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1. rui_mats

   29|Nov|2009 1

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   An audio amplifier is an electronic amplifier that works with audio frequencies (generally 20 hertz to 20,000 hertz).
   Overview
   An audio power amplifier is the final stage in the audio playback chain and its purpose is to amplify the electrical audio signal from its preceding stage to a level that can drive the loudspeaker(s). The preceding stages are low power audio amplifiers which perform tasks like pre-amplification, equalisation, tone control, mixing/effects, etc or audio sources like record players, CD players, cassette players, etc. By “low power” it is meant that the signals are usually limited to those required to drive headphones (less than 500 mW).
   While the input signal may be only a few hundred millivolts, the power amplifier’s output may have a power of many watts.
   Early audio amplifiers were based on vacuum tubes (also known as “valves”). Most modern audio amplifiers are based on solid state devices like transistors, FETs and MOSFETs, but there are still aficionados who prefer tube based amplifiers, as they have a ‘warmer’ sound. Audio amplifiers based on transistors became practical with the appearance of cheap transistors in the late 1960’s.
   Applications
   Important applications include public address systems, home and car sound systems. The sound card in a personal computer contains several audio amplifiers (depending on number of channels), as does every stereo or home-theatre system.
   Design parameters
   Key design parameters for audio amplifiers are frequency response, gain, noise, and distortion. These are interdependent, increasing gain often leads to undesirable increases in noise and distortion. While negative feedback actually reduces the gain, it also reduces noise, oscillation and distortion.
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