Class A audio amplifiers
A Class A audio amplifier is the most linear type. It uses a full signal cycle powered by an electricity-conducting transistor (output device). It tends to become hot because the output devices is always at full power, producing a “bias” or a regular signal. It is also extremely inefficient and only has an efficiency rating of about 20%.
However, Class A audio amplifiers produce the cleanest and clearest sound with little or no distortion at all. They come in 30 to 60-watt models of up to 8 ohm.
Class B audio amplifiers
A Class B audio amplifier has circuit parts that control the negative and positive halves of a signal. It uses at least two output devices with the transistor driven by a large input signal. It is also called a “push-pull” amplifier because it uses two output devices. One output device pulls the signal while the other pushes it. It is cooler than a Class A audio amplifier but produces sounds that are ‘less pure.’
Class B audio amplifiers can also produce distortions called “crossovers,” which is the effect of each output device turning off and on after each signal cycle.
Class AB audio amplifiers
A Class AB audio amplifier combines some of the characteristics of the Class A and Class B type. It produces excellent sound quality comparable to a Class A audio amplifier, but it remains more efficient and produces a lower output range. It also has some of the characteristics of a push-pull output.
The efficiency rating of most Class AB audio amplifiers is approximately 50%. They usually come in 75 to 300-watt models of two to four ohm.
Class D audio amplifiers
A Class D audio amplifier is also called a switching amplifier because each cycle requires each output device to be quickly turned off and on at least twice. The switching is controlled by audio signals that commonly use Pulse Width Modulation (PWM). Some models can operate on a true digital system, while others cannot. It can be turned on and off up to several million times in a second depending on its switching frequency.
Class D audio amplifiers are about 80% to 90% efficient (high efficiency gain at the expense of high-fidelity).