Historical look at amps

The amplifier evolution thread reminded me of the history of amplifier circuits that has occured over the last 20 years. Lots of changes but the one that stuck in my mind was the change in feedback circuits. In the early 1980s a good amp like Crown, McIntosh, Phase Linear etc all had large amounts of feedback and distortion levels of 0.00001% IM and THD. These amps sounded bad and the question was raised (and still is) why objective measurement didn't jib with listening tests. A Finnish engineer (OTTELA) came up with a new measurement called Transient IM Distortion (TIM). I wont go into the details but it did show that large amounts of feedback which made static IM and THD measurements good, made music waveforms bad. The result has been today's amps with low levels of global and local feedback, and better sound but with IM distortion levels of only 0.01% (and of course tube amps with more even then odd distortion harmonics). Just recently Ayre, and probably other companys are offering zero feedback designs. Feedback circuits have been with us since the 1920s and we are now just elliminating this basic design feature in modern amps and preamps.
Actually, the leader of that movement was/is Nelson Pass, whose patented STASIS amplifier designs of the 1980s and 1990s were revolutionary in their time. They featured Zero Global Feedback and were available in both Pure Class A, such as the SA-3, SA-2, and legendary SA-1; and high bias A/AB, such as the S-300, S-500, and S-1000.
I still find the following quote from Nelson Pass most amusing. It comes from the owner's manual for my Pass Aleph 3, written early in the 1990s:

"When I started designing amplifiers 25 years ago, solid state amplifiers had just achieved a firm grasp on the market. Power and harmonic distortion numbers were king, and the largest audio magazine said that amplifiers with the same specs sounded the same.

"We have heard Triodes, Pentodes, Bipolar, VFET, Mosfet, TFET valves, IGBT, Hybrids, THD distortion, IM distortion, TIM distortion, phase distortion, quantization, feedback, nested feedback, no feedback, feed forward, Stasis, harmonic time alignment, high slew, Class AB, Class A, Pure Class A, Class AA, Class A/AB, Class D, Class H, Constant bias, dynamic bias, optical bias, Real Life Bias, Sustained Plateau Bias, big supplies, smart supplies, regulated supplies, separate supplies, switching supplies, dynamic headroom, high current, balanced inputs and balanced outputs.

"Apart from digitally recorded source material, things have not changed very much in twenty five years. Solid state amplifiers still dominate the market, the largest audio magazine still doesn't hear the difference, and many audiophiles are still hanging on to their tubes. Leaving
aside the examples of marketing hype, we have a large number of attempts to improve the sound of amplifiers, each attempting to address a hypothesized flaw in the performance. Audiophiles have voted on the various designs with their pocketbooks, and products go down in history as classics or are forgotten. The used market speaks eloquently: Marantz 9's command a high price, while Dyna 120's are largely unwanted."
Be careful not to confuse global and local feedback. The comment:

"Feedback circuits have been with us since the 1920s and we are now just elliminating this basic design feature in modern amps and preamps"

is quite incorrect. There are designs with no global (aka "overall") feedback which does have some benefits - and thus some disadvantages too - but they definately still use feedback in the circuit. It is just applied around the transistors instead of around the circuit.

The marketing gurus have done a great job of convincing audiophiles that zero feedback is the way to go but no functional amplifier design actually exists without any feedback.
Aball, the Ayre products have no local or global feedback. That was my point. They now do exist and I don' believe Ayre will be alone for long. If you don't believe me go to Ayre.com (no I'm not an Ayre rep but I do own a K1xe.)

Ayre has zero feedback designs in their video circuits also.
The Ayre amps use local feedback, sometimes called degeneration, and no loop feedback.

The Stasis amps had no feedback from the speaker output to the signal input, but instead had feedback loops applied to "building blocks".

The new generation of so-called "digital" amps (the self-oscillating varieties) do have overall feedback loops from the output to the modulator. Some sense current, some voltage, some both.
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.
SS uses feedback to stabilise the circuit. In the more exotic designs this f/back is local. OTOH tube circuits can be stabilised w/out any type of feedback (even there it's not plain sailing -- it requires effort).

Objective measurements: let me insist that measurements are useful and can be reliable. Depends what you're measuring FOR, i.e. it's not always for sound reproduction.

If for marketing, then you go one way. If for guessing the precision of amplified signal, you go another way (i.e. checking into amp-speaker interface matters, et alia).
A Norwegian called R. Lian had looked extensively into this matter. To a certain extend so have we all: if I'm to check for amp performance, WHAT (and how) should I measure?? Cheers
How do you figure it has to do with stabilising?? Stabilising what?

And which way do you use for marketing, and which for precision? By precision, do you mean THD numbers of 0.00001%? Seems to me that is done for marketing, as the amps in the mid-70s using that approach sounded rotten.

Some of us design it so that it sounds best to us. Whether others agree or like it is another matter. ("Some" being most every amp designer that I know.)
Keis, you opened Pandora's box here & I expect there to be some heated debate! :-) This topic can degenerate into a "religious" battle.

Anyway to answer a few points:
Jameswei - nice cut & paste from your Aleph owner's manual. Show that there are so many diff ways to say the same thing & that there doesn't seem to be any consistency in the audio industry. They use whatever terminology that sells the most equipment!
Search thru that manual (I've read the X series manuals on-line & I know that it exists in there FOR SURE) & see if you can find this quote from Nelson Pass: "I ask anyone to show me an electronic circuit & I will show them the feedback path".

