class a?

[smopo24]

what makes a piece of audio equipment class a, b, and a/b?

[Nubus]

A good place for a wealth of knowledge like this is http://www.prosoundweb.com
CHeck there for the other classes.
Here is a, b, and ab, cut and pasted from their site:

Class A: A small signal modulates a larger current. This larger current is present when the small signal is not present. Efficiency up to about 26 percent. Excellent quality sound.

Class B: Uses a push-pull arrangement where one amplification device operates on the positive side of the waveform and another operates on the negative side. Efficiency up to 75%. Sounds bad because of distortion caused by switching from one device to another.

Class AB: A variation of Class B. Always has a small current flowing (class A operating region) and this eliminates the switching distortion inherent in Class B. Efficiency up to 65 percent. Sounds excellent—if well-designed