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An audio power amplifier (or power amp) is an
electronic amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It may increase the power (physics ...
that amplifies low-power electronic
audio signal An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of r ...
s, such as the signal from a
radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...
or an electric guitar pickup, to a level that is high enough for driving
loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or " ...
s or
headphones Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound. Headphones let a single user listen to an au ...
. Audio power amplifiers are found in all manner of sound systems including
sound reinforcement A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sound ...
,
public address A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
and
home audio Home audio systems are audio electronics intended for home entertainment use, such as shelf stereos, music centres and surround sound receivers. Home audio generally does not include standard equipment such as built-in television speakers, but ra ...
systems and musical
instrument amplifier An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical ins ...
s like
guitar amplifier A guitar amplifier (or amp) is an electronic device or system that strengthens the electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which a ...
s. It is the final electronic stage in a typical audio playback
chain A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A c ...
before the signal is sent to the loudspeakers. The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification of the signal (this is particularly associated with
record turntable A phonograph, in its later forms also called a gramophone (as a trademark since 1887, as a generic name in the UK since 1910) or since the 1940s called a record player, or more recently a turntable, is a device for the mechanical and analogu ...
signals,
microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
signals and electric instrument signals from pickups, such as the electric guitar and electric bass), equalization (e.g., adjusting the bass and treble), tone controls, mixing different input signals or adding electronic effects such as
reverb Reverberation (also known as reverb), in acoustics, is a persistence of sound, after a sound is produced. Reverberation is created when a sound or signal is reflected causing numerous reflections to build up and then decay as the sound is abso ...
. The inputs can also be any number of audio sources like
record player A phonograph, in its later forms also called a gramophone (as a trademark since 1887, as a generic name in the UK since 1910) or since the 1940s called a record player, or more recently a turntable, is a device for the mechanical and analogu ...
s,
CD player A CD player is an electronic device that plays audio compact discs, which are a digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such as music or aud ...
s,
digital audio player A portable media player (PMP) (also including the related digital audio player (DAP)) is a portable consumer electronics device capable of storing and playing digital media such as audio, images, and video files. The data is typically stored o ...
s and
cassette player A cassette deck is a type of tape machine for playing and recording audio cassettes that does not have a built-in power amplifier or speakers, and serves primarily as a transport. It can be a part of an automotive entertainment system, a part of a ...
s. Most audio power amplifiers require these low-level inputs, which are
line level Line level is the specified strength of an audio signal used to transmit analog audio between components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles. Line level sits between other levels of audio signals. ...
. While the input signal to an audio power amplifier, such as the signal from an electric guitar, may measure only a few hundred microwatts, its output may be a few watts for small consumer electronics devices, such as
clock radio An alarm clock (or sometimes just an alarm) is a clock that is designed to alert an individual or group of individuals at a specified time. The primary function of these clocks is to awaken people from their night's sleep or short naps; they ar ...
s, tens or hundreds of watts for a
home stereo system An entertainment center (or centre), also known as an entertainment complex, is a piece of furniture designed to house consumer electronic appliances and components, such as televisions. Evolution of home TV and stereo A stereo console (or "con ...
, several thousand watts for a
nightclub A nightclub (music club, discothèque, disco club, or simply club) is an entertainment venue during nighttime comprising a dance floor, lightshow, and a stage for live music or a disc jockey (DJ) who plays recorded music. Nightclubs gener ...
's sound system or tens of thousands of watts for a large rock concert
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sounds ...
. 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 A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sounds ...
professionals, most
consumer electronics Consumer electronics or home electronics are electronic (analog or digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications and recreation. Usually r ...
audio products, such as
clock radio An alarm clock (or sometimes just an alarm) is a clock that is designed to alert an individual or group of individuals at a specified time. The primary function of these clocks is to awaken people from their night's sleep or short naps; they ar ...
s,
boombox A boombox is a transistorized portable music player featuring one or two cassette tape recorder/players and AM/FM radio, generally with a carrying handle. Beginning in the mid 1980s, a CD player was often included. Sound is delivered through ...
es and
television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
s have relatively small power amplifiers that are integrated into the product.