Which leads me, nicely, into my next point:
Keis: you are DEAD wrong when you say that Ayre circuits have zero feedback! Sorry to be so blunt but this topic has been dealt w/ here on Audiogon N number of times. Just search the archives! Member Aball nailed it quite well - "The marketing gurus have done a great job of convincing audiophiles that zero feedback is the way to go but no functional amplifier design actually exists without any feedback".
Precisely correct!
This is also corroborated by member Gregm post about stabilizing.
Ar_t asks "stabilizing what?" Stabilizing the transistor bias point. This is of utmost importance - the bias point. Shift the bias point puts a transistor in Class-A or class-AB or Class-B or Class-C or Class-D, etc. When designing any electronic circuit, be it vacuum tube or s.s. - the designer spends 70% or more of their time ensuring that the bias point of the device(s) is(are) correct for the intended application & that the device will remain in class-A/AB/B/C/D, etc over the entire signal excursion. If the bias point is not stabilized, the device drifts with input signal (called signal level distortion, which is a really bad thing) & you can hear it.

No electronic circuit on Planet Earth exists w/o negative feedback! however small the negative feedback, it's always there. To that effect, the Ayre circuits probably use local feedback either around an individual device or around a small cluster of devices. I don't know which but it's got to be one of these if they are claiming "zero feedback".

Whoever came up w/ IM distortion tests for audio amps must have done it to get good test-bench performance numbers 'cuz it's generally know that THD numbers & sonic performance have little or no correlation today. large amounts of negative feedback gave excellent test-bench performance but the amp sounded like sh$$ in a home stereo system. Amps having negative feedback usually lack macro-dynamics & they usually have a flat (2-D) soundstage. That's because the negative feedback loop allows only so much excursion of the transistor bias point before limiting it. The bias point is never allowed to leave a certain region (a very tight region if feedback factor is very high & a looser region if the feedback factor is low[er]) thus the transistor doesn't distort much & THD measures very well!

The reason for inserting negative feedback is that there was a very cool device that was invented in 1948 called the BJT! After several years of development & use in other areas, it was "ripe" for use in audio in the late 1960s & early 1970s. It was supposed to be the next revolution & was supposed to replace the venerable vacuum tube. Like sh$$!!!
The vacuum tube is a device whose operation can be described by oridinary physics (the stuff you learnt in high school) & from these equations you see that gain is a linear function of current & other physical parameters of the tube.
OTOH, the semiconductor transistor is a square-law (MOS) or an exponential law (BJT) device! Thus, the distortions produced by s.s. devices is totally diff from that produced by a linear amplification device (vacuum tube). There is much more odd order harmonic content in s.s. amplified music than there is in vacuum tube amplified music. Plus, owing to the non-linear relationship between gain & physical parameters of the s.s. devices, this distortion rises more rapidly than it does for a tube circuit. So, how to curtail this distortion?? You guessed it - negative feedback!
This negative feedback is our best friend when we are designing electronics for most all other applications EXCEPT audio because we want to do signal processing that is usually some means to an end other than critical listening.
However, for critical listening, negative feedback is our worst enemy. Makes the amp test & measure excellent but makes it sound like cr**!
It's taken us over 30 years to come to grips w/ this - slowly admitting it w/ each new generation of s.s. gear until we come to year 2004-2005 when manuf are openly advertising zero global feedback. What if there was some real intelligent designer back in the 1970s who realized the "hazardous" effects of negative feedback after a few releases of audio gear that had it & he spoke up against using it, I think that his business would have long gone under even tho he would have been correct!! It's taken us this long.
Today s.s. gear is making some real strides in sounding "real"......................very much like vacuum tube gear always did before it!!! LOL!
The transistor was to replace the vacuum tube - like hell it did! Just like CDs were supposed to replace vinyl!
Vacuum tube circuits still sound the best & any top-notch system has tubes in it. It'll always sound the best to a human ear as long as it remains a linear amplification device.
And, vinyl is more coveted today than it was during it hey-days.

What peeves me the most is that the users in the audio community are not as informed about electronics as they should be esp. if they are pre-disposed to spending large amounts of money on gear. The marketing dudes still seem to rule the roost & they still seem to twsit & turn details to enhance a sale & even brain-wash a user into thinking what THEY want the user to think.
Even more pissing off is when you try to spend some time to teach them, they come back & bite you! This move suggests to me that they want to remain ignorant. Well, ignorance is not a bliss......................it's ignorance!!
Bombaywalla, that solid state most often uses feedback, may not be the perfect solution to a problem. On some level it works. On the other hand these problems are not the only ones that exist in audio amplification as we know it. All designs have problems that have less than perfect solutions. Yet, on some level they work. If a particular solution doesn't work satisfactorly for you, well, then you have options. Some may have measurable issues, that sound good enough for you. Some may have currently immeasurable issues that sound sound poor to you. Fine, but, "sh$$", "sh$$!!!" and "cr**!" is purely subjective and is also fine. To extrapolate that to mean that those who have made different choices than you, is due to ignorance is well, eh, ignorant. BTW, I plead guilty to some level of ignorance, but, I don't think I've been brain washed by the twists & turn of marketing dudes any more than I have been by this most recent post.
Read your post.
My post WAS NOT an attack on any person or persons. LET ME MAKE THAT CLEAR if it was not.
2ndly, I NEVER meant to say that my choices are better than yours (Unsound) or anybody elses. Let me MAKE THAT CLEAR as well, if it wasn't. (I should know better than that!)

