Zero Feedback Impedance Amplifiers

[Audiofanatic]

I had better try to keep my spelling in order - not one of my stronger points being dyslexic!

Dear Susan,

I just want to let you know, that you'r not alone

I'm also dyslexic, and I use a dictionary every time I post on the forum, so there is no need to feel self conscious

I admire you for who you are

All the best, and keep on the good work


Audiofanatic

[Susan-Parker]

Hi Graham,

Thank you for your reply and comments.

Originally posted by Graham Maynard
Hi Susan,

I guess the question ' What's inside your 'speaker ?' would also have been as relevent, because the amplifier-loudspeaker combination is more important than either the amp' or 'speaker when viewed alone in isolation.

Yes, I agree entirely and this is why I am less concerned with amplifier distorsion in absentia as it does not give the whole picture.

You quote 2.78 ohms output impedance. That would not satisfy me for use with 'conventional' loudspeakers.

My amplifer works as a current drive, not voltage.

"Son of Zen" is quoted as being 16 ohms output impedance.

I find that conventional speakers in the main show a noticable improvement when used with my amp - the exception being ones with over bright trebble.

My LS3/5As, which whilst very flat for frequency response (with a correspondingly complex crossover tuning circuit), are to my ears somewhat muddy driven by normal amps. With my amp the whole sound clarifies and a much improved sound stage definition is achieved.

All this is my own subjective experince of course, my ears being fussy about phase coherance as perhaps I may have mentioned previously?

Basically I want to be able to sit ten feet back with the speakers spaced sixteen feet apart and hear a complete sound stage in front of me without a hole in the middle.

Our living room is a bit smaller than this so I have to compromise.

However your Marble enclosures appear to house what looks like a Jordan JX92S, which can be 'bright' when driven by a low output impedance amplifier, this to the extent that many users prefer to use them with a series L//R 'baffle step' corrector. Now wouldn't that make for a super A-B comparison; your system, versus a good conventional amp plus baffle step, using the same source and loudspeakers!!

They are Jordan JX62s, unfortunantly no longer made.

I assume that you don't think a Quad 34/306 system is in the class of a "good conventional amp" being perhaps a bit long in the tooth?

You suggest small drivers like the Jordan are 'bright', but when you get 9 or 16 of them in a line, the brightness disappears due to physical spacing, which is why a tweeter becomes necessary at hf for non-'far-field' listening. But then who could afford 9 or 16 Jordans per channel ? I'm not so sure that parallel connection down to 0.5 ohms is going to sound any better than evenly or 'field' distributed series/parallel chains; as you say, even the Quad has distributive response tapering.

I couldn't afford them. I am currently looking at some cheeper wideband German drivers - initally as replacments for the Jordan contraflex drivers.

My amplifier is a curent driver, therefor works best if matched to the load. Because it is a current driver it can be configured to drive very low sub-ohm loads which simply wouldn't be possible in a conventional solid state amp.

The point with the line arrarys depends on the configuration.

People have attempted to make line arrays by stacking small bookshelf speakers on their sides one atop the other. Series/Parrallel chains with internal crossovers suffer HF loss, however true parallel drive does not have this problem.

Your amplifier does have its own internal voltage NFB - at the sources wrt input. Indeed, when there would be any transformer/driver resonance leading to momentarily high impedances at the sources there would be a potential for waveform rectification, though your amplifier controls this via its push-pull arrangement. Did you ever try any resistor damping at each source wrt ground, say 22 ohms ? I don't think your circuit would work so cleanly if it were single ended.

Yes, the followers do have internal NFB, but the bandwidth is very high. There is no global feedback to an input signal.

The followers have a gain of just under unity, hence rapid dampening of any "hiccups". This may be seen as a problem with compression at high power levels but I am not aware of it in my own listening (however that doesn't mean that it doesn't happen or that other people wouldn't notice).

Most of my listening is done with the first watt or two.

The resonance problem shouldn't accour as the amp is push-pull.

I haven't tried resistor dampening. The transformer is 1:1 so wouldn't the resistors still appear as 22 ohms / 2 to the loudspeaker? The output impedance is already down at 2.7 ohms.

A single ended version of my amplifier, as I have described in my notes, would have an asymetirc output impedance; the same as any single ended triode design - and as far as I understand these matters any AB amplifier once it is into it's B class operating region.

