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.

Craig,
if Audiogon didn't "expire" previouse and usefull archive information about classes of operation of amplifiers often appeared in discussions here than you'll find more detailed and deep explainations of classes ofoperation.

It's definitely something beyond sonic effects of one or another class of operation.

So far members explained to you differences between class A and class B but not realy class A/B.

Class A/B has a special diode placed between complementary pairs of output elements to bring the output devices from class A onto the B operation under demand of higher current or power. This actually may even be neccessary even on moderate volume levels depending on load i.e speaker. A complementary pair of transistors is defined to be opposite bias transistors(direct bias and reverse bias) with same input and output parameters. Such above described diode works as a gate between such complementary pairs and should stay "closed" when the input signal from the previous stage is less than peak and "open" once the peak input signal is reached by the previous stage transfering the output devices onto class B operation. Before the diode is "open" both groups of transistors direct and reverse bias are having their own zero-degree point of operation. After switching to class B both of output device groups transfer to one common zero degree point of operation.
Please note that previous stages in amplifier can also work as well in class A, B or AB operation.

The advantage of such A/B design is decreased distortions on higher volume levels and near-linear responce of class A on low volume levels.

Class B itself has higher distortions at low volume levels not only due to above said switching between complementary pairs but also due to some degree of parameter difference between such as well.