Courtesy of http://stason.org/TULARC/entertainment/audio/general/index.html
All of these terms refer to the operating characteristics of the output stages of amplifiers.
Briefly, Class A amps sound the best, cost the most, and are the least practical. They waste power and return very clean signals. Class AB amps dominate the market and rival the best Class A amps in sound quality. They use less power than Class A, and can be cheaper, smaller, cooler, and lighter. Class D amps are even smaller than Class AB amps and more efficient, because they use high-speed switching rather than linear control. Starting in the late 1990s, Class D amps have become quite good, and in some cases rivaling high quality amps in sound quality. Class B & Class C amps aren't used in audio.
In the following discussion, we will assume transistor output stages, with one transistor per function. In some amplifiers, the output devices are tubes. Most amps use more than one transistor or tube per function in the output stage to increase the power.Class A
refers to an output stage with bias current greater than the maximum output current, so that all output transistors are always conducting current. The biggest advantage of Class A is that it is most linear, ie: has the lowest distortion.
The biggest disadvantage of Class A is that it is inefficient, ie: it takes a very large Class A amplifier to deliver 50 watts, and that amplifier uses lots of electricity and gets very hot.
Some high-end amplifiers are Class A, but true Class A only accounts for perhaps 10% of the small high-end market and none of the middle or lower-end market.Class B
amps have output stages which have zero idle bias current. Typically, a Class B audio amplifier has zero bias current in a very small part of the power cycle, to avoid nonlinearities. Class B amplifiers have a significant advantage over Class A in efficiency because they use almost no electricity with small signals.
Class B amplifiers have a major disadvantage: very audible distortion with small signals. This distortion can be so bad that it is objectionable even with large signals. This distortion is called crossover distortion, because it occurs at the point when the output stage crosses between sourcing and sinking current. There are almost no Class B amplifiers on the market today.Class C
amplifiers are similar to Class B in that the output stage has zero idle bias current. However, Class C amplifiers have a region of zero idle current which is more than 50% of
the total supply voltage. The disadvantages of Class B amplifiers are even more evident in Class C amplifiers, so Class C is likewise not practical for audio amps.
Class A amplifiers often consist of a driven transistor connected from output to positive power supply and a constant current transistor connected from output to negative power supply. The signal to the driven transistor modulates the output voltage and the output current. With no input signal, the constant bias current flows directly from the positive supply to the negative supply, resulting in no output current, yet lots of power consumed. More sophisticated Class A amps have both transistors driven (in a push-pull fashion).
Class B amplifiers consist of a driven transistor connected from output to positive power supply and another driven transistor connected from output to negative power supply. The signal drives one transistor on while the other is off, so in a Class B amp, no power is wasted going from the positive supply straight to the negative supply.Class AB
amplifiers are almost the same as Class B amplifiers in that they have two driven transistors. However, Class AB amplifiers differ from Class B amplifiers in that they have a small idle current flowing from positive supply to negative supply even when there is no input signal. This idle current slightly increases power consumption, but does not increase it anywhere near as much as Class A. This idle current also corrects almost all of the nonlinearity associated with crossover distortion. These amplifiers are called Class AB rather than Class A because with large signals, they behave like Class B amplifiers, but with small signals, they behave like Class A amplifiers. Most amplifiers on the market are
Some good amplifiers today use variations on the above themes. For example, some "Class A" amplifiers have both transistors driven, yet also have both transistors always on. A specific example of this kind of amplifier is the "Stasis" (TM) amplifier topology promoted by Threshold, and used in a few different high-end amplifiers. Stasis (TM) amplifiers are indeed Class A, but are not the same as a classic Class A amplifier.Class D
amplifiers use switching techniques to achieve even higher efficiency than Class B amplifiers. As Class B amplifiers used linear regulating transistors to modulate output current and voltage, they could never be more efficient than 71%. Class D amplifiers use transistors that are either on or off, and almost never in-between, so they waste the least amount of power.
Obviously, then, Class D amplifiers are more efficient than Class A, Class AB, or Class B. Some Class D amplifiers have >80% efficiency at full power. Class D amplifiers can also have low distortion, although theoretically not as good as Class AB or Class A.
To make a very good full-range Class D amplifier, the switching frequency must be well above 40kHz. Also, the amplifier must be followed by a very good low-pass filter that will remove all of the switching noise without causing power loss, phase-shift, or distortion. Unfortunately, high switching frequency also means significant switching power dissipation. It also means that the chances of radiated noise (which might get into a tuner or phono cartridge) is much higher. If the switching frequency is high enough, then less filtering is required. As technology improves, industry is be able to make higher switching frequency amplifiers which require less low-pass filtering. Eventually, Class D amplifier quality could catch up with Class A amplifiers. Some believe that it already has.