You, Unsound, might not have been brain-washed by the marketing dudes, which is probably good for you. My statement was a general one. Most of the people fall for the marketing ploy & you often read posts supporting the marketing propaganda. Of course, there are people who have learnt & know better than to succumb to it.

Indeed what sounds "correct" to me might be/will be different from what sounds correct to someone else. I do find, however, that the longer a person stays in this audio hobby & gets many chances to hear diff gear & make direct comparisons of s.s. vs. tube gear in the same system, the better are his/her chances of realizing the positive attributes of linear amplification devices over non-linear ampl devices. That person might still not want to own tube gear - fine! - but the realization does set in.
This point might have been missed by you?
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.

Maybe the guys who design tube gear do, but I don't hang around with any.

But since this was supposed to be about a "historical look at amps", and feedback...........let me ramble on some about the stuff that I have done.

Mid 80s.......overall feedback, but much less than usual. Maybe 10-20 dB.

Late 80s.....no loop feedback in the voltage gain stage, loop feedback for the outputs. (Similar to Stasis.)

Mid 90s.......no loop feedback anywhere.

Early 00s........"digital" amps. Tons of loop feedback.

Looks like we have come full circle.
Bombaywalla, perhaps the point was missed by me. As far as realization, others may indeed come to the same conclusion and others may not. How ever I think it important to realize that our systems are complex and some attributes of sound that may be particularly pertinent to an individual may be better served due to technical or budgetery needs to use certain components that that may be better served by alternate means of amplification. Just so we undertand each other, I don't take any personal offense by your opinion. I just thought that another perspective was in order. Best Regards.
We may have come full circle but that circle has some waves. For example, one of the more respected SS amps that has received great reviews is the Parasound JC-1's. Well, guess what, it has 30db of GLOBAL feedback! (and to me, it sounds like it---it's soundstaging is relatively 2 deminsional) Global feedback is being used more than you think. Ayre, Theta, some Pass and maybe another or so don't use global feedback. It's still fairly uncommon. These amps have higher distortion values and are generally lighter in the bass than higher FB amps. They act somewhat like a tube amp and clip similarly.
I agree completely with Bombaywalla. Marketers try to influence you with BS but it is up to the individual to educate themselves. All designs have tradeoffs. As one remains in the hobby, your tastes with change---usually with age. There is no doubt in this "Old fart's" mind that no global feedback amps offer a higher level of sonic purity (and notice here I didn't say anything about sounding better---that's for you to decide.) If you listen to voices and instruments, you'll find a higher level of natural sound.
I played sax for 30 years and I can tell you without hesitation that the low feedback (0 GFB) amps sound more realistic (they may not meet the dynamic criteria or impress with a "Balls to the wall" sound some favor.) They are certainly not for everyone. I can also say GOOD tube amps are even closer. They offer that special bloom but not all. Some are noticably soft and bloated. I think the perfect amp would be tube mids and highs with SS bass.
I always thought the "High end" was about accurate portrayal of a source. If it's not and it's about "Good sound" then I need to get out because it all becomes a moot point. There's a huge difference between what offers good sound and what is an accurate portrayal of the recorded venue. Some stuff just doesn't sound good and shouldn't sound good.
So much equipment is offered because of the tastes of the individual. However, some of it is wrong and a little of it is right. It does take years to figure a lot of this out. I've been at it 35+ years and it is amazing how my tastes have changed since the beginning.
BUT---to each his own! Everyone knows what they like.
With all that said, I will add one last thing. It is my held belief that some of the differences in sound of no feedback and feedback amps is do to time domain distortions created by out of phase global feedback---but this is a discussion for another day.
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.
THis is fun. Bombaywalla you need to quit SHOUTING. I was not DEAD WRONG. I was relating what Ayre says. I hold no religious conviction. I live 50 miles from the Ayre factory in Boulder CO and my comments were taken from my discussions with engineering and technical staff members at their factory, not just their marketing hype. I'm not saying your wrong in your opinions which if you read my original post showed the problems of negative feedback. Some reading this post might think this is all a mine-is- bigger-then-yours arguement so I am going to explain the non-circuit design reason for feedback being good and bad..
Feedback is almost always NEGATIVE feedback. Positive feedback is used for oscilators. YOu don't want a amplifier to be an oscillator...its hard on speakers and your ears. Negative feedback takes a bit of the amps output and applies it to the input but inverted (negative sign) This tends to damp oscilations and cancel output errors.
The real problem with feedback is the amp or even the transistor have a non zero propagation time delay. Given the perfect amp a wire with gain the wire itself delays the signal. (Signals do not travel at the speed of light in electronics. The signal travels at just a small fraction of the speed of light, the drift current speed.) All circuitry including the bare wire, take a finit amount of time for a signal presented at its input to reach its output. Feedback circuits regardless of their design therefore are adding an "old" signal that has propagated through the device to the new signal at the device input. For a steady state signal that's not a problem but for music even a few miliseconds of delay mean the feedback signal applied doesnt match the new input signal. Global feedback has this problem in a major way because there is significant delay accumulated through all the amps circuitry. Local feedback would just have to cope with the delay in one FET or transistor. TIM as I mentioned in the first message of this thread is sensitive to this time delay distortion.
By the way there are lots of electronics without feedback. But we are talking about amplifier circuits here so the arguement is whether all or most use negative feedback which is good for stability and bad for music.
Ayre does not use any loop feedback. You don't have to ask them............

As for the sound of amps without global feedback: yes.

