AMPLIFIER CLASSES - Class A, Class B, Class A/B
What do they mean?

In addition to the two basic types of amplifiers, BTL and SPS, amplifiers also come in classes. Each class handles the actual waveform amplifying differently, and each class has its own set of advantages and disadvantages. Most amplifiers fall into one of three classes. Class A, Class B, Class A/B.

Class A amplifiers have transistors that are biased (running) all of the time. Class A ampls offer the least distorted sound, but they are inefficient and run very hot. Although they are considered sonically the best class of amplifier, there are very few pure Class A amplifiers in mobile audio use.

Class B circuits are very efficient, since they use seperate transistors to amplify the positive and negative parts of the signal. With one exception, switching noise, they are also fairly low in distortion.

Because the transistors switch on and off rapidly in response to the changing polarity of the signal, Class B amps generate switching distortion, which is usually only audible during very soft musical passages. Since the mobile audio environment is relatively noisy, passages soft enough to have audible switching distortion present are usually lost in the general road noise anyway. Thus, there are quite a few Class B mobile amplifiers.

Class A/B is almost as efficient as Class B since the transistors in Class A/B never quite shut off. Because of this, Class A/B does not have audible switching distortion. Most main line home audio amplifiers and most better car amplifiers use Class A/B amplification. Today’s top A/B designs allow the amplifiers to operate during considerable amounts of music in Class A, while having the overall efficiency of the higher distortion Class B.