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An audio power amplifier is an electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between , the human range of hearing) to a level suitable for driving loudspeakers. It is the final electronic 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 (this is particularly associated with record turntable signals), equalization, tone controls, mixing/effects, or audio sources like record players, CD players, and cassette players. Most audio power amplifiers require these low-level inputs to adhere to line levels. While the input signal to an audio power amplifier may measure only a few hundred microwatts, its output may be tens or hundreds of watts for a home stereo system or thousands or tens of thousands of watts for a concert sound reinforcement system. == History == 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.〔http://nobelprize.org/educational_games/physics/transistor/history/ The Transistor in a Century of Electronics〕 Early audio power amplifiers were based on vacuum tubes (also known as ''valves''), and some of these achieved notably high quality (e.g., the Williamson amplifier of 1947-9). Most modern audio amplifiers are based on solid state devices (transistors such as BJTs, FETs and MOSFETs), but there are still some who prefer tube-based amplifiers, and the valve sound. Audio power amplifiers based on transistors became practical with the wide availability of inexpensive transistors in the late 1960s. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Audio power amplifier」の詳細全文を読む スポンサード リンク
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