Historical look at amps

Damn, we're really getting somewhere now. Thanks Ar_t! I love it when I actually learn something.

Someone asked me a question via e-mail that is probably too technical for everyone here, but.......

As I was formulating my response (it was about current-feedback vs voltage-feedback), some thoughts came to me that might help to clear up this "zero feedback" subject.

Let's put semantics aside. Whether "current-feedback" is really current feedback, or a special condition of voltage feedback is not the issue. I said "Hell, let's call it a bean bag amp.......anything, but we need to have an agreed upon term to call this type of amp."

The crux: "current-feedback" amps have a bandwidth that remains constant, regardless of gain. Traditional feedback amps do not: as the gain increases, their bandwidth goes down. There has to be a way to define this type of amp. It may not qualify as a unique situation, but it is very different in marked ways from typical amps.

And this leads to the "zero feedback" concept, as used by Ayre.

Amps without loop feedback.........any loop feedback.........have a distinctive sound. It is unmistakable. If you hear one side by side any other amp, you will understand immediately what I mean.

The issue here seems to be concern that Ayre is the one playing fast and loose with buzzwords, created just for marketing measures. I disagree. I believe that is others who are guilty of it, and perhaps Ayre is being cast in with them.

Here are some examples of ways to fudge "zero feedback" when it really isn't anything close.

Rowland, Threhold/Forte, and others (me, at one time) used an output stage that used a feedback loop around it, but had no connection to the input. So, overall feedback free? Yes. Zero feedback free? No way.

(If you compared one of our amps with the zero feedback output stage to the one with a local loop, you would have no difficulty hearing the distinct sound of zero feedback. I modded every one that I could track down, and every single owner liked the modded version. Despite higher THD and output Z.)

Ok......let's take the analogy one step further...............to say...........a Boulder amp.

Most use two separate gain cells, each one has a feedback loop around it. No feedback from one to the other. Now, overall loop feedback free, but definitely not feedback free!

Ok.....let's go the Eagle (Electron Kinetics).............

An integrator input, driving a transresistance stage. Feedback loop around from the output to the transresistance stage, but the integrator is outside the feedback loop. Again, overall feedback free, but nowhere near zero feedback.

Ok, here is one:

Someone builds an integated amp, with a typical amp that has a feedback loop around the amp stage, but a separate buffer for the preamp section. You could stretch the point that it is overall feedback free. No feedback from output back to input........just like the Eagle. I hardly think that anyone would believe that it would qualify as a feedback free design.

But that sounds exactly what Maxim is doing!

Here is an excerpt from the Maxim data sheet, and it does not sound like zero feedback to me.

"Since these devices operate without negative feedback, there is no loop gain to transform the input impedance upward, as in closed-loop buffers."

Ok.........sounds like no feedback from output to input. No loop feedback design.

They go on to make the same claim that there is no feedback to decrease output Z. OK, still sounds like no loop feedback.

But get this!

"The MAX4200–MAX4205 include local feedback around the buffer’s class-AB output stage to ensure low output impedance and reduce gain sensitivity to load variations."

Ah-ha! Caught in the act!

Well, the datasheet goes on to talk about the advantages of not being "closed loop". I agree with their assertions. But they openly claim that it has local feedback. What do we call it? Ok......no overall loop feedback, but they admit it has a local feedback loop.

So, as C. Hansen pointed out in his e-mail response, there is no agreed upon dictionary definition of what constitutes a feedback free design. Hell, engineers can not even agree upon what to call circuits that do have feedback. (There was a heated exchange on one of the DIY nerd forums months ago on the current- vs. voltage-feedback terminology.) How can we expect you guys to be able to sort out who is making bold statements about their gear, and who is just making b*** s*** about their gear?

Well, the only way to know who is telling the truth is to listen. I have not heard an Ayre design in around 10 years. But I can attest that it was definitely free of any loop feedback. I seriously doubt that stance has changed.

Ok.......so how can we tell, you ask?

To me, the front-back soundstage is the first clue. Designs without any loop feedback have much more separation here. Feedback tends to have the effect of compressing things from front to rear. The other things that I notice is that the bass "seems" to be more lifelike. Maybe not have the punch feedback amps have, but bass notes seem to be more lifelike, and each one stands out individually from the others. Heavy feedback amps may have serious "crunch factor" but to my ears, the bass notes tend to all sound the same.

Who likes which one, and why, means nowt to me. Just be assured that there are distinct differences between so-called "zero feedback" amps, and all other amps that do employ ANY type of loop feedback. What may sound like marketing hype, double talk, or just plain crapola could well be. Except in the case of Ayre. Their claims are an accurate reflection of their products.

Well, enough if that. I would rather exchange thoughts on why "digital" amps have to be more rolled off so that prospective customers won't kvetch that they sound bright. But, if you guys want to talk about other amp topics, I will do so as time permits. (I know that tomorrow is out, maybe evening.)

Thanks again Ar_t, I do agree with Unsound that you are the MVP here (Most Valuable Poster.) The most honest and straightforward answers I’ve seen here from the manufacturing community.

I followed your last post OK except for your conclusion about Ayre amps. What caught my eye about Hansen’s response was the fact that he is using the term "zero feedback" in a marketing campaign aimed at your average audiophile, and then justifying the use of it by quoting data sheets that are aimed at electrical engineers. All of this smacks of “baffle them with BS” and it is preposterous on Ayre’s part to expect the average audiophile to pick up on this. Not only that, he defended his position by selectively quoting the Maxim data sheets and ignored the part about their use of local loop feedback that both of us caught and quoted in previous posts.

So my question to you is this; since I am not privy to the Ayre’s schematics and I doubt they are interested in releasing them, unless I misinterpreted your comments, you agree with Ayre’s description of their amps as using zero feedback.

Oops, that wasn’t a question. The real question, why do you feel Ayre is justified in using the term zero feedback? No loop feedback?? If so, do you know this as a fact or just based on listening to them?

i, too, thank Ar_t for his candid & informative posts. Such info sharing has been a long time coming (we got a lot of this in the speaker forum from Roy Johnson when one member 'innocently' asked what the diff between time & phase coherence was & which one was more important. Many of you might remember that valuable thread) & much appreciated by us. The glimpse of the real inside scoop.

Herman, well put in your last post. I join you in asking your question.

"The issue here seems to be concern that Ayre is the one playing fast and loose with buzzwords, created just for marketing measures. I disagree. I believe that is others who are guilty of it, and perhaps Ayre is being cast in with them."
True, "others" have coined these terms. however, these "others" were EE & maybe even the IEEE! They were *not* engineers/designers making audio equipment to the general consumers. Amongst the EE, who are schooled in the art of ckt design, it is fine to use these terms. One is amongst peers of similar knowledge bases. But.......the consumer?
what i was insinuating in my last posts re. Ayre & other manuf loosely using the feedback terminology is that Ayre has done nothing to educate their clients on negative feedback, done nothing over time to dispel the doubts on this topic, done nothing to educate them on the types of feedback, done nothing to show them how their products are any diff from the others in a way that the layman comprehends it. All of this while they seemingly have been the longest term practising proponent of "zero feedback". good companies are responsible for not only good products but also for educating their customer base. Great companies are even reformers.
With audio hobby in a decline, the companies surviving in this realm should think of this (honestly educating the customers) as a self-preservation act.

Bombaywalla,

Ar_t may have topologies of tube, and solid state down, but his "bright" characterization of D amps is off the mark. I don't know of any owners of sophisticated class D amps complaining about HF.