An audio amplifier is electronic, and amplifies low-power audio signals (signals composed primarily of frequencies between 20 – 20,000 Hz, to a level suitable for driving with loudspeakers, and is the final stage in a typical audio playback chain. The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification, equalization, tone control, mixing/effects, or audio sources like record players, CD players, and cassette players. While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens, hundreds, or thousands of watts.
The audio amplifier was invented in 1909 by Lee De Forest when he invented the triode vacuum tube. 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 triode vacuum amplifier was used to make the first AM radio. Most modern audio amplifiers are based on solid state devices which are known as transistors. A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. Because the controlled output power can be much more than the controlling input power, a transistor can amplify a signal. Audio amplifiers based on transistors became practical with the wide availability of inexpensive transistors in the late 1960s.
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