Class-D or switching amps, any opinions on??

Magfan: The new NAD M2 ... operates in the digital mode, which I expect to be the real definition of 'digital amp'.

Agreed 100%. My previous post was not worded as precisely as it should have been, to the extent that it implied anything different than your statement quoted above.

The signal path through a digital Class D amp will consist entirely of either digital signals or switched waveforms, from the amp's input to just before the low-pass reconstruction filter which reconstructs an analog waveform at the amp's output.

However, the literature at the Tact site appears to indicate that their amplifiers do exactly that. For instance, see the section on page 8 of the 2150X Manual entitled "How Does It Work."

I know of no other Class D amplifiers besides the Tact's and the NAD which you mentioned which can properly be called digital amplifiers, although there certainly may be a few isolated exceptions that I am not aware of.

Best regards,
-- Al

Hello Kijanki,

May be word "inventor" in not the most accurate - however, as I have illustated with the quote from "50 Greatest A/V Innovations" over last 50 years - John ULrick is attributed for bringing class D into HIGH FIDELITY audio, specifically in conjunction with his Infinity speakers which I believe were considered one of the best in 1970's. I believe that Spectron amplfiiers today are also one of the best among powerful amplifiers but this is besides the point.

All The Best
Rafael

Rafael - Absolutely agree.

Al - I'm not sure what they do to obtain decent resolution. In order to get 16 bit resolution and 20kHz bandwidth with traditional PWM clock has to be 65536x20e3=1.3GHz. It is way to fast and no room to filter out carrier. They might play games with modulating power supply at the same time or creating more states (more Mosfets and more voltages). It is getting very complicated to get real digital amp. Pure digital might be not bad - just look at HDTV.

Pure digital might not be bad? It's not that good- yet. As for HDTV- I saw a demonstration of Sony's analog HDTV at the Chicago CES in 1989. It was amazing. Today's digital HDTV has a ways to go yet. Just look at the lack of low level detail in dark scenes. I also see pixelation in fast moving scenes in HDTV broadcasts, but not in 1080p blu-ray material.

Al - I'm not sure what they do to obtain decent resolution. In order to get 16 bit resolution and 20kHz bandwidth with traditional PWM clock has to be 65536x20e3=1.3GHz. It is way to fast and no room to filter out carrier. They might play games with modulating power supply at the same time or creating more states (more Mosfets and more voltages). It is getting very complicated to get real digital amp. Pure digital might be not bad - just look at HDTV.

That's an excellent question, Kijanki. I'm not particularly knowledgeable in that area, but I believe that what is done to provide adequate resolution is oversampling + noise shaping, conceptually similar to what is done in the so-called 1-bit cd players. I found the following paragraph here:

The PWM resolution is typically much more coarse (than that of the incoming pcm), in order to keep switching frequency above maximum audio sample rates of 192 kHz, whilst limiting MCK frequency. Typical ratios are between 32 (5 bits) and 256 (8 bits), giving a best dynamic range of 50 dB. Clearly, this means that other techniques are required to provide sufficient dynamic range, and the answer is oversampling with noise shaping. This technique essentially allows a time averaging of the PWM pulse widths to be applied, to deliver fractional resolution in the amplitude of the output signal.

The theory behind that is esoteric and also somewhat counter-intuitive, and I'm not sufficiently familiar with it to be able to explain it well, but I believe that is basically the answer. Note that the Tact manual I referenced indicates an 8-bit pwm resolution, within each pulse period (corresponding approximately to the 50db dynamic range mentioned in the quote above); a pulse rate of 384kHz; and a master clock rate of 98MHz.

Best regards,
-- Al