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Class D is generally only used in power amps because that is where it'll do the most good. The efficiency is much better and the whole thing is lighter with less heat sinking required. Couple that with a switched mode power supply and you can have many watts that will fit in the palm of your hand. Look up "Icepower", they are made by B&O. Eventually financial and environmental constraints are going to make such designs more common, but the one big drawback is that they are practically unrepairable. Imagine buying a car and when it gets a flat tire you have to buy a new car, that is where it is going. (in fact with those skinny wheels it really is, hitting a bump can crack the rim and there goes $600 or maybe even more) Like a SMPS works by varying the on time of a transistor instead of controlling it variably, it dissipates virtually (ideally) no power. Like a light switch in your house, when it is on there is current but no voltage, when it is off there is voltage but no current. Power is the product of volts and amps so it is a multiply by zero, thus no heat is generated. In fact if you have light dimmers they turn on and off very fast, they don't really vary the power, they interrupt it at a controlled rate. An incandescent lamp filament or the persistence of vision in your eyes will schmooze that out, though some people can see the flicker. Well a class D amp does pretty much the same thing but they generally chop it up at like 100 KHz, not 60 Hz like a light dimmer. This gives them pretty accurate control of the output by varying the duty cycle of the positive and negative outputs. This results in full blown AC, in square waves which needs to be filtered out. The design of that filter is one of the crucial things. Some want to filter very well and there will be phase shift at the high end. You are not supposed to be able to hear that but that doesn't mean it is not there. Years ago a line of hearing aids came out that used AC bias at a high frequency. It was observed that the hard of hearing actually heard better in the vicinity of ultrasonic motion detectors. I am not sure if they still sell them but the point is even though they could not hear the sound per se, it was still there. It had a noticeable effect. Problem with class D is the filter. If they get the switching frequency up to like 1 MHz this will no longer be a problem. However that poses its own set of difficulties. When you get to swinging all that voltage that fast there are what's called dv/dt problems, and if you use MOSFETs, they turn on faster than they turn off. You don't want that. With bipolar, the drive requirements become significant. It is not all that practical. So if you switch at 100 Khz that is only five times the 20 KHZ you want to accurately reproduce. People say the sound is not quantized but I say it is, but only on one axis. The voltage is not quantized but it is still a little sample of time. It has been found that some class D amps put out as much as a volt and a half of their switching frequency at idle. Not sure which but it was maybe a post here or at AK or maybe on Usenet. The guy was asking if it was normal. I didn't respond because I would need more direct knowledge. I doubt it was a Crown, which has their own little special design that is supposed to be even more efficient. But some people don't like them either, except maybe the DC-150s and whatever. (not class D) These days, in TV sets, low power amplification for some small speakers inside the cabinet, they don't even use a filter, rather depending on the inductance of the speakers to provide the filtering. You can't even take one apart and hook up external speaker wires to it, or run them to an amp. Most people here will agree that class D does not belong in high end audio. But I do believe that we are going to see more of it in mid-fi and whatever. Energy becoming the new currency will dictate that, and really, 75 % of the people will not notice a damn thing. And really, if you are talking battery powered stuff, what, outside by the grill or in your car ? Why not ? A garage stereo or some type of boombox ? Oh, and I can't stand those new boomboxes. I want one made like in the 1970s. New ones are all chrome n shit, "beauty rings", one note bass. I remember a few that used to sound pretty good. And those could do with a class D amp because they weren't really hifi. They were like utility-fi. I haven't worked on a boombox in a long time but I would bet that some of them have class D in them. And I found class D in pro audio equipment. But pro audio is by no means hifi. This is similar to the argument about audio formats. Sure, you get the sampling (switching) rate up high enough that the golden ears either can't hear the difference or even like it, then you have accomplished something. Until then as long as you got the switch and they sit in the chair and can accurately tell you which is which, they are not going to accept it. But then this is like a hobby. It is like driving around in a car with a bigblock V8. You don't take that thing to work. (unless you got secured parking) It will probably be some time before the audiophiles accept class D in their listening environment, but in the cars and bars, and outside cooking the steaks, or in the garage burning one, it will be fine. Just my considered opinion.
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