Classes of Amplifiers
An amplifier is classified according to its circuit design and the way its output stages are powered. Although some may assume that for every portion of the input signal there is corresponding 100% output from the amplifier, power dissipation (in the form of heat) and distortion of the audio signal are two key factors in determining the efficiency and fidelity of an amplifier. Each class has its own performance characteristics and advantages.

* Class A amplifiers are desirable for the high quality of their sound, but, because of the configuration of its transistors, a pure class A amplifier is not very efficient and runs very hot. This is because even when there is no audio signal, the output transistors always have current running through them. The current flowing through the output transistors (with no audio signal) causes the amp to heat up. The advantage of class-A is that the circuit holds the same temperature indifferent of the load. In other circuit types the temperature shifts constantly (with the amplified signal) and that results in shifting capacities in transistors. Hence the bandwidth and phase shift constantly. Not so with class-A. The efficiency is around 25%. A class-A amplifier has a relatively high price because of the large heat sinks and large power supply.

* The output transistors of Class B amplifiers actually turn "almost" off for half of every signal cycle. This improves efficiency and saves energy, but introduces a special kind of "cross-over" distortion during the switching periods.

Class AB amplifiers also allow current to run through the output transistors when there is no audio signal, but at a very low level. A class AB amplifier runs cooler, and therefore, more efficiently than a class A, with low distortion and high reliability. Efficiency when amplifying a signal is around 50%. Class-AB comes at a lower price then class-A because the power supply circuit and the heat sink can be smaller.

* Class D amplifiers use output transistors as switches to control power distribution — the transistors rapidly switch on and off at least twice during every signal cycle. Class D amps boast higher efficiency, produce less heat, and draw less current than traditional Class AB designs. Class D amplifiers produce higher distortion than AB designs due to the high-speed switching on and off of the transistors, but this distortion occurs at high frequencies that are typically removed by a low-pass filter. Class-D is cheaper then other circuits because the power supply can be smaller and there's hardly a need for a (large) heat sink. The efficiency of a class-D amplifier is typically over 90%.