Class D amplification works in swichmode. This is to say that the power transistors turn on and off in sequence as they are driven by control circuitry.
The downside is roughly the same as comparing a CD player with a turntable. Yes, less noise on CD, yes, less scratches, yes, a guaranteed wider and more vene bandwidth, but somehow, it never sounds quite like a decent turntable.
The upside is that it is extremely efficient. Having an efficiency of 90% in D class is no big problemo, whereas you'll be lucky to get more than 10% efficiency (i.e. 90% of drawn power is wasted producing heat) in class A, and in class AB, 30...40% is about right. This means kess wasted energy, hence less heat, hence MUCH smaller, almost token heatsinks - large area heat sinks are far from being cheap, they take up space, they are heavy, etc. Since it works in switchmode, well, one can do the power supply in switchmode as well, again, with more efficiency, immediately accepting anything from 110 to 240 VAC (good for you because it doesn't give a damn about varying grid voltage, whereas a classic transformer does react to lower voltage by lowering its output voltage), and eventually, they can be made to be cheaper to much cheaper than classic power supplies.
In theory, class D is all but perfect - how many times were we told that one?
In practice, it appears that not all is perfect, after all. Maths show that using fast power MOSFETs should be just fine, given that their turn on time is just 35 nS, or, 0.000,000,035 seconds, but since most class D amps tend to tire one out after a while in me upper mid and treble range, it would seem that we need faster still. MUCH faster!
They come by the truckload beuase they are cheap and easy to make. A board of the size of a say cigarette pack could deliver several hundred watts - try that with analog and see how big and expensive the result would need to be.
Also, even if digital, these amps suffer from more or less same problems their analog counterparts did and do, namely their ability to drive low impedance loads is most questionable. That can be done only by using more powerful output stages and beefier power supplies, but that costs money, and is in fact counterproductive to the idea of cheap mega amplification.
All that notwithstaning, let's be frank, that is the way audio is going, and eventually, our worst audiophile nightmare WILL come true and all audio will end up as a chip in a PC. The trouble is, I'll still be alive when that happens.
