Audio amplifiers and audio preamplifiers are used to increase the amplitude of sound signals.  Preamplifiers or preamps are designed to amplify low-level signal from a high-impedance device such as a microphone or instrument pick-up. After pre-amplification, the signal is sent to a power amplifier or amp, which provides the higher current necessary to drive the speakers.

Types of Audio Amplifiers and Audio Preamplifiers

The GlobalSpec SpecSearch database contains information about lettered classes of amps and preamps. These classes differ in terms of slew rate, the maximum rate of change of the output; linearity, the degree to which the output signal strength varies in direct proportion to the input signal strength; and specific applications.

• Class A amps and preamps are output stage devices that pass currents at all times, even when the input stage is idle. Their slew rate is quick and there is only a small delay between the introduction of a signal at the amplifier's inputs and outputs. Because of the full-bias-current-on state of Class A amplifiers, they are the most inefficient but also the most linear.
• Class B amps and preamps are the opposite of Class A devices. No currents flow when the output devices are idle, and thus Class B amps and preamps must turn on from a zero-current state when signal is present. Class B amplifiers are not generally used for professional audio equipment, but are reserved for low-power operations such as radios.
• Class AB amps and preamps operate like a Class A devices for part of their output, and turn on an additional current for the rest of its output. The amplifier's slew rate is slower in Class AB operations than in Class A because there is a measurable length of time between the appearance of input signal and the appearance of output signal. Class AB is the most popular type because of its increased efficiency and excellent linearity.
• Class C amps and preamps are used for radio-frequency (RF) transmissions. This class is similar to Class B operations in that each output stage device (negative, positive) is turned on for less than one-half cycle, and pulsed on and off through the duration of the half-cycle. Class C amplifiers can produce large amounts of output power, although the distortion is great. RF circuitry has been developed and tuned to alleviate the effects of this distortion.
• Class D amps and preamps are also switched. The output devices are switched on and off at least twice per cycle. Because the output devices (negative and positive) are completely on or completely off, no power is dissipated. In reality, however, they are less than 100% efficient.

Other classes of audio amplifiers and audio preamplifiers are also available.

Performance Specifications for Audio Amps and Preamps

In addition to slew rate and linearity, there are five main performance specifications for audio amplifiers and preamplifiers: frequency response, output gain, power, number of channels, and nominal impedance. Other parameters to specify include gain, bandwidth, efficiency, noise, output dynamic range, slew rate, rise time, settling time and ringing, overshoot, and stability.

References

Techlib.com – Audio Amplifiers

How Stuff Works – How Amplifiers Work