Class D switching amps
From DRC
A perfect amplifier should behave "like a wire with gain". ie you put a small signal in one end and you get a perfect copy, but scaled up, out of the other end
In practice it is relatively hard to build a good amplifier with this property into general speaker loads. The speaker does not present a uniform load to the amplifier and in the case of tube/valve amplifiers this can cause a filter effect which causes a frequency dependent sound cutoff. Or the amplifier can clip quite easily into difficult loads. Rising levels of harmonic distortion are not uncommon in amplifiers driven hard with insufficiently sized powersupplies.
There are a whole bunch of theories on how to build the best amplifiers.
- Valve amplifiers
- Transistor amplifiers (also called solid state)
Also:
- Class A
- Class B
- Class A/B
- Class D
Class A and B are different ways of biasing a transistor amplifier. The transistor is only linear over a small part of it's operating range so the obvious solution it to bias it so that it is always operating in it's linear range. Unfortunately this consumes massive gobs of power continuously...
Class D is a whole new way to build an amplifier and is sometimes called a "digital amplifier". The basic idea is that the amplifier only ever switches full power on or off (actually it only ever switches full power positive or negative, but this is not entirely important for our basic explanation). However, it switches each way very fast, typically 400,000 times per second. So if you want 50% power at any given instant you can get this by switching the amp on as often as you switch it off. If you want 80% power then just switch it on for 8 pulses and off for 2 pulses, and so on.
Class D raises a whole bunch of technical problems like spewing out RFI. However, these problems are all surmountable and the resulting amplifier can have many useful properties. In particular they tend to be extremely "stiff" and handle difficult speakers well. In my experience they also work extremely well for room correction in that room correction requires heavy demands from an amplifier and the class-D amps seem to have reasonable performance right up to their design limits.
In theory of course any good amplifier should be the same as any other good amplifier. Room correction is just another stiff test for an amp and tends to show up the duffers more easily...
