Q. Why does 'Class' matter in an amplifier?

By Hugh Robjohns

I saw a mic preamp advertised as 'Class A' and 'Transformerless.' What do these terms mean and why exactly are they a good thing?

SOS Forum Post

Technical Editor Hugh Robjohns replies: The 'class' of an amplifier refers to the circuit topology used, and is independent of whether the circuit uses valves, transistors or FET as the active devices.


In a Class-A circuit the output device is arranged to pass the entire audio waveform — both the upper and lower halves of the signal waveform. This provides the cleanest, most transparent sound, but the necessary biasing arrangements makes this kind of circuit power-hungry, and it tends to generate a lot of heat as a result.


A more efficient circuit design is the Class B, which uses two output devices, one to handle only the upper portion of the sound waveform and another to handle the lower half. The benefit is that only one device is working at any time, and when there is no input, both are switched off, allowing huge savings in power consumption and heat generation. The drawback is that at the zero crossover point between the positive and negative halves of the waveform, one device might have switched off before the other has come on, and that results in 'crossover distortion' — which isn't a good thing in high-quality audio circuits.

 
The Buzz Audio MA2.2 (top), Focusrite ISA220 (middle) and TL Audio VP1 (above) all feature Class A circuitry, sacrificing efficiency for superior sound quality.

The compromise solution is a combination of both topologies (Class A and Class B), and it's called... Class AB. This also employs separate devices to handle the upper and lower portions of the sound waveform, but they are biased in such a way that both are operating when the signal is close to the zero crossover region, and thus crossover distortion is much less of a problem.
These basic circuit topologies can be employed in any amplifier design, whether it's a power amplifier to drive loudspeakers, a microphone preamplifier, or a line driving amplifier, as well as in discrete-component or integrated (IC) circuits. However, Class A remains the best choice for audio systems where the power consumption can be tolerated.

The term 'transformerless' refers to the absence of a transformer within the circuit. Transformers can be useful things in audio systems, providing 'galvanic' isolation between circuits and systems, or impedance-matching and the balancing (or unbalancing) of audio circuits, or even providing a 'free' voltage gain, depending on the application. However, transformers also have disadvantages, such as large size and weight in audio applications, and the introduction of large phase shifts which can become audible under some circumstances and therefore undesirable.

Many modern electronic circuits have been developed to replicate some of the desirable characteristics of transformers, without their associated disadvantages, and this is often championed as an overall advantage. Hence the 'transformerless' term is generally seen as a good thing, along with Class A. However, there are some circumstances where transformers still provide the best solution, and the inherent sonic qualities are often deliberately sought.

Published October 2003