The amps we made in the mid-90s (where I got rid of the feedback loop in the output stage) really opened things up. It did sound much more lifelike, although it did lack that certain "punch" that is needed to make a product marketable to a wider market. Fortunately, we were able to find the small segment of the market that wanted something else in their amps.

As for the "closed-in" sound. Yes, that seems to be a direct function of lood feedback. I am not going to try to claim that I know why, but I know that is is true for conventional amps. I can verfiy this by taking the average amps, and lowering the loop gain. This can easily be done by placing a resistor from the collector to the base in the VAS (voltage amplification stage). Yes, the same place that you will find a Miller compensation cap. As the loop gain is lowered, the soundstage opens up, things become more lifelike.

And the bottom end and impact drops off............

(Modding amps this way lead to me to think "Why am I working on the other guy's stuff, and trying to fix the obvious problems? I can do better from scratch." So, I did. 20 years later I wonder what the hell I was thinking.)

BTW........I know from my discussions from those 2 guys who design C-J gear, that they spend a lot of time carefully changing loop gain, to where things just fall into place. Too much, or too little, things don't sound the way they like.

Now.......the obvious question:

Class D had lots of feedback. Yep. Bottom end and punch, right? Yep.

Closed in soundstage?

Nope. Don't ask. I will be the first to admit I have no clue why.
I was not shouting! I used capital letters to emphasize a word/words. In an all-lower case post, it seemed reasonable to use capital letters to stand out. Sorry that you thought I was shouting.

I re-read you orig. post - certainly appears that you succumbed to Ayre's marketing hype. There was no way to know otherwise. Your latest post suggests otherwise. Wish that you had written the words in your latest post in your original one!

Re. feedback in bias ckts - yes, I agree that you do not need to used feedback to design a bias circuit. That's *not* what I meant to say! I re-read my orig post & I cut & paste from there:
"That's because the negative feedback loop allows only so much excursion of the transistor bias point before limiting it".
what I was trying to say was the transistor's bias point's movement along the load line. That's the excursion I'm talking about. Not the static/DC bias point, which is what I think Ar_t & Herman are talking about.
Negative feedback curtails the effective load line movement keeping the transistor in it's linear region of operation. Just trying to clarify my point.

True, today's class-D power amps of tons of negative feedback. I'm wondering (without actually having studied a class-D amp in any details) if today's negative feedback is diff from yester year's negative feedback OR if feedback is feedback is feedback?
In yester year's feedback, as Keis pointed out, the output signal is fedback to some point in along the forward gain path. So, this feedback is directly messing w/ the wanted output signal & botching up the sound.
Today's class-D power amps seem to be using a high-speed switching power supply that makes an attempt to follow the envelope of the music signal while the transistors are merely turned on/off at that same rate. By making the switching speed 64X or 128X or 256X 20KHz, the power supply is able to quantize the music envelope quite well.
There must be an error signal generated to ensure that the power supply is correctly tracking the envelope. It *appears* that the fedback signal is not directly in the music signal path (maybe I'm wrong & it is?!)? Maybe that's why these digital amps (despite the tons of feedback) sound much better than amps of the prev many generations? (I have a friend who seems to love his Rowland 201 mono blocks more than his prev Rowland Model 2).
Technically, you guys are way over my head. I am just a consumer. It seems you techies agree on most of everything. I'm glad that class D has finally made it into this amp history exploration thread.

I do not agree Class D amp's performance value depends on digital power supplies. The best I've heard have analogue power supplies.
Hi Ar_t
reading your post suggests very clearly that you are a manufacturer of audio equipment. However, you did not make this disclaimer. We love to hear from you audio industry insiders but we also would appreciate if you would be open about it & let us know that you are directly involved.
AudioAsylum forces this in a certain way by putting "(M)" after the moniker (sure one could lie but it wouldn't go over well when one is caught). We don't have that system here on Audiogon & so we rely on the honour system. So, if could please honour the honour system, we'd appreciate it. Thanks!
I registered as a manufacturer, and I initially thought that would have been sufficient. On my profile, there is no such indication, so any advice on how to better identify my connection would be welcome.

As Ross Perot would say: "I'm all ears."
Since I am doing my PhD on Class D amplifiers, I thought I would weigh in on the feedback discussion of this topology. In general, there are two feedback loops - take that! haha. It depends on the complexity of the control system but often there is a current feedback and a voltage feedback. The tracking error is controlled via both methods since the modulation frequency varies with the input (usually the case for audio class D amps) and thus makes regulating the gain tougher (done with a comparator). Having two feedback loops allows better tracking of the signal since the switches are more-or-less decoupled from the signal - unlike a linear amp. You can even go more complex and have feedforward loops that will compensate for changes in input line voltages thereby creating built-in power conditioning.

I am leaving out a lot of details because there are zillions of topologies and I am not familiar with all of them. I mainly focus on Pentium power supplies which are have even higher switching frequencies than audio amp topologies and even then, timing error is really not that bad. What I dislike about Class D audio amps is the fact that the signal is quantized...and there isn't any real way around that - and has a greater impact than feedback delays on the signal integrity. The CD doing it is enough for me. I have however read the CI white papers and have attend talks by the Philips engineers that created the basis of many of today's audio amp Class D circuits and they undoubtably work.

The effect of feedback in audio will continue to be a debate until we have better equipment to measure the differences (or discover new variables) that, apparently, only our ears know about today. One thing for sure is that less feedback is used in linear amplifiers today than was used in the 80s mainly because transistors have gotten much better - which is also part of the reason the sound is much better too...zero global negative feedback is getting more praise than it deserves IMO.

