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An audio power amplifier (or power amp) amplifies low-power electronic , such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving or headphones. Audio power amplifiers are found in all manner of sound systems, including sound reinforcement, public address, home audio systems and musical instrument amplifiers like . It is the final electronic stage in a typical audio playback signal chain before the signal is sent to the loudspeakers.
The preceding stages in such a chain are low-power audio amplifiers, which perform tasks like preamplifier of the signal, equalization, mixing console. The inputs can also be any number of audio sources like , , digital audio players and . Most audio power amplifiers require these low-level inputs, which are line level.
While the input signal to an audio power amplifier, such as the signal from an electric guitar, may measure only a few hundred watt, its output may be a few watts for small consumer electronics devices, such as , 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. 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, many consumer electronics audio products such as an integrated amplifier, a receiver, clock radios, and have both a preamplifier and a power amplifier contained in a single chassis.
Audio power amplifiers based on became practical with the wide availability of inexpensive transistors in the late 1960s. Since the 1970s, most modern audio amplifiers are based on solid-state transistors, especially the bipolar junction transistor (BJT) and the metal–oxide–semiconductor field-effect transistor (MOSFET). Transistor-based amplifiers are lighter in weight, more reliable and require less maintenance than .
The MOSFET was invented at Bell Labs between 1955 and 1960. (2026). 9783540342588, Springer Science & Business Media. ISBN 9783540342588 was adapted into a power MOSFET for audio by Jun-ichi Nishizawa at Tohoku University in 1974. Power MOSFETs were soon manufactured by Yamaha for their hi-fi audio amplifiers. JVC, Pioneer Corporation, Sony and Toshiba also began manufacturing amplifiers with power MOSFETs in 1974. In 1977, Hitachi introduced the LDMOS (lateral diffused MOS), a type of power MOSFET. Hitachi was the only LDMOS manufacturer between 1977 and 1983, during which time LDMOS was used in audio power amplifiers from manufacturers such as HH Electronics (V-series) and Ashly Audio, and were used for music and public address systems. Class-D amplifiers became successful in the mid-1980s when low-cost, fast-switching MOSFETs were made available. Many transistor amps use MOSFET devices in their power sections, because their distortion curve is more tube sound.
In the 2010s, there are still audio enthusiasts, musicians (particularly , , Hammond organ players and Fender Rhodes electric piano players, among others), audio engineers and who prefer tube-based amplifiers, and what is perceived as a "warmer" tube sound.
Until the 1970s, most amplifiers used . During the 1970s, tube amps were increasingly replaced with transistor-based amplifiers, which were lighter in weight, more reliable, and lower maintenance. Nevertheless, tube preamplifiers are still sold in , such as with home hi-fi enthusiasts, and (who use tube preamplifiers in studio recordings to "warm up" microphone signals) and electric guitarists, electric bassists and Hammond organ players, of whom a minority continue to use tube preamps, tube power amps and tube . While hi-fi enthusiasts and audio engineers doing live sound or monitoring tracks in the studio typically seek out amplifiers with the lowest distortion, electric instrument players in genres such as blues, rock music and heavy metal music, among others, use tube amplifiers because they like the natural overdrive that tube amps produce when pushed hard.
The class-D amplifier, which is much more efficient than class-AB amplifiers, is now widely used in consumer electronics audio products, and sound reinforcement system gear, as class-D amplifiers are much lighter in weight and produce much less heat, but may introduce some noise.
TIM did not appear at steady state sine tone measurements, helping to hide it from design engineers prior to 1970. Problems with TIM distortion stem from reduced open-loop frequency response of solid-state amplifiers. Further works of Otala and other authors found the solution for TIM distortion, including increasing slew rate, decreasing preamp frequency bandwidth, and the insertion of a lag compensation circuit in the input stage of the amplifier.Discussion of practical design features that can provoke or lessen slew-rate limiting and transient intermodulation in audio amplifiers can also be found for example in In high-quality modern amplifiers, the open loop response is at least 20 kHz, canceling TIM distortion.
The next step in advanced design was the Baxandall Theorem, created by Peter Baxandall in England.Baxandall, Peter (February 1979) "Audio power amplifier design" , Wireless World magazine This theorem introduced the concept of comparing the ratio between the input distortion and the output distortion of an amplifier. This new idea helped audio design engineers to better evaluate the distortion processes within an amplifier.
Power amplifiers are available in standalone units, which are used by hi-fi audio enthusiasts and designers of public address systems (PA systems) and sound reinforcement systems. A hi-fi user of power amplifiers may have a stereo power amplifier to drive left and right speakers and a single-channel (mono) power amplifier to drive a subwoofer. The number of power amplifiers used in a sound reinforcement setting depends on the size of the venue. A small coffeehouse may have a single power amp driving two PA speakers. A nightclub may have several power amps for the main speakers, one or more power amps for the monitor speakers (pointing towards the band) and an additional power amp for the subwoofer. A stadium concert may have a large number of power amps mounted in racks. Most consumer electronics sound products, such as TVs, , home cinema sound systems, Casio and Yamaha electronic keyboards, guitar amplifier and car stereos have power amplifiers integrated inside the chassis of the main product.
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