What’s the difference for the listening experience when choosing between audio amplifier classes (A/B/AB/D)?


What’s the difference for the listening experience when choosing between audio amplifier classes (A/B/AB/D)?

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Class A amplifiers sound really good, but use a lot of energy and can get very hot.

Class B amplifiers are more efficient but can sometimes add noise or distortion, especially at quieter parts.

Class AB amplifiers are a good balance between sounding good and being efficient.

Class D amplifiers are very efficient but some people might notice added noise that isn’t in the original recording.

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.

The technical definition is that a class A amplifier uses a single element (tube or transistor) that conducts 100% of the time and amplifies the whole signal. A class B uses two elements that conduct 50% of the time, and each only conducts half the signal (positive and negative portions). A class A/B is a hybrid that uses two class A amplifiers but configured like a class B.

The differences between these are that class A is low distortion, low efficiency (at most 25%). Class B is high distortion and high efficiency (at most 75%). And class A/B is low distortion and moderate efficiency (at most 100%).

For decades, class A/B was used everywhere for audio power amps as it was the only sensible choice. Class A had its uses (often as a separate stage of the amp). Class B is basically a waste of a transistor or tube, so you don’t see it as much.

Class D operate entirely differently than tube and transistor amplifiers, but they’re lower distortion than any of the others and have peak efficiencies close to 99% and are cheaper at scale. So most devices use class D today, unless you have some other design constraints.