The choice between Class A operation vs Class AB comes down to a simple decision between:
(A) Inherent low distortion and absence of device switching over the entire waveform, and also the least efficient mode of operation.
(AB) Significantly higher efficiency, greater power output (typically 3X or more) and an implied requirement for local or global feedback to linearize the switching transition in the output devices.
So if you’re building a transistor or tube power amp, the only difference from a design perspective is the rail or B+ voltage and the operating point for the output devices. Class A operation involves lower voltages but a high standing current ... in effect, the amplifier draws the same amount of power from the AC line, whether it is idling or at clipping. So it runs hot all the time. They are room heaters, thanks to low efficiency.
Class AB has the interesting property that AC power draw varies with signal level; this has implications for power supply design, since current flow through the supply is program modulated, and not in a simple way. This is a roundabout way of saying Class AB amplifiers need better power supplies, and full regulation isn’t a bad idea. Otherwise, 100/120 Hz rectification buzz in the audio signal will be program modulated, which is extremely undesirable.
There is an additional complication which applies to solid-state but not tube amplifiers. Tubes don’t need heat sinks, since device characteristics are not affected by operating temperature, and they are designed to radiate heat on their own without assistance.
Transistors can be destroyed by high device temperature, or have their useful lives shortened. The failure mode is complete destruction, which happens without warning, and might expose the loudspeaker to full rail voltage, which will destroy it.
In practice, transistor amps operating in Class A need substantial heat-sinking and effective temperature monitoring to prevent runaway thermal faults. These additional circuits must be highly reliable, since runaway thermal faults will destroy the output section before the user can get to the power switch.
In tube amps, Class A versus Class AB is a simple decision between power output versus linearity. Not much to it. In transistor amps, in addition to a significant power derating, Class A implies good thermal management if the amp is to be reliable.
In practice, transistor amps that operate in true Class A are thermally limited. How big a heat sink will the consumer accept, and are fans acceptable?
(Note: there are various sliding-bias schemes, which have been around since the Seventies, that claim to be Class A. They’re not. Non-switching is not the same as Class A.)