I am not sure we know everything about their circuits but if Ayre uses no local feedback, I won't be buying one of their amps. The bias would have to be reset every month - unless you listen to it everyday in which case you probably wouldn't even notice...
I indeed see that you are a commercial user but that's only when I do a "member lookup".

How about 1 of these suggestions:-
* put a disclaimer in each post you make saying that you belong to such-&-such audio outfit.
(this is tedious, I agree)
* you can sign off each post with you name & audio outfit like, say, Roy Johnson does in his posts: http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1062780030&openusid&zzRoyj&4&5#Royj
* Another example: Ralph Karsten of Atmasphere uses his company's name as his moniker "atmasphere". The name is well-known & it's easy to see from who the post is from.
Only problem with the first 2 suggestions........

I know of some people (on other forums) that make countless useless posts, only so that they can get their name in front of as many people as possible. Again.

I would rather not appear to be one of them.

Not many people are familiar with my company. We keep a low profile.

I think the best approach would be some sort of indicator for commercial posters. Maybe someone who knows the powers that be can suggest that. I do not want to e source ofd confusion, any more than you guys need any.
I see that you are being extremely difficult on a simple matter like this!
I note an extreme resistance on your part to openly cite your affiliation w/ your company thinking that somehow by revealing it we are going to jump on you.
For such a simple matter as this you are making excuses.
Whether you are low profile or not is of no concern to me. What I/we want to know from your posts is that you are an audio industry person & weight your comments accordingly.
And, I see that you are trying to avoid this at all costs.
Something does not seem correct to me! No wonder AudioAsylum designates their dealer sub-forum as "Shady Lane"!

Forget about the other people who "I know of some people (on other forums) that make countless useless posts, only so that they can get their name in front of as many people as possible. Again."

You post sensibly + make very clear your association with the audio industry & you will *not* end up looking those other people & neither will you create any confusion for us. You haven't been long enough on these Audiogon forums - we can handle quite a bit of confusion before we cry "uncle"!

You are *not* coming across clean, Ar_t - something is not right here!
I would have never thought of the Mc amps from the 80's as bad sounding though..
Jsujo, On Mac amps, he means in his opinion to his ears. Millions of people think otherwise and have voted with pocketbooks. At the end of the day it ONLY matters what sounds good to THAT person.
I am not being difficult. I have a long-standing feud with someone that does exactly what I am commenting on. It will just be a matter of time that he finds this through Google, and the feud will spill over to here. So, while it may be beneficial to you, it may give him fodder to slander me even further by using my posts here as ammo.

Trust me.......ok.

Ok.......my company is Analog Research-Technology. We used to have an entire line of electronics, way back when. Way back when we had dealers. The migration to HT put them all out of business, so we went even more underground than usual.

With the recent introduction of the new "self-oscillating" Class D amps (digital amps to some of you............), I decided to gear up and start making 5-channel amps for the HT crowd. Somehow, I have a hard time getting excited about that market, and since we were amazed at how good they sounded, we became sidetracked and made some stereo versions.

One of my customers mentioned here that he has one of amps......well, actually, he has a 5-channel one also......and next thing that you know, my phone is ringing, wanting info, etc. If it were not for that, you never would have heard of me.

Is that help any???

Or do you want:

I live in Texas. I eat really hot peppers and curry. I have a '73 'Cuda hidden away in the garage. I don't drive it much because gas costs too damn much, it rides hard as hell, and I am too damn old to get excited about trying to operate a 4-speed with a clutch that is even stiffer than the suspension.


I don't like the sound of feedback. I guess that you have already figured that out. I tend to build stuff that is full of discrete circuitry. Trying to find ways to add all that extra stuff onto a Class D amp module was a minor challenge. The next ones will have even more discrete circuitry. (Hint: it is either power supply, or input stages.)

Anything else that you may care to ask?
This is not the first time poor Ar_t has had to deal with the issue of his business. Audiogon certainly isn't making it any easier. I don't believe Ar_t is trying to hide the fact, it just becomes redundant after a while. All the same for those not in the know, a heads up might be in order.
Ayre's response. I asked Ayre the following question. Please not this is not hype but a reponse from Ayre's president and chief circuit designer.

My question........
> I’ve been corresponding with other
> audiophiles on Audiogon.com in the discussion forum under
> amps/preamps. Lots of discussion and disbelief about whether Ayre
> products are really zero feedback. May say no global feedback but its
> not possible to build an amp or pre without some local feedback and
> that Ayre’s claim is just a sales pitch. The thread is my (Keis)
> thread titled amp history.

Charles Hansen's reply

I briefly read through this thread, and this is basically an argument about semantics. (You will find that the people that argue about topic this generally have an ax to grind.)

The problem stems from the fact that there is no master dictionary that has a list of agreed-upon terms for circuits. So if someone calls a circuit a "current conveyor" or a "transconductance amplifier" or a whatever, you can probably find people that want to argue about the definition of those terms. These arguments can never be settled because there is no master dictionary that can be referred to.

