Explain Class A amp to non audiophile friends

In Class A, the bias current is turned on to the transistors all of the time. Meaning that the transistors are always on and ready to reproduce the signal. In a Class AB amp, the signal triggers the current to the transistors, turning the transistors on when louder passages are coming and shutting bias off when signal quites down. The end result is that in a Class AB amp the current to the transistors is turned on and off, thereby sparing electricity. Those that enjoy Class A sound say that it's due to not hearing the byproduct of what they call switching disorder. That in essence is the sound of transistors turning on and off.

The down side to Class A is that the amp runs very hot when bias current is on all the time, which also increases the electric bill.

http://sound.westhost.com/class-a.htm

First you have to think about the waveform of a sound, it has a peak and a trough and it cycles from peak to trough over and over at a certain frequency.

With a Class A, the amplifing device is drawing more than enough current at all times throughout the whole cyle and is being called on to amplify the whole cycle from peak through trough. There can be one or more amplifying devices -- a tube, a transistor, etc.

With a Class B there has to be at least two amplifying devices per channel -- a pair of transistors or tubes (or two pair or three pair etc).

The signal is split going into the output devices so that one device is amplifying the peak of the waveform and one is amplifying the trough of the waveform. When one is working the other is switched off. This results in crossover distortion at the point when the devices switch.

In Class AB it's like Class B but the "resting" device isn't ever fully switched off. This reduces the crossover distortion.