Historical look at amps

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.

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.

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...