Historical look at amps

Keis, I believe it is impossible to build a practical amp using discrete fets that doesn't use some type of feedback. I'm going to need more proof than the fact their web site uses the term "zero feedback." I sent an email to Ayre asking them to clarify. I'll let you know if they respond.

Aball, my functional amp uses zero feedback. That is the beauty of the triode vacuum tube. They are linear enough to use without any feedback.

I don't know of any designer that looks to feedback as a solution to bias stabilisation in a SS amp. Lowering noise, distortion and output impedance, yes. Increasing bandwidth and input impedance, ditto.

I agree. Feedback is not used for bias stability in transistor or for that matter tube amps. There are numerous stable bias schemes for amps and they don't involve feedback.

Some amps use DC servo control to maintain zero potential at the output of a push pull amp to avoid using coupling caps. This might be interpreted as feedback since the output potential is monitored, and a correction voltage fed back to a prior stage, but this is usually not considered as feedback since it works well outside the audio band, and is used for an entirely different reason than those listed above.

Still no response from Ayre on my inquiry about their claim of zero feedback.

Charles Hansen should run for political office. He never did say for sure whether or not they use feedback. Only that

We use the term "zero feedback" to describe these circuits because this is the most accurate, generally accepted way to describe them.

A great non-answer.

Hi Ar_t, I'm not trying to get in an argument here, I'm just looking for a bit of clarification on what you meant by your last post. I'm sure the answer to your question is obvious to you but not to me. Either you think we all realize how difficult it is OR you think we are incapable of understanding this.

In either case, does your statement refer to the need to use feedback to overcome the limitations of SS devices such as their nonlinearity in order to have a practical circuit, OR the feedback paths inherent in the devices themselves?

Thanks Ar_t, I didn't think you meant any disrespect by it. I was really just making sure you weren't thumbing your nose at us with your comment "do you have any idea...I didn't think so." I taught electronic theory at a junior college for 10 years including transistor biasing and circuits so I do understand what you are saying. I also appreciate the effort that went into your last response and getting some insight from someone trying to design a practical, marketable amplifier. It is obvious you have a passion for what you are doing and I wish you luck in your endeavors.

I use tubes in my amps but the theory is pretty much the same. The amp I'm using now has 3 stages; all are common cathode with a bypassed cathode resistor which gives it a lower cutoff frequency of below 10 Hz. To me that is zero feedback, but as you point out there is some small amount and more as the frequency decreases

That brings us back to Hansen's defense of using the term "zero feedback," an idea which was an integral part of the original post. He is correct that there is no textbook definition so he defines it to suit his marketing needs. I don't need my lower cutoff to be any lower and the feedback in the audio band in my opinion is negligible so I describe my amp as having zero feedback. While I think he is taking liberties with the term to the point of being deceptive, a point born out by Keis' belief that Ayre wasn't using any feedback whatsoever, a belief based on Ayre's advertising claims, Hansen could also point his finger at me and accuse me of the same so it is an argument that can't be won. However, I have nothing to gain by using the term and I truly believe that for all practical purposes my amp is zero feedback. On the other hand, it is just as obvious to me that Ayre is twisting the term to capitalize on the current "feedback is bad" frenzy gripping the audiophile community, and relying on the fact that most audiophiles are non-technical and will therefore believe they aren't using any feedback at all.

This is also born out by his reference to the Maxim 4200 data sheets. By selectively quoting from the sheet it gives the impression that his use of the term is an accepted industry practice. If you actually read the sheet and take the phrase "without negative feedback" in context, it is obvious they are only talking about a global feedback loop from output to input. Further reading from the same data sheet:

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.

This shows that they do employ feedback and that they, unlike others, are not trying to hide the fact.

I don't want to blow this out of proportion. Like Bombaywalla, I have no real axe to grind with Ayre. I admire their products and even went to the unprecedented length (at least for me) to purchase one of their CD players new from a dealer because I could not find a used one. Even paying retail I thought it was a comparative bargain. It is just that Hansen’s defense of his marketing campaign reminds me of Bill Clinton asking for the definition of the word “is” when defending himself in the Monica Lewinsky scandal. I suppose I shouldn't worry about this advertising claim any more than I worry about Miller's claim that their lite beer tastes great.

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?

Well, it has been very informative but I think my time spent on this topic has come to an end. I still think Ayre is being deceptive by using the term "zero feedback," but unless we get some more information on their amplifier design I'll never know for sure. Ayre never responded to my email asking them to clarify and I did not expect they would.

I sure don't expect Hansen to jump in at this point and clear it up even though you can be sure he is watching this thread (I'm suprised he answered Keis' question at all even though it didn't tell us anything), and Ar_t evidently knows something that he can't or doesn't wish to share about the design. I understand that.

In any case, thanks for the history lesson Ar_t. I know more about amplifier design and the business of bringing them to market now than I did a few days ago so I suppose it was time well spent.