Hi:
My recent experiences purchasing an audio amp afford some points for reflection on the electronics of such.
I purchased a NAD C325BEE amplifier, basically the cheapest 2-channel integrated amp on the market ($399). One might surmise then that I don't accept the idea that amplifiers "sound different" presuming they are of less than some threshold of distortion considered to be "hi-fi" (let's say about 0.1%) and not operating at clipping.
Now I discover that the amp runs very hot when doing nothing. I recorded temperatures on the top cover over the centrally placed heatsinks of about 47C when the front panel power button was off, and up to 52C when operating (but not producing any meaningful output power, so basically idling).
This seriously bugs me. I have a Kyocera A-710 which is twice the power, and it is barely perceptably warm almost always, but particularly when idle. Granted, the Kyocera has two massive sinks on either side of the chassis, with total area probably 2-4 times EACH the area of the single sink in the NAD used for all four output transistors (two for each channel, 4 total all on one sink).
I am worried that the amp was not correctly adjusted for bias when it was made. Another possibility is that it was designed this way.
I posted in rec.audio.high-end:
"I have personally designed a very high power (480W cont., 650W peak) class-D amp and some single ended motion-control servo amplifiers in the several hundred Watt level. When designed thoughfully, thermal cycling is not a detrimental reliability factor. I'm afraid perhaps the designers erred on the side of greater bias current rather than higher negative feedback and open-loop BW to servo out crossover distortion. While this might make some audiophiles (not myself, I think low distortion can be acheived with low quiescent) happy, it is not ideal from an idle efficiency standpoint. "
To which "BEAR" responded:
BEGIN QUOTE: Secondly, you can not achieve the same results with (as you say) "negative feedback and open-loop BW to servo out crossover distortion" as you can with a higher bias point (there may be exceptions, of course). In general, higher levels of negative fb yield undesireable artifacts in terms of higher order harmonics.
It is certainly a good idea to have a wider than not wide open loop BW, whenever possible. Of course.
Overall better sound comes when each stage in the amplifier is made as linear as possible before applying feedback (if you do at all) and when the details of each stage's operational pros and cons are considered fully. It's not a simple thing, even though there are "standard ways" of designing each stage of an amp. The "standard ways" do not always yield the intended or optimal result.
(Let's ignore for the benefit of this discussion those who would posit that all amplfiers not clipped, and below some threshold of distortion are not audibly different) END QUOTE.
As I implied my limited design experience is more toward motor amps rather than audio. Yet I have read considerable material on the subject over the years. All I really have to go on is the fact that my Kyocera amp is twice the power yet runs extremely cool, and sounds no different than the NAD amp when not at its limits.
What is the view on the fellows assertion that NFB is bad? Even if the THD harmonics are not distributed evenly, what difference does it make if the amp still acheives 0.02% or something like that?
Comments appreciated.
P.S. All this is making me curious to try some experiments sometime, starting with a butt-simple class-B puch pull emitter follower driving a speaker, then followed by an op-amp power booster topology with the op-amp cleaning up the crossover distortion. I wonder if just going that far would be enough to where the distortion is no longer audible.