Classes of Amplifiers
People sometimes assume that for every portion of the input signal there is corresponding 100% output from the amplifier. However, power dissipation (in the form of heat) and distortion of the audio signal are two key factors in determining the efficiency of an amp. The design of an amp's circuitry determines the class of operation of an amplifier, and each class has its own performance characteristics.
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 inefficient and runs very hot. This is because even when there is no audio signal, the output transistor(s) always have current running through them. The current flowing through the output transistor(s) (with no audio signal) causes the amp to heat up unnecessarily, and "waste" input energy. Most car amplifiers that boast "Class A" circuitry are really Class A/Class AB hybrids.
By far the most common car amp design, Class AB amplifiers also allow current to run through the output transistors when there is no audio signal, but at a much lower level. A class AB amplifier runs cooler, and therefore, more efficiently than a class A, with low distortion and high reliability.
Class D amplifiers use output transistors as switches to control power distribution — the transistors "turn off" when there is too much voltage across them. Class D amps boast higher efficiency, produce less heat, and draw less current than traditional Class AB designs, but they tend to have more distortion. Since low-frequency distortion is hard to detect, compact Class D amps are ideal for bass amplification.
Class Tamplifiers provide the sort of sonic advantages of conventional Class AB designs, combined with the high power efficiency and low heat production of a Class D design. Class T amps are able to generate 2-4 times more power than a comparably-sized Class AB amp.