History

The audio amplifier was invented around 1912 by
Lee de Forest Lee de Forest (August 26, 1873 – June 30, 1961) was an American inventor and a fundamentally important early pioneer in electronics. He invented the first electronic device for controlling current flow; the three-element "Audion" triode va ...
, made possible by his invention of the first practical amplifying electrical component, the
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 19 ...
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
(or "valve" in British English) in 1907. The
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 19 ...
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 AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transm ...
. Early audio power amplifiers were based on vacuum tubes and some of these achieved notably high audio quality (e.g., the
Williamson amplifier The Williamson amplifier is a four-stage, push-pull, Class A triode-output valve audio power amplifier designed by D. T. N. Williamson during World War II. The original circuit, published in 1947 and addressed to the worldwide do it yourself c ...
of 1947–9). Audio power amplifiers based on
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s 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 Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their use ...
transistors, especially the
bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar ...
(BJT) and the
metal–oxide–semiconductor field-effect transistor The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(MOSFET). Transistor-based amplifiers are lighter in weight, more reliable and require less maintenance than
tube amplifier A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by sol ...
s. The
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
, invented by Mohamed Atalla and
Dawon Kahng Dawon Kahng ( ko, 강대원; May 4, 1931 – May 13, 1992) was a Korean-American electrical engineer and inventor, known for his work in solid-state electronics. He is best known for inventing the MOSFET (metal–oxide–semiconductor field-effe ...
at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mult ...
in 1959, was adapted into a
power MOSFET A power MOSFET is a specific type of metal–oxide–semiconductor field-effect transistor (MOSFET) designed to handle significant power levels. Compared to the other power semiconductor devices, such as an insulated-gate bipolar transistor (IG ...
for audio by
Jun-ichi Nishizawa was a Japanese engineer and inventor. He is known for his electronic inventions since the 1950s, including the PIN diode, static induction transistor, static induction thyristor, SIT/SITh. His inventions contributed to the development of in ...
at
Tohoku University , or is a Japanese national university located in Sendai, Miyagi in the Tōhoku Region, Japan. It is informally referred to as . Established in 1907, it was the third Imperial University in Japan and among the first three Designated National ...
in 1974. Power MOSFETs were soon manufactured by
Yamaha Yamaha may refer to: * Yamaha Corporation, a Japanese company with a wide range of products and services, established in 1887. The company is the largest shareholder of Yamaha Motor Company (below). ** Yamaha Music Foundation, an organization estab ...
for their hi-fi audio amplifiers.
JVC JVC (short for Japan Victor Company) is a Japanese brand owned by JVCKenwood corporation. Founded in 1927 as the Victor Talking Machine Company of Japan and later as , the company is best known for introducing Japan's first televisions and for ...
,
Pioneer Corporation commonly referred to as Pioneer, is a Japanese multinational corporation based in Tokyo, that specializes in digital entertainment products. The company was founded by Nozomu Matsumoto in January 1, 1938 in Tokyo as a radio and speaker repair s ...
,
Sony , commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
and
Toshiba , commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. Its diversified products and services include power, industrial and social infrastructure system ...
also began manufacturing amplifiers with power MOSFETs in 1974. In 1977,
Hitachi () is a Japanese multinational corporation, multinational Conglomerate (company), conglomerate corporation headquartered in Chiyoda, Tokyo, Japan. It is the parent company of the Hitachi Group (''Hitachi Gurūpu'') and had formed part of the Ni ...
introduced the
LDMOS LDMOS (laterally-diffused metal-oxide semiconductor) is a planar double-diffused MOSFET (metal–oxide–semiconductor field-effect transistor) used in amplifiers, including microwave power amplifiers, RF power amplifiers and audio power amplif ...
(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 HH Electronics is a British amplifier manufacturer, that was founded in 1968 by Mike Harrison, Malcolm Green and Graham Lowes in Harston near Cambridge, England, where its first solid state TPA and MA range of studio quality amplifiers were desig ...
(V-series) and Ashly Audio, and were used for music and
public address system A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
s.
Class-D amplifier A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, and not as linear gain devices as in other amplifiers. They operate by rapidly ...
s became successful in the mid-1980s when low-cost, fast-switching MOSFETs were made available. Many transistor amps use MOSFET devices in their
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
sections, because their
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
curve is more tube-like. In the 2010s, there are still audio enthusiasts, musicians (particularly
electric guitar An electric guitar is a guitar that requires external amplification in order to be heard at typical performance volumes, unlike a standard acoustic guitar (however combinations of the two - a semi-acoustic guitar and an electric acoustic gui ...
ists,
electric bass The bass guitar, electric bass or simply bass (), is the lowest-pitched member of the string family. It is a plucked string instrument similar in appearance and construction to an electric or an acoustic guitar, but with a longer neck The ...
ists,
Hammond organ The Hammond organ is an electric organ invented by Laurens Hammond and John M. Hanert and first manufactured in 1935. Multiple models have been produced, most of which use sliding drawbars to vary sounds. Until 1975, Hammond organs generated s ...
players and
Fender Rhodes The Rhodes piano (also known as the Fender Rhodes piano) is an electric piano invented by Harold Rhodes, which became popular in the 1970s. Like a conventional piano, the Rhodes generates sound with keys and hammers, but instead of strings, th ...
electric piano An electric piano is a musical instrument which produces sounds when a performer presses the keys of a piano-style musical keyboard. Pressing keys causes mechanical hammers to strike metal strings, metal reeds or wire tines, leading to vibrations ...
players, among others), audio engineers and
music producer A record producer is a recording project's creative and technical leader, commanding studio time and coaching artists, and in popular genres typically creates the song's very sound and structure.Virgil Moorefield"Introduction" ''The Producer as ...
s who prefer tube-based amplifiers, and what is perceived as a "warmer"
tube sound Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube amplifier (valve amplifier in British English), a vacuum tube-based audio amplifier. At first, the concept of ''tube sound'' did not exist, because practically ...
.


