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Q. How do the different amp classes work?

Published August 2010

I'm trying to learn a little more about amp design. One thing that really baffles me is the different classes available. What does an amp's class mean, and how does this affect the way it is used?

Via SOS web site

SOS Technical Editor Hugh Robjohns replies: In a Class‑A circuit, the active device (whether valve or solid‑state) passes current regardless of the polarity of the input signal; in other words, in an audio application, it is 'biased' so as to pass both the positive cycle and the negative cycles of an audio signal. The side effect of the biasing is that the active device has to pass current all the time, making it relatively inefficient.

In a Class-B circuit, the active device only passes current for one polarity of input signal — which polarity depends on the circuit design — and this makes it a much more efficient way of working. So, in this case, where it is required to pass a symmetrical audio signal using a Class‑B circuit, the circuit will need two active devices, one to handle each polarity. This is an arrangement often also known as 'push‑pull'.

Class C is a format that only conducts on signal peaks and is rarely (but occasionally) used for audio in situations where power efficiency is more important than distortion. Class D — which is now becoming very popular in audio applications — works by generating a stream of high-voltage pulses at a very high frequency. These pulses are modulated in such a way that the average energy they convey follows the wanted audio waveform.

Returning to the Class-B design, this exhibits a problem called crossover distortion for audio applications, because both of the active devices in the push‑pull pair turn off as the signal nears the zero line. The solution is to bias the devices so that they don't turn off. They actually continue to pass signal as it crosses over into the opposite polarity. In other words, it works a little more like a Class-A device (but without the same levels of power inefficiency).In a push‑pull amp design, each active device handles one polarity of the input signal. Hence the compromise name Class AB; it is a Class-B design biased to operate in a similar way to Class A around the crossover region. However, it should also be remembered that push‑pull designs can also be operated fully as Class A if required, and some high‑power amps do work in that way. This is also a handy technique for cancelling out even-harmonic distortion products in tube-amp designs.  

Published August 2010