The way Ayre uses the term "zero feedback" is in the context of a complete amplifier circuit and are completely consistent with the generally accepted terminology as used by major semiconductor manufacturers and experienced designers around the world (and not just in the field of audio). For example:

From the Burr-Brown data sheet for the BUF600: "The BUF600 and BUF601 are 3- stage open-loop buffer amplifiers consisting of complementary emitter followers with a symmetrical class AB Darlington output stage.... The amplifiers use no feedback, so their low-frequency gain is slightly less than unity and somewhat dependent on loading." (emphasis mine)

From the Maxim data sheet for the MAX4200: "The MAX4200–MAX4205 are ultra- high-speed, open loop buffers....Since these devices operate without negative feedback, there is no loop gain to transform the input impedance upward, as in closed-loop buffers." (emphasis mine)

Words are used to communicate ideas. (Remember that the map is not the territory.) The circuits in the Ayre products operate differently from the circuits used in other audio products. (This is obvious because they *sound* different than amplifiers that use feedback.) We use the term "zero feedback" to describe these circuits because this is the most accurate, generally accepted way to describe them.

If someone wants to argue about the definition of "feedback", they are welcome to. However, you will probably notice that those people have quite a bit less experience with circuit design than the engineers at Burr-Brown and Maxim.

Hope this helps,
Charles Hansen
Back to a subject touched by Bombay & Bigtee:
I always thought the "High end" was about accurate portrayal of a source. If it's not and it's about "Good sound" then I need to get out because it all becomes a moot point.
I agree -- yet, much of the time we see "pleasing" prime over "accurate". I.e. many audiophiles are searching for the most pleasing to the ears.
Case in point: lately we sat down with friends to audition equipment. As usual, the commentary edged upon "correct", "better", "more realistic" -- all relating to "accurate" rather than pleasing.
The music was electronic w/ some a'phile "girl with double-bass" thrown in. Quality of recording notwithstanding, how are we to gauge "accurate" or "precise" with electronic music??? A violin, say, (whether one listens to classical or not) offers some easy real-life reference -- as do many other sounds -- but electronic, or even electrical instruments???
Thanks for posting the response from Charlie Hansen. His mostly detailed post does clarify his intentions & it is exactly what I envisioned his circuits to be - use mostly or only local negative feedback.

He is correct in writing that there is no standardized terminology for describing circuits. Often a circuit topology will acquire a certain name if it is used often enough & over a period of time. If one is in the business of creating electronic circuits, then, it is generally understood that 'feedback' pertains to 'negative feedback' unless otherwise stated & also that it pertains to some degree of 'global feedback' - either around the entire ckt or part of it. It is tacitly understood, once you graduate from EE school, that no device works w/o local feedback. so, if you are in the biz of making ckts, why even bother talking about it 'cuz it's a for-gone concl that it's omni-present.
HOWEVER, the audio market does not consist of merely EEs - it's got all walks of life. So, is it OK to assume that these people, non EEs, will know (& even care) about "generally accepted terminology as used by major semiconductor manufacturers and experienced designers around the world"? Under my breath I say "bullshit! they won't".
So........here you are, a well-known audio brand, selling seemingly good sounding equipment to many, many non-EEs BUT.........using "generally accepted terminology as used by major semiconductor manufacturers and experienced designers around the world"!!!
IMHO, the manuf (& I'm *not* singling out any 1 in particular 'cuz I feel that they are all guilty of this) is doing the minimum possible to enlighten the public about his/her product on his/her website. Hot issues within the audio community (& global negative feedback is one of them) are high-lighted in the marketing text almost the same way it would have been written in a technical data sheet.
Hell, the techincal data sheet is read by trained eyes (in the art of circuit design) while the marketing text is read by mostly untrained eyes!
It boils down to what is convenient to the manuf in the sale of the product. If you end up being less educated about, it's probably better for the manuf.
You fell for his "zero feedback" marketing stuff & really believed that he had no feedback in his circuits. If you had not asked for a more detailed explanation from the manuf, none would have been offered!
THAT IS my issue & is what I tried to highlight in my original post - Don't just eat what they give you hook, line & sinker. Ask for a more detailed explanation. They do not need to reveal any trade secrets or proprietary info for you to better understand their product & its benefits to you.
So, I'd like to ask - in this audio market, who is serving who? Is the manuf serving us the consumer or is the consumer serving the manuf?
I think, it's a bit of both - it has to be a closed loop (i.e. there has to be negative feedback between manuf & consumer. I'm almost laughing as I write this but I'm quite serious) system in that the manuf makes product to sell to the consumer so his/her tastes must be taken into account + the consumer should be able to feedback his/her preferences to the manuf & have the products evolve. However, in audio, where electronics is used to perform the signal processing, the consumer has to learn the appropriate language to have a meaningful conversation w/ manuf. To that effect, the manuf has a vested interest to teach the consumer some aspects of electronics. If that doesn't happen, the loop opens & you begin to get cr** electronics entering the market & marketing types taking ownership telling you what you should have & what you shouldn't have.
From a EE perspective I agree w/ Charlie but from a strictly consumer point of view I do not.
I have no ax to grind as I'm strictly a consumer & I'm free to choose which ever brand I like.
I might have less experience than the Burr-Brown engineers but the repetoire is fairly diversified.
(BTW, I've heard Ayre designs on several occasions & I have always liked Charles Hansen's designs tho' I have never owned one myself. I have no quarrels w/ Ayre or Charlie Hansen - just for the record. Ayre appears repeatedly in this post simply because you posted something about their latest product offering & for no other reason).

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.
Do you guys have any idea how hard it is to NOT use any (local) feedback, anywhere?

Thought so.........
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?
I will defend Ayre in two regards on this issue. First off he has tried to educate us all. In Stereophiles Ayre Factory Tour article Hansen discusses zero feedback at length.

