos, many of them GOSS to reduce EM emissions. He's a lovely guy, patient and sincere, and can ship them to you in Sydney. He also does all my power transformers for my GK1 preamp.

Strongly recommended.

Cheers,

Hugh