Explain Class A amp to non audiophile friends

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.

Well the whole goal of running class AB instead of Class B is to pretty much eliminate crossover distortion by judicious biasing of the output devices and usually also the use of negative feedback. I'm not sure if it can be eliminated to the point where it never appears on a 'scope under any circumstances, probably not would be my guess, though it is worse at lower power, better at higher power. Still, that's a great question for a circuit designer with real tech knowledge. Here's a link to a bit of an explanation of crossover distortion: http://www.electronics-tutorials.ws/amplifier/amp_7.html

I guess the crossover distortion in a Class B or Class AB amp is measured as part of the overall THD spec, so I'm sure, like any departure from linearity, it's audible in some form or another, but it may be so small as to be a tiny component of the sound. You know, no amp is perfectly linear. I'm sure there are single-ended Class A amps with higher THD levels than some well designed push-pull AB amps -- it's just different distortion. With circuit design there are always compromises, it's always a matter of picking your poison.