On a separate note I guess I am guilty of emphasizing the wrong thing. My real reason for starting the thread in the first place was to point out that amp circuits have been getting better and better and I believe one major reason is our understanding of what feedback does to sound. I think the argueements have been enlightening but I'd summerize them as an arguement about whether zero feedback is local or global and whether it's evolutionary or revolutionary in terms of a company's claim to having a unique design.

I think Ayre equipment sounds wonderful and jumped to a conclusion about it being the next evolutionary rung on the ladder to sonic perfection.
I know you aren't trying to argue. But it does get frustrating to us technical types when it seems like we speak, but no one listens. I know........maybe we don't put it in terms the layman can grasp. But all too often the layman's response comes across to us as it is us who is the one that "no capisce".

OK.......let us look at some very elementary building blocks, and you will understand what I mean. I hope.

Almost all SS amps use emitter-follower outputs. Even ones that use what is called a "complementary feedback pair", like Rowland and Threshold/Forte used in the 80s, still have an emitter follower at its core.

The emitter follower, but its very nature, has 100% local, degenerative feedback. Simple as that. Followers are all over the place in a typical amp. Even ones with ICs, use lots of them. A very handy building block. Sometimes it used to amplify current (output stage), sometimes to isolate stages or even just to shift voltage levels. So, any circuit with one in it anywhere has feedback.

Obviously.......it is local, not loop. That is the point that the guy from Ayre was trying to make.

Another example:

Take a single 1 transistor circuit......make an amplifier with it. Well, to do so, you need to stick a resistor in the emitter leg in order to bias it on. Guess what........more local feedback.

You say that you don't want feedback there.......ok......here are your options:

Take the resistor out. Connect the emitter to ground. Great, now you have something that won't bias on in a linear manner. You have what is called a Class C amp. Great for RF, useless for audio.

Bypass the resistor with a capacitor. Fine, except that it won't look like 0 ohms at all (or really any) frequency. So, there will always be a small amount of local feedback.

Come up with some bias scheme that allows the emitter to be at ground wrt AC, but not DC. You need 2 supplies......and what eventually ends up is you take the easy way out and make a differential amplifier. Ok......great.....now you have done away with the local feedback, but to make something functional, you have to apply loop feedback if want it to incorporate it into an amp design. Or apply local degeneration to make it work without overall loop feedback.

Now.....if you really want to get confused.......get a couple of amp designers with very different views and ask them if they prefer voltage feedback or current feedback. One guy will claim that current feedback is a made up term, that what the other guy calls current feedback is really voltage feedback. No, the other will claim that current feedback does exist, and that it is something that is used in conjunction with a certain topology that is characterised by bandwidth that does not change with gain......etc., blah, blah........enough squabbling to drive even me mad.

In that case, I would have to side with the layman and tell both of them to shut up. But I would not suggest that either try to run for political office. If they ran against each other, we would have to find some way to void the election, since one would have to win.

(Amps using "current feedback" have been made. Analog Devices has a paper called the "Alexander Amplifier". You can find it on their site. Rowland made an amp using that scheme.......Model 8(?), maybe. I made a CD player that had a current feedback circuit inside. The dealers hated it, and I almost lost all of them.....a story for some other day.)

But back to the original subject.......historical look at amps. You would need to include "current feedback" types to have a complete perspective. Another subject of discussion could be bandwidth........how much does an amp really need, and does it help? Some amps......Spectral........have tons of bandwidth.......and some will say that they sound bright as a result.

(BTW.....if you do build amps with overall loop feedback, the more bandwidth, the better. It just gets very hard to increase it beyond a certain point. No sure how Spectral does what they do, except the designer is a sharp dude.)

I bring this part up because I have found that Class D amps, with the same bandwidth as a typical SS amp, will sound bright to almost all listeners. You have to lower the bandwidth to make it "sound" the same. Honestly, I am not sure why. On one hand it is interesting, the other frustrating as hell for an amp designer.
The next rung??? Don't know that I would characterise it that way. I would say it is just another branch on the tree, and trust me, I have climbed all over it.

OK.....at this point someone scratches their head and goes "Doesn't this guy make digital amps now? Did he abandon "zero feedback?"

Yes, and no.

Making "digital" as a way to segue into the HT market. "Zero feedback" designs are not going to go far there. At least not for amps. Small, light, efficient, powerful, lots of "oomph"; Class D is the way to go.

As for anything else that we make.......still has "zero feedback" inside. In fact, a lot of "zero feedback" thinking in the auxillary parts of the "digital" amps.

One thing that has been missing so far is the role of transisor evolution in all of this. Most of it is way too technical, but there are a few major developments in transistor design that have led us to where are. And a few side branches, like MOSFETS. Not only do they allow for kilowatt Class D amps, but different processes led to amps like the Acoustat line (and others) which found favor with electrostat owners. The Trans Nova series comes to mind.
Just one comment re. Ar_t's prev post:
"You say that you don't want feedback there.......ok......here are your options:

Take the resistor out. Connect the emitter to ground. Great, now you have something that won't bias on in a linear manner. You have what is called a Class C amp. Great for RF, useless for audio."

even when that resistor in the emitter leg is removed there IS local feedback in that device. It's the intrinsic emitter resistance called "little re" + any package lead resistance. The issue is that it's really very small & of no practical consequence i.e. not large enough to bias the transistor in its linear region.

