Transistor-based amplifiers are lighter in weight, more reliable and require less maintenance than tube amplifiers. Since the 1970s, most modern audio amplifiers are based on solid-state transistors, especially the bipolar junction transistor (BJT) and the metalвЂ“oxideвЂ“semiconductor field-effect transistor (MOSFET). Early audio power amplifiers were based on vacuum tubes and some of these achieved notably high audio quality (e.g., the Williamson amplifier of 1947вЂ“9).ДЄudio power amplifiers based on transistors became practical with the wide availability of inexpensive transistors in the late 1960s. The triode vacuum amplifier was used to make the first AM radio. The triode was a three terminal device with a control grid that can modulate the flow of electrons from the filament to the plate. The audio amplifier was invented around 1912 by Lee de Forest, made possible by his invention of the first practical amplifying electrical component, the triode vacuum tube (or 'valve' in British English) in 1907. While power amplifiers are available in standalone units, typically aimed at the hi-fi audiophile market (a niche market) of audio enthusiasts and sound reinforcement system professionals, most consumer electronics audio products, such as clock radios, boomboxes and televisions have relatively small power amplifiers that are integrated into the product.Дe Forest's prototype audio amplifier of 1914. While the input signal to an audio power amplifier, such as the signal from an electric guitar, may measure only a few hundred microwatts, its output may be a few watts for small consumer electronics devices, such as clock radios, tens or hundreds of watts for a home stereo system, several thousand watts for a nightclub's sound system or tens of thousands of watts for a large rock concert sound reinforcement system. Most audio power amplifiers require these low-level inputs, which are line level. The inputs can also be any number of audio sources like record players, CD players, digital audio players and cassette players.

The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification of the signal (this is particularly associated with record turntable signals, microphone signals and electric instrument signals from pickups, such as the electric guitar and electric bass), equalization (e.g., adjusting the bass and treble), tone controls, mixing different input signals or adding electronic effects such as reverb. It is the final electronic stage in a typical audio playback chain before the signal is sent to the loudspeakers. Audio power amplifiers are found in all manner of sound systems including sound reinforcement, public address and home audio systems and musical instrument amplifiers like guitar amplifiers. The internal view of a Mission Cyrus 1 Hi Fi integrated audio amplifier (1984) ДЄn audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspeakers or headphones.

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