Class D Technology

but nobody talks about phase shifts.

I do and all the time. The ear can't hear phase shift in single notes, but over a spectrum it can hear phase shift in a variety of ways- as a tonality or as a change in the soundstage. The engineering rule of thumb to prevent phase shift is to have bandwidth exceeding the highest frequency to be amplified by about 10 times.

Now that is generally interpreted to be a bandwidth of 200KHz (but might be higher- for example nearly all LPs have bandwidth well past 30KHz in record and in playback; usually this is limited by the source tape or file or the microphones themselves...). However you need the bandwidth in the playback electronics even if the source lacks the bandwidth if you expect to reproduce all the phases correctly! We get that out of our amplifiers but to do that with class D is still a bit of a trick. We are barely getting switching speeds much past 250KHz which means true 200KHz bandwidth isn't there yet.

Now the AES says that if you can get about 2 octaves above the limit of human hearing (80KHz IOW) that you are doing OK for a monitor amplifier. Audiophiles routinely operate gear that has performance in excess of that. And one of the things I don't like about many of the class D amps I've heard has been the lack of speed and spaciousness on the top end. Like mapman, there's no way my hearing is as good now as when I was in my 20s but I still notice this stuff nevertheless.

So if you're going to get that 200KHz response you need a switching speed of around 1 MHz and even then its going to be a bit spotty (2Mhz would be much better). There's no problem getting the converters to run that high. A prototype we're building here in the shop can do conversion at 3MHz easily no worries (the chips we're using have a bandwidth product of about 50MHz). The real problem is the switching speed of the output devices (and whatever junk occurs between the output of the converter and the inputs of the output devices...). If one were to spend the extra cash to get the really high speed stuff then one of the primary advantages of class D goes away real fast: low cost/high profit margin. Really fast, powerful switching transistors aren't cheap and you don't see them in 99% of all class D amps! So to compete against traditional solid state and tube amplifiers in a nutshell Class D has to improve bandwidth, and you don't have to be a bat to appreciate the difference.

This is one reason why class D is still a rising star. Its also why some class D amps are quite diss-worthy and others are not too bad. 

Atmasphere why does 200khz bandwidth matter if there are no sources practically (record or CD res digital) that can even come close to delivering it today?

Seems like a purely theoretical argument.

I agree and have stated on many occasions that higher bandwidth alone is better theoretically all other things aside and Class D can still get much better there.   It certainly cannot hurt  assuming it is done well/right like most things.

But practically I am less sure. I expected my Class D amps to not sound nearly as good as they do given the theory. I’m glad I did not let that stop me from trying because it seems to be a non issue already practically, at least for me.

Thanks.

Well said, atmasphere. One of things that stands out, at least to me, about the H2O Class D amp is that it uses an extremely robust power supply comparable to the great SS/Tube amps.

http://www.6moons.com/audioreviews/h2o2/1.html

http://iceh2oaudio.com/

What prominence does the heft of the power supply play in the sonic performance of Class D amps relative to SS or tube amps?

Best to you atmasphere,
Dave

Power supply is no less important with class D than with other amps.   A lot of vendors focus on optimizing it and it is a common theme among better reviewed and received designs. 

Atmasphere why does 200khz bandwidth matter if there are no sources practically (record or CD res digital) that can even come close to delivering it today?

I did explain this in my post but here it is again:

In order to reproduce phase correctly bandwidth is a requirement. Put another way, if the bandwidth is not there phase shift is the result. You are correct that digital doesn't have bandwidth and that is one of the reasons that the LP sounds better and is still around after all these years of being 'obsolete'. Nevertheless, to prevent phase shift in the amp you need bandwidth even it its not there in the recording. In fact the recording doesn't matter.

To put this another way: phase shift components can be heard to 1/10th of the cutoff frequency. So if you are cutting off at 20KHz there will be phase shift to 2 Khz. 100KHz cutoff means that phase shift will exist down to 10KHz. We older adults don't hear so well at 10KHz so you can sort of get by with 100KHz bandwidth but if you really want to do it right you better have 200KHz so your kids will like the stereo (unless you're the type that just wants them off your lawn...).

What prominence does the heft of the power supply play in the sonic performance of Class D amps relative to SS or tube amps?

The supply should be clean else IMD components related to the noise in the supply and the scan frequency will show up in the output. IMD is pretty audible to the human ear (shows up as brightness and grit or 'grundge'). Since the scan frequency is pretty high the power supply had better be properly bypassed to be effective at the scan frequency! But it can't have any sawtooth going on either. Otherwise the supply has to be able to support the operation of the amp at full power, even though most of the time the supply will see a fraction of the current draw that a traditional amplifier supply would see, so most of the time the power draw from the wall is minimal.

Traditional power supply design or SMPS is irrelevant so long as the supply is quiet (SMPSs have the advantage of being easier to regulate...).