Class D at low volume

03-03-15: Kijanki
Class D (PWM) can be easily constructed without negative feedback at all.

I wasn't aware of this. how does the PWM system know that it's tracking the input if there is no feedback? Can you please point me to some references?

but current modules resemble more of Delta-Sigma A/D converters.

so, in current class-D modules, they noise shape the input music signal to move the noise out of band & the resulting pulse train from the delta-sigma block is used to drive the power switches? I suppose that delta-sigma technology makes sense for audio because the ear is very sensitive to distortion & noise? what frequency is the delta-sigma clocked at - also 1MHz like the new gen SMPSs? thanks.

03-04-15: Mitch2
I think the audio SMPS has finally developed to a point where it is has a low enough noise floor & can handle large currents in a compact size.
The Acoustic Imagery Atsahs use the Hypex/Ncore SMPS 1200 switch mode power supply and in my system they sound at least as good as similar powered, high quality solid state, Class A and Class A/B amps I have owned. They are dead nuts quiet and I notice absolutely no listener fatigue. When I listen, I sometimes wonder why some are trying to use linear supplies with the NC1200 amp module, when it sounds this good with the stock SMPS.

Someone i talked to last year about class-D amps & this someone I have some respect for informed me that the best part of the Hypex/NCore modules was their SMPS.... ;-)

SMPS have line and load regulation. Amplifier with SMPS keeps composure during power peaks since voltage does not sag, like it often happens in linear power supplies.

linear power supplies have line & load regulation as well, don't they? one could argue that it might be easier to design better line & load reg for SMPS compared to linear power supplies but i believe that a good linear power supply can also have very good line & load reg. I think a few such supplies are available for Empirical Audio gear from a Paul XXX (I'm forgetting his name) & I also see a good linear power supply in Sanders Magtech & ESL power amps.
Yes, traditionally the manuf of class-A, AB amps have not bothered to put regulated supplies hence the poor line & load reg.

Bombaywalla, Linear power supplies have regulation in preamps, DACs etc. but MOST of power amps are unregulated because of the amount of heat dissipated in power supply (and loss of efficiency).

AFAIK older REF1000 and newer REF1000M use the same module, but REF1000M has additional separate power factor correction module.

Look at fig. 2 showing how PWM signal can be obtained by just using linear ramp. Icepower modulator uses sinewave, has two feedbacks etc. - but only for improvements.
http://en.wikipedia.org/wiki/Pulse-width_modulation

One limiting factor in class D is bandwidth. Latest Rowland class AB amps have bandwidth of 350kHz while newest class D model 925 50kHz. My small 102 extends to 60kHz. It is limited by the speed of Mosfets currently available especially high current devices, but it is improving every single year. We definitely cannot hear 350kHz but wide bandwidth prevents phase shift thus preserving harmonics within audible band. My amplifier with bandwidth of 60kHz shifts phase of the 20kHz signal by 20 degree.

The difference in sound, with class D, I observed is that sound is more immediate, faster but at the same time there is less of brightness in high frequencies. Cymbals sound less "splashy" but more brassy with more body. Perhaps it is related to lower TIM distortions or it is just the nature of the beast. It is definitely different but I like it.

03-04-15: Kijanki
Bombaywalla, Linear power supplies have regulation in preamps, DACs etc. but MOST of power amps are unregulated because of the amount of heat dissipated in power supply (and loss of efficiency).

agree. There might a small change happening here - Sanders Sound Systems Magtech amps have a regulated power supply (& I believe Roger Sanders has applied for a patent for that?).

Look at fig. 2 showing how PWM signal can be obtained by just using linear ramp. Icepower modulator uses sinewave, has two feedbacks etc. - but only for improvements.
http://en.wikipedia.org/wiki/Pulse-width_modulation

thanks for this reference, Kijanki I'm quite familiar with class-D PWM using a ramp waveform.
My question was using delta-sigma instead. There might be some info on that same page you sent the link for. I'll read up.

Also had a question re. how class-D PWM can be designed without feedback like you wrote in your initial post. Got any references that I can read?

My amplifier with bandwidth of 60kHz shifts phase of the 20kHz signal by 20 degree.

agree! The "funny" thing here is that I wrote something very similar in another thread but had Almarg state a contrary view stating that it wouldn't be such a big deal given that the hi freq energy is much less at these frequencies. Perhaps that is true - it's true that the there is generally less energy at the high frequencies. So, even tho' your amp produces a 20 deg phase shift at 20KHz, it might matter very little(?).
What I wrote in that thread specifically was

....This also means that since the ARC Ref 150 bandwidth is just 3X (rather than 8X or 10X) the music bandwidth (of 20KHz) one can expect to hear the amp impart its own phase shift onto the higher frequencies of the music. This can manifest itself in a few ways - the highs could sound rolled-off or they could sound warmer or there could be less sparkle/shimmer compared to an amp of higher bandwidth.

.
Almarg's comments to my post:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1423113432&openflup&50&4#50