IMHO, there are several reasons that amps s.s. amps (or sand amps as I like to call them) sound better:
* firstly, the transistor devices themselves have gotten radically better than they were 5, 10, 20 years ago. Ar_t did cite this as well. The distortions from these newer devices is much lower than what it used to be so amps using them can play louder for longer. Tighter manuf tolerances also makes it easier to match them. Many manuf like Pass, Rowland, Symphonic Line & a whole host of others use several of them in parallel for high current outputs.
* 2ndly, the s.s. amp designers themselves have grown in their skill set to design these amps. I bet that Ar_t can testify to this! :-) It takes a while & several generations of products to understand how the semiconductor devices *really* works & how to coax the best from it.
You can see this in the realm of CD music too. When was the CD 1st introduced? If anyone of you has a CD, say, from 1985 or so & you compare it to a recent re-issue of the same music, you can tell that the people re-mastering the CD have a vastly better understanding of the whole process.
The growth of the individual amp designers to skillfully use the semiconducting device to its inherent strengths is what I consider a more important reason for better sounding amps. You can have the best BJT or MOS but if the implementation is poor, you'll still get bad sound.
* 3rdly, *more* s.s. amp designers understand & believe within themselves that there is no or very little correlation between THD, TIM measurements & sonic character of the amp. Thus, applying large amounts of negative feedback to ensure that the amp measured superbly in some reviewer's lab is not a top priority any more. There have always been s.s. amp designers thru the yester years that believed in less global feedback & we consumers have voted w/ our money by owning these products.
* 4thly, there is a lot of admission from the s.s. amp design camp that the vacuum tube, tho old & to some unreliable, was & remains a really fantastic amplification device for audio where lack of harmonic & inter-mod distortion is king. You'll often find people seeking a "tubey sounding s.s. amp" - the forums are littered w/ such posts. Why are these people seeking such an amp?
Also, note that some of the best s.s. amps sound like a good tube amp. Anytime I have ever tried to find an adjective to describe a good sounding s.s. amp I've mostly come up with "sounds like a tube amp!". Fancy that!!! I wrote this in my orig post (which Unsound picked up on). The vacuum tube might be a real old fart but it remains the most linear amplification device that we can work w/ practically. Any world class system will have it somewhere in the chain. Real good s.s. amps only approach that quality of sound.
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?
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.

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.
i'm afraid that I'm unable to make any concrete comments on class-D amps. i've not heard one yet.
Vince......don't start again, ok?? I have too much research on this subject to be off my rocker. Remember, I have building stuff commercially for around 20 years. I am not some young upstart just getting his feet wet. We all know that Henry's amps sound just wonderful on your Apogees. Great. We are happy for you. I doubt that you have dragged a half dozen or so amps, of varying topologies, to as many different systems as we have. Trends emerge........

Well, I can not speak for C. Hansen. I do not know his motivations. I can only surmise that his inclusion of the Maxim part was to show that there really isn't any dictionary accepted definition.

As for how I know.........

At one time, they had the schematic silk-screened on the inside of the lid. I listened to it. Confirmed what needed to be confirmed. Besides, you see someone at CES or RMAF......you talk; you know someone who used to work there; you seem to have a lot of dealers in common; lots of ways that stuff gets around. None of us design in a vacuum, and secrets have a way of not staying secret. I had a dealer in Chicago once call me and 'fess up that he may have been the one who gave Mark Brasfield the idea for a transimpedance amp as an I/V stage. After he heard mine, and I 'splained how it worked. I dunno.......maybe Mark came up with the same idea on his own, just 6 moths later.

Is that both clear and evasive enough? I'm thinking of running for office! Actually, I am fixin' to head out into the hot Texas sun, and bake what little is left of my brain. If I don't return, you will know why. Just look for my dry, withered body..........

Listening to all those bright Class D amps has made me bonkers to start with. It's a joke.......you're supposed to laugh, ok. It won't kill you. Try it.

Maybe when I return tonight, I can relate a story about a buddy in the speaker business, who had a hard time deciding on the tweeter level on his new creation. Seems that he had to change it 0.25 dB depending on what brand of SS amp he listened to it on. And this was before Class D! Wonder how is faring now........I'll have to send him one and drive him over the edge! (Yeah, another attempt at humour.)
Muralman1, I have not done any serious listenig of digital amps, but, it is a characteristic that I have noticed on occassion. More importantly I think that Ar_t is suggesting that many if not most digital amps seem to take measures that allieveate this phenomonon. As Ar_t has made clear, he currently manufactures digital amps. I seriously doubt he would make and then advertise a product as having a trait that is the most damning in the business. Furthermore he has opined that this particular tratit in this particular technology seems to be more system dependent than usual. I don't have the experience as to whether digital amps have become the new standard bearer presently or if they will in the future. I am hopefull that they will. At the very least they show great promise.
Well, Unsound, that is exactly what Ar_t is doing, "Expounding on a trait that is most damning," and it confounds me.

I am not talking about class D amps on my speakers. Sajran reviewed the amp with Gallo speakers, for gosh sakes.There are plenty of speakers, of all types, being run by class D. I also am not talking about the proliferation of module in a box amps.

If Ar_t can build an amp like Henry's, since nothing is a secret, then why doesn't he? His commercial products do not reflect the same class D amp philosophy, as Henry's.

Another thing is, I don't think a commercial entity should prop himself up as the high mucky muck of amps, holding a thumbs up, or down on his competitors, unheard. I don't see any other amp builder doing that.

Do a search on Rowland 302, or H2O, and see if the word "bright" is a common tag. I can't find any.