Audio Amplifier

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An audio amplifier is an electronic amplifier that amplifies low-power audio signal (information theory) (signals composed primarily of frequencies between 20 hertz to 20,000 hertz, the human range of hearing) to a level suitable for driving loudspeaker 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 preamplifier, equalization, Tone control circuits, mixing console, or audio sources like record players, CD players, and cassette players. Most audio amplifiers require these low-level inputs to adhere to line levels.

While the input signal to an audio amplifier may measure only a few hundred watt, its output may be tens, hundreds, or thousands of watts.

History Early audio amplifiers were based on vacuum tubes (also known as "valves"). Most modern audio amplifiers are based on solid state devices like transistors, field effect transistors and MOSFETs, but there are still aficionados who prefer tube-based amplifiers, due to a perceived 'warmer' valve sound. Audio amplifiers based on transistors became practical with the wide availability of inexpensive transistors in the late 1960s.

Design parameters Key design parameters for audio amplifiers are frequency response, gain, signal 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, and distortion. Most audio amplifiers are linear amplifiers operating in class AB.

Filters and preamplifiers Historically, the majority of commercial audio preamplifiers made had complex filter circuits for equalization and tone adjustment, due to the far from ideal quality of recordings, playback technology, and speakers of the day.

Using today's high quality (often digital) source material and speakers etc, such filter circuits are usually not needed. Audiophiles generally agree that filter circuits are to be avoided wherever possible. Today's audiophile amplifiers do not have tone controls or filters.

Since modern digital devices, including CD and DVD players, radio receivers and tape decks already provide a "flat" signal at line level, the preamp is not needed other than as volume control. One alternative to a separate preamp is to simply use passive volume and switching controls, sometimes integrated into a power amp to form an "integrated" amplifier.

Phono (vinyl record) "equalization" Phono pickups provide such a weak signal that preamps are necessary. The poor noise margin of the vinyl record has also resulted in the use of "equalization", where the treble and bass are recorded with different gains. It is therefore necessary to both boost and also correct this frequency response ("equalize") of a phono signal prior to feeding into the rest of the (flat) replay chain. In recent times the RIAA equalization has been standardized, but 78's and other earlier records used a large number of other equalizations.

Applications Important applications include public address systems, theatrical and concert sound reinforcement, and domestic 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.

See also

Valve audio amplifier
Valve sound
Audiophile
Single-ended triode
Tone control circuits

References "Circuits" by Ceapa and Constantin.

External links

Audio Design Guide

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