Digitally controlled class D initially delivered a success in the form of the Tact Millennium. However, by its mere existence this device (and another one of my own making, the "PPDSD" which performs marginally better) proves that obtaining good performance from such a contraption is largely an analog design exercise - a very complicated and expensive one at that. After all, the distortion phenomena that stand in the way between a perfectly formed digital control signal and a perfect analog replica are inherently analog. Similarly,cheaper digital class D’s (such as SonyS-Master and TI’s) go on to show that at practical price and complexity levels, performance is quite abysmal (better than 0.1% THD is unusual and be sure it ain’t just third harmonic!)
One should ask the question: would any D/A converter designer in his right mind build a DAC using power components? Probably not. Then how about the old argument that digital-to-the-end is best? Well, I should think the D/A barrier is best put precisely where it allows the whole signal chain to perform at its best and why should we believe that this is necessarily right at the end? Quite obviously the concept of a digital class D amplifier was dreamt up by DSP folks who presumed that the signal should be kept out of the big bad analog world as long as possible, at the same time expecting the power stage, power supply and filter (all highly analog in nature) to perform flawlessly.
Analog controlled class D is quite adifferent story. Although mostcommercially available implementationsare well short of this ideal, proper error control can be used to makeanalog class D amplifiers with performance figures giving thedigital variety a run for their money, at a price well below that of even the cheapest digital class D’s.They can have vanishingly low output impedance right across and beyond the audio range (which the digital ones can’t!) while frequency-independent distortion (for that “zero-feedback sound”) is actually easier to achieve than with digital ones.
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"Digitally controlled class D: dead end street.Analog controlled class D: definitely the future, allthough you shouldn’t expect it to flatten competition from traditional solid-state and tube amps by a tremendous margin.
In the very high-end segment the three are bound to coexist for a very long time. In mainstream gear, class D is certain to take over the scene completely, although one serious problem remains: building a good class D amp is an order of magnitude tougher than a linear amp, and the knowledge required is much more diverse. It may take long before each large company has at least one knowledgeable designer. It won’t stop them from putting class D based products on the market, but until then and unless they buy completed amplifier modules from specialist vendors (which eastern companies rarely do, they’d rather commit harakiri than having to swallow their pride), they will be putting out seriously substandard products for years to come."