Design parameters

Key design parameters for audio power amplifiers are
frequency response In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and phase of the output as a function of input frequency. The frequency response is widely used in the design and analysis of sy ...
,
gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...
,
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
, and
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
. These are interdependent; increasing gain often leads to undesirable increases in noise and distortion. While
negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...
actually reduces the gain, it also reduces distortion. Most audio amplifiers are linear amplifiers operating in
class AB In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active ...
. Until the 1970s, most amplifiers used
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
s. During the 1970s, tube amps were increasingly replaced with
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
-based amplifiers, which were lighter in weight, more reliable, and lower maintenance. Nevertheless, tube preamplifiers are still sold in
niche market A niche market is the subset of the market on which a specific product is focused. The market niche defines the product features aimed at satisfying specific market needs, as well as the price range, production quality and the demographics that it ...
s, such as with home hi-fi enthusiasts,
audio engineer An audio engineer (also known as a sound engineer or recording engineer) helps to produce a recording or a live performance, balancing and adjusting sound sources using equalization, dynamics processing and audio effects, mixing, reproduction, ...
s and
music producer A record producer is a recording project's creative and technical leader, commanding studio time and coaching artists, and in popular genres typically creates the song's very sound and structure.Virgil Moorefield"Introduction" ''The Producer as ...
s (who use tube preamplifiers in studio recordings to "warm up" microphone signals) and electric guitarists, electric bassists and
Hammond organ The Hammond organ is an electric organ invented by Laurens Hammond and John M. Hanert and first manufactured in 1935. Multiple models have been produced, most of which use sliding drawbars to vary sounds. Until 1975, Hammond organs generated s ...
players, of whom a minority continue to use tube preamps, tube power amps and tube
effects unit An effects unit or effects pedal is an electronic device that alters the sound of a musical instrument or other audio source through audio signal processing. Common effects include distortion/overdrive, often used with electric guitar in el ...
s. 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 Blues is a music genre and musical form which originated in the Deep South of the United States around the 1860s. Blues incorporated spirituals, work songs, field hollers, shouts, chants, and rhymed simple narrative ballads from the Afr ...
,
rock music Rock music is a broad genre of popular music that originated as " rock and roll" in the United States in the late 1940s and early 1950s, developing into a range of different styles in the mid-1960s and later, particularly in the United States an ...
and
heavy metal music Heavy metal (or simply metal) is a genre of rock music that developed in the late 1960s and early 1970s, largely in the United Kingdom and United States. With roots in blues rock, psychedelic rock and acid rock, heavy metal bands developed a ...
, among others, use tube amplifiers because they like the natural overdrive that tube amps produce when pushed hard. The
Class-D amplifier A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, and not as linear gain devices as in other amplifiers. They operate by rapidly ...
, which is much more efficient than Class AB amplifiers, is now widely used in
consumer electronics Consumer electronics or home electronics are electronic (analog or digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications and recreation. Usually r ...
audio products,
bass amplifier A bass amplifier (also abbreviated to bass amp) is a musical instrument electronic device that uses electrical power to make lower-pitched instruments such as the bass guitar or double bass loud enough to be heard by the performers and audien ...
s and
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sounds ...
gear, as Class D amplifiers are much lighter in weight and produce much less heat.


