What’s the difference for the listening experience when choosing between audio amplifier classes (A/B/AB/D)?
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This is very subjective and there is a lot of BS in audiophile discussions so be careful if anybody claiming that there is one universal difference between these designs and/or claiming one is always better.
Class B amplifiers would be prone to something called crossover distortion because different devices (either tubes or transistors) are used to produce the positive and negative halves of the signal, so when the signal is near zero there is an awkward shutoff or switching over behavior. This is why they are barely ever sold.
As for the rest of them… if it’s been properly tested with real equipment (not some person on the internet saying it sounded good to them) and the distortion and noise are extremely low, they’re all going to pass the input signal through unchanged and will sound similar.
There are differences other than the sound quality. Class A can be the simplest circuits with as few as one vacuum tube, but they are also inefficient. So they either have low output power or they are very large and power hungry.
Class AB is much more efficient, as much as 66% of the power consumed by the amplifier could be delivered to the speaker, but in reality it’s lower due to losses in power conversion and other practical limitations.
Class D uses a circuit similar to a switch-mode power supply to construct an audio signal without many of the inefficiencies of traditional amplifiers. They can be over 90% efficient and therefore very small, cheap, and put out little heat. However they are effectively digital computer circuits which can create a lot of unwanted noise if not designed properly, so they can be very good, relatively affordable amplifiers, or they can just be cheap junk.
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