but many hear some thing not quite right with Class-D in the upper
mids/highs with many speakers, and this is it’s Achilles heel.
Technics
(SE-R1/SU-R1000) claim to get rid of it by making the switching
frequency 3 x higher, 1.5mhz, so that then the output filter is also 3 x
higher and therefore it has little or no influence down into the upper
mids/highs.
Most Class-D’s that have around 600khz switching
frequencies, have output filters that have effects down into the upper
mids and highs.
And yes throwing a **** load of feedback may fix it,
but the best amp "theoretically" would be ones that use no feedback if
it can be done without distortions, or very little or local not global.
Complete rubbish.
I've explained this many times on multiple threads. Its apparent that faced with facts, George is at a loss; his comments are far too inclusive.
The output filter will have an effect on phase shift in the audio band **if the feedback is insufficient**. So if you see a class D amp that has lots of phase shift in the audio band now you know why.
To prevent phase shift you can do it two ways: have loads of bandwidth (that's how we do it in our OTLs, by going well past 100KHz at full power) **OR** by using enough feedback that phase shift is corrected (which is in excess of 35dB).
Some class D amps that switch at 500-600KHz sound great and others do not. Its very much like the difference between a type 45 SET and and 750 watt behemoth push-pull tube amp- they are two different things and sound different as a result. Class D in general is no different in this regard.
The last paragraph is really terrible. You don't 'throw a load of feedback' in an amp to fix the amp, you throw a load of feedback in the amp to *fix the feedback itself*. There is a profound difference; to start with you can't just keep adding feedback as you need gain in the amp to do that and each stage of gain degrades the phase margin of the circuit. So it requires a bit of finesse.
To get the required amount in a class D, you have to resort to self-oscillating circuitry (which is also helpful for keeping noise down as opposed to a zero feedback class D design). As a challenge to George: why not work out the oscillation criteria and get back to us here? If you can't don't feel bad, just allow the lesson to sink in that there is far more to this than you think!