Class D at low volume

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

Went to class D 12 years ago when replaced my Bryston 3B-ST with Sunfire 300(class T which is same as D) and never looked back.

Bombaywalla, fig. 2 shows exactly how it can be done. For
any input voltage corresponding duty cycle is created.
Feedback would only improve performance. Modulator itself
might be not 100% linear but most likely linearity error is
created by the Mosfet switches. Their positive and negative
slew rates are different and vary a bit with the load
(creating timing errors). Negative feedback improves timing
but also lowers output impedance increased by the presence
of common mode choke. Early D class amps (tripath) had this
choke outside of the feedback and had loading problems with
many speakers. Icepower uses two different feedbacks. One
is from the output of modulator (improving timing), the
other from the speaker output. At least that's what I
remember from Karsten Nielsen doctorate.

Icepower uses single supply. Speaker (thru Zobel network)
is placed in the bridge made of 4 N-channel Mosfets. Bridge
is connected to GND and +Vs. At any given moment speaker is
always connected between +Vs and GND only direction changes.
Mosfets are turned on in pairs diagonally. At 50% duty
cycle average, filtered voltage on the speaker is zero. Half
of supply voltage is always present on the speaker's
terminals and the output cannot be bridged. Hypex used only
two Mosfets switching speaker between GND and +Vs or -Vs.
This allows output to be bridged.

As for the phase shift - 20 deg is a substantial phase shift
but I have no idea how much it (summing of harmonics) is
audible. Obviously Jeff Rowland wants no phase shift,
extending bandwidth to 350kHz.

I've read few times people saying "I know that class D
requires a lot of negative feedback" Now I hear that it
cannot even operate without feedback. How do they know
that??? If anything, class D requires less feedback since
voltage-duty cycle conversion seems to be more linear than
nonlinear bipolar transistors in class AB output stage.

SACD is an example of class D without any feedback.
Incoming 2.8MHz train of pulses is averaged and becomes a
sound.