Historical look at amps

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.

Yes, the intrinsic emitter resistance does constitute local feedback, but I was trying not to get too technical.

Ok, speaking of emitters and such, the development of "ring emitter" transistors lead to a radical change in transistor design. Back when I started getting serious about amp design, you had 2 choices: RCA and Motorola. RCA (for whatever reason) did not make high-power PNP devices. Motorola did. With those, we had amps like the SWTP "Tiger" series. A bit unstable, but probably the first step towards modern amp design. Even then, the transistors were made with diffused processes, and were not the most rugged in the world. Eventually, they learned how to make epitaxial processes, and things started to take off. Some firms, Bedini as example, stuck to using only NPN devices in the outputs.(The RCA approach.) But most everyone else went to complementary devices. However, out of that grew the 0.000001% THD wars, and the resultant bad sound.

(Looking back......in hindsight......there may not be a convincing reason to use complementary devices in closed loop amps. Remember, the little 20 watt Bedini did sound good.)

I suspect that the guys who came up with the ring-emitter concept were probably used to designing RF transistors. Sanken, Toshiba, and Fujitsu all had strong contenders. Linear, fairly rugged, and perhaps most important: low capacitance. This allowed designers to push the bandwidth higher, as we were all concerned with TIM, SID, and a host of other "new" mechanisms that we were becoming convinced explained why our amps all sounded like doo-doo. Somewhere, things had gone horribly wrong.

To me, the thing that really got my attention was not only the linearity and low capacitance, but the new packing concept. WOW! You can bolt the transistor to the heat sink, on the inside, bend the leads 90 degrees, and hook it right to the PCB! No more drilling hole through the heat sinks, using nasty sockets, steel cases with screws going through them to make electrical contact, etc.

But let me pause and give praise to the guy who may have been the first to "think outside of the box", when it came to using ring-emitters, and in an entirely different manner.

John Iverson.

Not only did he incorporate the new transistors, mounted in a different manner, but he came up with an usual input stage, followed by an even more unusual gain stage. I had not seen anything like it before. He refined the gain stage somewhat in the later versions of the Eagle amps.

OK......what was so great about it?

Some will argue, but transistors are basically current controlled devices. (Yes, you have to create a voltage to have current......not the point here.) If you think as the input/control signal as a current, and design with current linearity, not necessarily voltage linearity, as the parameter to optimise, you come up with ideas that have not been used before.

At least not in audio power amplifiers. I suspect John may have worked on some military/government electronics somewhere in his career. Regardless, guys who thought like him gave us things like folded cascodes, and other techniques that increased both linearity and bandwidth.

The more linear it is to start with, the better it will sound if you use feedback to lower it. Likewise with bandwidth: the higher you can get it, the more stable an amp should be.

So, a lot of factors came along that made it easier to build amps that were inherently more linear than the junk we designed in the 70s. Some of us decided that designing by specs was even more meaningless than the rest of the crowd, and we got rid of all the loop feedback. But none of it would have possible 25-30 years ago. The semiconductors did not exist, we had our head(s) screwed on backwards, and it took some cock-eyed ideas (which may have been invalid!) to get them oriented back in the right direction.

Actually......now that I think about it......Audio Research was on the leading edge in SS design with the notoriously unreliable D-100. It used a "zero-feedback" output stage....what was it...mid 70s?......long before anyone else thought of that concept. (The problem was mostly a heat sink issue. The amp could have been reliable with about 2, maybe 4, times the heat sink surface area.) The input stage may have been bad.........I don't know, the modules were potted, but the output stage concept was a good one. I know..........I have used it the last 10+ years. With decent transistors on much larger heat sinks.

OK.....that ought to be enough to digest for a while. I appreciate the encouraging e-mails. Thanks.

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?