High fidelity audio amplifiers fall into one or more of three classes, A, AB and B. The class of an amplifier governs not only its design topology, but also the expected output quality.
A class A design is one in which the output devices conduct over the
full cycle (360 
) of an output signal. Essentially, class A
requires the output devices to conduct all of the audio signal
without turning off. This topology is inherently inefficient, because
the output devices must be biased up to a level where the input signal
can't dip negative enough to shut the device off, and this bias
manifests itself in a large DC quiescent current that must be dissipated by
the output devices. With class A, we can approach a miserable 25%
efficiency, but we get inherently high quality output.
A class B design incorporates at least two output devices where each conducts for exactly half of the output cycle. This translates into having one output device handle the positive voltage portion of the signal, and the other handling the negative. A class B output signal always suffers from crossover distortion, which occurs at the zero-crossings of the signal, where one device must turn off and the other turn on seamlessly. It never works out seamlessly, and we end up with gaps at the zero voltage level in the output. Class B is extremely efficient, but yields poor quality output due to crossover distortion.
A class AB design incorporates at least two output devices that conduct for more than half of the output cycle. Class AB fixes the problem with class B by biasing the output devices a bit higher, so that there is a period where both devices are conducting at the same time. There is little or no crossover distortion in class AB. Two things need to happen for class AB to work well: the output devices need to be biased ``just right'' and they need to be matched. Class AB offers quality and efficiency that are very design dependent. Most modern amplifiers are class AB.
Audio power amplifiers employ one or more of three kinds of output devices: vacuum tubes, power BJT's or power MOSFET's. The output device is responsible for the final stage of amplification and for supplying power to the loudspeakers. Audio amplifier designers approach output devices with equal amounts of knowledge, black magic and religion.
Vacuum tubes sit in the middle of an incredible controversy in the audio world. Historically, they were the first electrical audio amplifiers, starting with Lee DeForest's Audion tube that made ``talkies'' possible. When the transistor hit the market, it ruined the audio amplifier market for tubes. Recently, interest has flared in tube designs. Die-hard fleets of audiophiles proclaim that tubes have a superior sound in comparison to transistors. (See Hamm[1] for an excellent discussion of this) I didn't even consider tubes for this design.
Bipolar junction transistors were the first type of transistor available. They are able to handle large currents, supply lots of power and can provide a great deal of gain in a single stage. They have a positive temperature coefficient that will need to be compensated for at high levels. BJT's are cheap and readily available, and are found in the bulk of amplifiers existing today. Bob Carver, known through his company of the same name, swears by BJT's. The BJT sound is normally associated with modern pop music with testosterone inflicted bass levels and crisp highs. BJT's are behind most high-powered amplification systems.
Power MOSFET's entered the market later than BJT's. They offer excellent linearity, and have a negative temperature coefficient. Power MOSFET's have lower gain (transconductance) than BJT's, can't handle as much power and are more expensive. Nevertheless, power MOSFET's are gaining popularity within the audio community and are being used in many new designs. Hafler made one of the first MOSFET amplifiers, and its sound has been described as ``tube-like.'' MOSFET's are most often used in class A designs because of their negative temperature coefficient, and low power class AB designs.
The power supply will play an integral role in any audio amplifier. Care must be taken to insure that all components are well overrated for the power that they will need to supply to the amplifier. Characteristics of most high-fidelity amplifiers are: