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Compound Amplifiers
With compound amplifiers, the input and driver stages are usually run in Class A, as Class A operation produces the least distortion and does not severely task these devices. (The classification of a compound amplifier is usually based on just the mode of operation of the output stage.) The output stage of the compound amplifier might be run in Class A or, more commonly, Class AB.
The advantages to Class AB are efficiency and fairly low distortion. Class AB, which only makes sense in a push-pull output stage, works by allowing both devices to conduct enough current at idle to allow some overlap in the handling of the output waveform. In other words, Class AB is partially Class A up to some percentage of the total power output.
Here is where specifying the load is so important in evaluating an amplifier's mode of operation. For example, a mighty Krell Class A amplifier might be rated at 100 watts, but working into a 1 ohm load, would yield only 13 Class A watts before the output stage devices would stop to conduct over the entire portion of the output waveform, i.e. before entering Class AB operation. On the other hand, a small receiver's Class AB output stage would never stray from Class A operation while driving 600 ohm headphones, as the full power supply rail voltage developed across the 600 ohm load would not induce a current flow greater than twice the idle current flow of the output stage.
While popular with public address amplifiers, Class B is seldom used in home audio systems (McIntosh amplifiers excepted). Class B is much more efficient than Class A or Class AB operation, as the output devices are "approximately" turned off in the absence of a drive signal. Unfortunately, the distortion is so great in this mode of operation that a great deal of feedback is required to tame the output stage' deviations.
What follows is taken directly from GlassWare's SE Amp CAD user manual; it is
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the most successful explanation of amplifier classes I have found for the tube novice.
The single-ended amplifier predated the push-pull amplifier. It was quite simple and straightforward: input signal cascaded from input to driver tube to output tube, no phase splitter and, originally, no feedback. Class A from start to finish. To understand Class A, AB, B, and C amplifiers, imagine one lumberjack sawing a tree down. He must extend the saw blade through the tree's width and retract it afterwards. If he stops, the sawing stops. This is like single-ended, Class A operation: whether the signal swings positive or negative, the sole output device handles all of the output waveform.
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Now, should another lumberjack grab hold of the saw at the other end and work equally hard, both will both push and pull the blade through the tree. Although the work goes twice as fast, the net number of felled trees would equal the same number of trees the two would fell in the same time period working independently. This is like push-pull, Class A operation: whether the signal swings positive or negative, the two output devices equally handle all of the output waveform.
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