A preamplifier is an electronic amplifier which precedes another amplifier to prepare an electronic signal for further amplification or processing. The preamplifier circuitry may or may not be housed as a separate component.
In general, the function of a preamp is to amplify a low-level signal to line-level. A list of common sources would include a pickup, microphone, turntable or other transducer. Equalization and tone control may also be applied.
In a home audio system, the term 'preamplifier' may sometimes be used to describe equipment which merely switches between different line level sources and applies a volume control, so that no actual amplification may be involved. In an audio system, the second amplifier is typically a power amplifier (power amp). The preamplifier provides voltage gain (about: 10millivolts to 1volt) but no significant current gain. The power amplifier provides the higher current necessary to drive loudspeakers.