Historical look at amps
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... |
Ar_t, 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. |
Ar_t, 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! |
