Class D amps are often referred to as "digital amplifiers," which is a misnomer. They are not digital. They are "switching amplifiers." They can be digitally controlled, but most are not.
In Class A amps, the output stages are "on" all the time. This creates a lot of heat, and the efficiency, well, it sucks. They can lose 70% of their output power in the form of heat, which is why you see huge heat sinks on those big Class A solid state amplifiers. Most of the weight is heat sinks. But they're the best in terms of the amplified signal- they introduce the least distortion, and generate an amplified signal of the input wave that most closely matches it.
Class B amplifiers have the output stages conducting 50% of the time- although the non-conducting stage is not actually turned "off" when it's not in cycle, so to speak. More efficient design, but subject to crossover distortion if the transition from one output element to the other is not perfect. Less heat generation, more distortion.
AB amps have the output stages on over 50% of the time. This lessens the crossover distortion, but you'll still get fairly rotten efficiency and a good bit of heat production. The output signal is not an exact replica of the input signal, especially at higher power, when some of the more sophisticated AB amps go from "pure Class A" mode to AB mode.
A Class D amplifier has one output stage fully "off" for half the cycle. This makes for a very efficient amplifier- only about 10% of generated power is lost as heat. These amps use pulse width modulation to take the input signal and generate a square wave analog signal. This is in contrast to the typical sine wave generated by a Class A amplifier. The square wave contains spectral inaccuracies, which is rectified by running this signal through a low pass filter. The resulting signal is a near perfect sine wave amplification of the original input signal.
The result is much higher efficiency. The switching elements are off half the time, so they generate less heat and suck less power than they would in a linear (on all the time) mode.
MOSFETS used to be used as the switching elements, but many of the newer D amps use integrated chip assemblies to act as the switching elements. They are more efficient switching devices than are MOSFETS.
The result? A powerhouse amplifier in a tiny package. You can get a 500 wpc D amp that weighs about 20 pounds. A similar Class A power amp would weigh in somewhere between 60-100 pounds, have huge heat sinks and have to be placed somewhere that they can freely ventilate. The Class D amps generate almost no heat whatsoever, and are designed to be left on all the time. At idle, they draw practically no current at all, so there's no need to power off after you're done listening. Heat dissipation is not an issue, so you get a relatively tiny package that can be placed anywhere you like. They don't need to "breathe."
They sound pretty darn good, too. I prefer tubes, but the Class D amp I own (Rowland 102) is no slouch. It sounds quite good. Weighs about seven pounds. No heat sinks. Minimal hassles. 100 wpc into 8 ohms.
If you're looking for a tiny amp with a huge bang that generates practically zero heat, the Class D amps are for you. Purists may bitch that they're jacking around with the input signal (and they are), which means that somewhere in the path, you have to introduce signal inaccuracies. However, as the technology improves, those inaccuracies are becoming less and less. Run 'em side by side with a Class A solid state amp or a tube amp, and you might find a difference, but it's not much. They don't sound processed or mechanical to my ears. In fact, to me, they sound quite a bit like a tube amp. Not quite as good, but when the 845 amp is heating up to 200+ degrees this summer, and I'm sweating like a pig with the AC on full blast, I may very well switch over to the Rowland and stay cool without sacrificing as much sonic quality as I once thought. I've been using the Rowland more and more lately, and it's getting more difficult to find a huge difference between it and the MastersounD tube amp I use most of the time. Make no mistake, the difference is there, but it's subtle, and something I could definitely live with in the name of comfort. If you live in the northeast or midwest, where air conditioners are more of a luxury that a necessity (as they are here in the deep south), yeah, I can see myself easily switching over to the Rowland during the dog days of summer.
