Difference between Class a and a/b

Craig, In class a operation,the output devices are constantly on.In a/b,the output devices turn on and off {generally causing "switching distortion".The pure class a operation usually draws more power due to the devices not cycling on/off.In my opinion,it sounds better,closer to analog.Digital is an on/off medium and I'm sure you have heard the complaints there.Some amps will provide class a operation to a certain power output,then slide a/b {IE:Pass Labs X series}.I hope this helped,I know I over-simplified the explanation..But.

simple terms.

Class A - sounds much refine, good in resolution, focus on high and mid. Bass quality tends to develop vertically (deep deep deep)

Class A/B - sounds less bright (relatively), much bass and tends to develop horizontally. Punchy or boomy bass for extreme case.

Craig,
I will try to explain it more technically, but maybe it will make more cents. Your amplifier is working to deliver some signal to the output. If you draw imaginary line through the center of that signal you will divide it into a positive half and a negative half. In Class A amplifier a single output device controls the flow of the entire signal. To do so it has to be biased in such a way that the lowest point of the negative signal still has to be above 0 level, at which point output device (transistor, tube, FET, etc...) shuts off and clips (distorts) the signal.
In class AB amplifier the output signal is controlled by a pair of output devices, where each one of the pair controls only the positive or the negative half (push-pull) of the signal. So each one is biased to be shut-off at zero crossing and stay off while the complimentary one is working on the opposite half of the signal (hence the crossover distortion).
This is basically how they work. I hope that my explanation was clear enough to follow.
Well... I tried...:)

zoya,

your explaination is incorrect. Your description of class A is actually a single ended design(i.e. one output device handling the entire signal). class A can also use two output devices, one handling each phase. however both devices stay powered up at all times, even when not used to drive the speakers (i.e. each device stays powered up for the entire 360 degrees).

your description of class a/b is actually a description of class b (i.e. the out device powers down at zero crossing or at 180 degrees). In class a/b each device stays powered up beyond zero crossing, but not for the entire signal (say 200 to 300 degrees). when you hear manufacturers claim a "high class a bias", they are talking about how far past zero crossing the output device stays on.