You mention
1 pre-amp cables............ not a problem if the pre-amp designer knows to buffer properly with resistor feed and loading.
2 NFB .............. not a problem if the amp designer examines for amplifier response to predictable crossover/loudspeaker induced back EMF.
3 cables .............. not a problem if the crossover is before the amplifiers or driven at genuinely low impedance then with inexpensive cables going to the loudspeaker drivers.
You have covered these aspects with your own design in a way that most commercially available systems have yet to do.
It is not signal path capacitors or NFB that are the problem, indeed there are those of us who could equally complain that transformers are a problem, but then you know your own design in a way that we cannot.

The nice thing about audio is that there arn't any absolutes right or wrong. Transformer based impedance amplification and current drive just happens to be my hobby horse.

"One should not increase, beyond what is necessary, the number of entities required to perform a given task."

(Parker after Occam)

I thank you for your offer for remote listening, but alas I have my own as well as family health probs to cope with. You would not believe the difficulties I had just completing the text for my article; my own hands on work has become almost non-existant. Hows about comparing your amp at home with a class-A like the JLH, for Jordan loudspeakers can be well driven by any of the JLH class-A's ?

I am sorry to hear of your health and hope that things improve quickly for you all.

I could do the comparison, however I would have to build a pair and this will take me a week or three as I am somewhat pushed for time due to my own work commitments.

Never let us wear you down,

Cheers ............ Graham.

Thanks, I will try to remember this.

Best wishes,
Susan.

P.S.

Have to go do some work now, back this evening.

[Circlotron]

Hello Susan.
Have you ever changed your first name?
If so, what was it before?

[Circlomanen]

Is this something similar??? If so it is a very nice review.

http://www.hifichoice.co.uk/archive/...rintreview.htm

[Graham Maynard]

Hi Susan,

You write - "My amplifier works as current drive"

But I don't view it that way ........... the Mosfet sources that are driving the output transformer are following input transformer secondary voltage.
The Mosfets are like behaving like perfect 'current' amplifiers followed by a 2.78 ohm resistor (as measured).
The 'Son of Zen' amplifier you mention is much more a current driving amplifier.
Also maybe the LS3/5As would sound better with a conventional amp plus 2.7 ohms ? I don't know.

And this is what concerns me with your design, because audio trafos (saves writing it in full all the time) tend to have a self capacitance/inductance peak at some hopefully supersonic high frequency, though with a phase change that comes in at about one third to half of that frequency. I'm not sure whether that phase change has ever been investigated/documented in audio papers, but your input trafo is susceptible to generating this effect then passing it on through to the output devices plus output trafo and load.

I suggested the 22 ohm resistors as a 'try' because they will ensure that Mosfet current will always retain a degree of resistive current flow, even when dynamically induced resultant driver/output tranfo/load currents become phase shifted wrt input voltage.

Now Circlomanen's link is interesting, for it implies a similar mode of operation to your amplifier, and it mentions a hint of 'metallic' sound on highs. Could this be trafo resonance ?... the very reason why I have mentioned split primary and secondary resistor damping at input, and say 22 ohms to ground per source. Jordan drivers tend to cut off more sharply around 20kHz than do normal tweeters.

I am discussing possible probs and possible improvements, rather that being a downer on your technological concept.


Cheers for now ........ Graham.

[Susan-Parker]

Hi,

For those that might be interested...

http://www.susan-parker.co.uk/susan-cd-player.htm

Best wishes,
Susan.

[EUVL]

So it turns all quiet all of a sudden.

I think it is now the moment of truth -- time to build something and report results.

Any volunteers ?
I'm sure you'll get all the help necessary from Susan. : )


Patrick

[Netlist]

I'm in, although posting test results could take some time.
As well as building.

/Hugo

[EUVL]

I wasn't suggest to hear results within 24 hours, but a first volunteer after a few minutes is speedy enough. Now the rest (supply of transformers and quote for prices) is up to Susan. : )

I'm just trying to be the catalyst. : )


Patrick

[darkmoebius]

Yep, I can't wait to hear the first tester's impressions. It will be interesting to see how much the trannis will cost.

I love the new ideas that pop up here every week. There's a never ending supply of things to learn and think about.

How's this for a quote:
diyAudio.com is brainfood