Class A, Class A/B and Class D amplification WHAT DOES IT ALL MEAN?

We explain why some amplifiers run hot and others don't

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If you have read an amplifier review or looked at the specifications for one, the chances are that you have come across a term such as ‘Class A/B’. In fact, this is just one of a number of different amplifier classes, the most common being Class A, Class A/B and Class D.

Most amps have the same basic building blocks in their electronic design. They typically use transistors as the core amplification component along with a supporting cast of resistors, capacitors, inductors, wires and transformers to make the whole thing work. But it is the amount of current that flows through the main output devices (the output transistors) when there is no input signal that defines the class of the amplifier.

This single engineering decision sets the product’s basic parameters. It decides the power output, the amount of energy the amp consumes, the level of heat generated and even the minimum size that the product can be.

Class A amplification

A transistor needs a certain amount of bias current flowing through it to actually work. In a Class A circuit, the level of that current is always set to maximum. If that amplifier design is working at full volume then the bulk of that energy is used to drive the speakers. (Note that no amplifier is 100 per cent efficient, so there is always some wastage of the input energy, but that isn’t significant in what we are describing here.)