Class "D" amp? I do not have a clue. Do you?

Kij has it correct. The RDon is very low for MOSFETs. Resistance Device on.
This is a big selling point for discrete power devices. less resistance is less power lost, or rather delivered to the load.....whichever you prefer.

Now, on to the full /half bridge argument.
http://www.irf.com/product-info/audio/classdtutorial.pdf

Above is the Internation Rectifier article on 'd' amps. Very good read. But, this is only part 1 of 2.
The full/half bridge configurations are well covered as are some of the rf issues.

QUESTION: how do the 2 different kilowatt Bel Canto amps differ? Don't they both use the B+O ASP module? Is there only a difference in input circuitry or the Zobel?

Here is a link to the B+O ASP site.
http://www.icepower.bang-olufsen.com/files/solutions/icepower500asp.pdf

Looking at the technical specs you will see the power output is specified for 60 seconds.....not FTC continuous. The ASP1000 module has only a 30 second time limit.

I like my 'd' amp. No question about it getting the job done.

Kij, How DID you know my speaker wires were 197 feet long? Just kidding!

As for the PS switching noise at the output, that is removed with the low pass Zobel. However, there are special filters used by the measurement crowd which test equipment. Some problems occur when measuring 'd' amps.

"Switching power supplies generate high frequency noise that is easy to filter out and non audible. For this reason Rowland uses switching supply in Capri preamp and he is very noise conscious guy (he used batteries in preamps). "

Hello Kijanki. By chance I owned Capri before I got my "heavenly" full tube Joule-Electra LA-300ME preamp. As strange as it may sound, this tube amp is more silent then Capri.

Next, I spoke with Simon from Spectron yesterday regarding asserion that high-frequency noise (or signal) is inaudible.
He says that then you emit hi-freq test signal it is indeed inaudible. However, then this hi-freq signal is a part of the mixture with other signals i.e. real music then due to the non-linearity it effects human perception of the music and effect listener fatigue.

He agreed with you, however, that future audio equipment will have only (perfected) switching power supplies even in class A ss amps - which will not have a weight of huge transformers and somewhat less of the heatsinks; the same with high power tube amplifiers.

fascinating topic !

Mike

"QUESTION: how do the 2 different kilowatt Bel Canto amps differ? Don't they both use the B+O ASP module? Is there only a difference in input circuitry or the Zobel?"

ref1000 is stock Icepower module in a nice box.

ref1000m has additional input buffer and power supply circuits. If you open up the box, the 3 separate circuit boards/modules are clearly visible. It is dead quiet and most nonfatiguing.

I have read that ref1000m effectively addresses RF noise concerns in comparison to ref1000.

I have never owned or heard any other Bel Canto amp so I cannot offer an assessment of sound differences. From what I read however, others sem to concur that there are differences.

Mike - Problem with switching power supplies is that most of them are designed to save money and space and not to improve performance. REF1000 for instance in improved version REF1000m has additional power supply with a lot of capacitors on separate board. I believe that with the same resources (cost and space) better PS can be achieved in switching than linear technologies.

I remember that there was an argument that high frequency can modulate audible frequencies on nonlinear (at this range) transducer - tweeter. Argument is valid under condition that tweeter membrane can move at 500kHz - not likely.

Larger Icepowers have a little lower carrier frequency than small ones and therefore lower -3dB frequency (50kHz vs. 60kHz). It is because stronger Mosfets are slower. Wide frequency range is necessary to provide smallest phase shift in audible range. Even with 60kHz amplifier shows about 30deg phase shift over 5Hz-20kHz.

This is first avalanche of class D amps (remember first SS amps?). Next generations will have faster Mosfets, higher frequency range, better power supplies and better noise filtering. I see it as technology of the future.

Magfan - this Zobel, can be improved as well. In Icepower it is simply common mode choke two filtering capacitors and one resistor. This Zobel network is responsible for time/power limitations and available power at higher frequencies. In practice musical power is only few percent of peak power (unless one listens to sine-waves)and high frequency energy is minimal. Should I pay for larger Zobel toroid to allow high frequency power that would make me deaf? Thanks for the link to IRF primer. I read opinion that full bridge sounds like very good tube amp while half bridge sounds more like high quality class AB amp (less tubey).