class D - anyone?

[petercom]

Just to add fuel to the thread, as it has gone quiet again, what do we actually call a Class D amplifier? Class D does not necessarily mean digital, it just happened to be the next letter in the alphabet after A, B, C. Class D refers to using a Pulse Width Modulated (PWM) signal where the envelope of the width of the pulses follows the signal to be amplified. The high frequency PWM output is then filtered leaving the required signal to go to the loudspeakers.

There is nothing to say that Class D has to have a digital input or that it is really digital at all. All that it means is that the amplifying devices switch between one voltage state and another and stay on, or off, for a time that varies according to the desired signal. As such you can apply analogue feedback from output to input or, as is more common these days, digital feedback where the output is re-sampled, DSP applied, and then the input corrected accordingly. Either way the feedback needed is considerable.

If you want a small, highly efficient, low heat amplifier then PWM can be combined with a switching power supply. The problem then is that you have two sources of high frequency interference to contend with - the muck coming out of the switching power supply and the remains of the pulse waveform coming out of the outputs.

To get an idea of what this looks like check the following link:
http://stereophile.com/solidpoweramp...ro/index4.html

This is one of the better PWM amps where the designer has sought the goal, as epitomised in his analogue designs, of 'vanishingly low' distortion. Whether low distortion is a better goal than high frequency interference I'll leave you to decide.

The interesting comment in the listening notes in this review is the emphasis on 'flat' two dimensional sound. To that I would add 'musically uninvolving' too. In fact these type of amps sound to me exactly like good, high feedback, ss amps.

How far removed these are from the valve amps that this site promotes. The distortion of a 300B SE is palpable in terms of both measurement and audibility. Yet I prefer the harmonically related distortion prone signal from a 300B SE anyday.

What we have to consider IMO is that the level of distortion, and its harmonic relation to the music, is still low enough to be hardly detectable. We have become so caught up in the 'lower is better' spec war that we have forgotten that it is really difficult to hear harmonically related distortion at levels of around 1%. Our forebears knew this in the '40s and '50s but, before subjective reviewing, hi-fi mags went by measurement as a measure of goodness.

Funnily enough there is an article on this in the same issue of Stereophile
http://www.stereophile.com/reference...ard/index.html

Whilst I think Keith misses the point, the message is clear that a relatively high level of harmonically related distortion where the harmonics are naturally graded from the second harmonic downwards is preferable to distortion where odd harmonics predominate. And what it doesn't point out is that non-harmonically related distortion, such as intermodulation, has a pretty nasty effect on the quality of the listening experience.