Historical look at amps

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.

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'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

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.