Filters and preamplifiers

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 a volume control and source selector. One alternative to a separate preamp is to simply use passive volume and switching controls, sometimes integrated into a power amplifier to form an
integrated amplifier An integrated amplifier (pre/main amp) is an electronic device containing an audio preamplifier and power amplifier in one unit, as opposed to separating the two. Most modern audio amplifiers are integrated and have several inputs for devices such ...
.


Power output stages

The final stage of amplification, after preamplifiers, is the output stage, where the highest demands are placed on the transistors or tubes. For this reason, the design choices made around the output device (for single-ended output stages, such as in
single-ended triode A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted sig ...
amplifiers) or devices (for push-pull output stages), such as the Class of operation of the output devices is often taken as the description of the whole power amplifier. For example, a Class B amplifier will probably have just the high power output devices operating cut off for half of each cycle, while the other devices (such as differential amplifier, voltage amplifier and possibly even driver transistors) operate in Class A. In a transformerless output stage, the devices are essentially in series with the power supply and output
load Load or LOAD may refer to: Aeronautics and transportation *Load factor (aeronautics), the ratio of the lift of an aircraft to its weight *Passenger load factor, the ratio of revenue passenger miles to available seat miles of a particular transpo ...
(such as a loudspeaker), possibly via some large capacitor and/or small resistances.


Further developments

For some years following the introduction of solid state amplifiers, their perceived sound did not have the excellent audio quality of the best valve amplifiers (see
valve audio amplifier A valve audio amplifier ( UK) or vacuum tube audio amplifier ( US) is a valve amplifier used for sound reinforcement, sound recording and reproduction. Until the invention of solid state devices such as the transistor, all electronic amplifica ...
). This led
audiophile An audiophile is a person who is enthusiastic about high-fidelity sound reproduction. An audiophile seeks to reproduce the sound of a piece of recorded music or a live musical performance, typically inside closed headphones, In-ear monitors, open ...
s to believe that "tube sound" or
valve sound Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube amplifier (valve amplifier in British English), a vacuum tube-based audio amplifier. At first, the concept of ''tube sound'' did not exist, because practically ...
had an intrinsic quality due to the vacuum tube technology itself. In 1970, Matti Otala published a paper on the origin of a previously unobserved form of distortion: transient
intermodulation distortion Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of Signal (electrical engineering), signals containing two or more different frequencies, caused by non-linear, nonlinearities or time variance in a system. ...
(TIM), later also called
slew-induced distortion Slew-induced distortion (SID or slew-rate induced distortion) is caused when an amplifier or transducer is required to change output (or displacement), i.e. slew rate, faster than it is able to do so without error. At such times any other signals ma ...
(SID) by others. TIM distortion was found to occur during very rapid increases in amplifier output voltage. 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 In electronics, slew rate is defined as the change of voltage or current, or any other electrical quantity, per unit of time. Expressed in SI units, the unit of measurement is volts/second or amperes/second, but is usually expressed in terms of m ...
, decreasing preamp frequency bandwidth, and the insertion of a lag compensation circuit in the input stage of the amplifier. 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.


Applications

Important applications include
public address A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
systems, theatrical and concert
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sounds ...
s, and domestic systems such as a
stereo Stereophonic sound, or more commonly stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration ...
or home-theatre system.
Instrument amplifier An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical ins ...
s including
guitar amplifier A guitar amplifier (or amp) is an electronic device or system that strengthens the electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which a ...
s and electric
keyboard amplifier A keyboard amplifier is a powered electronic amplifier and loudspeaker in a wooden speaker cabinet used for amplification of electronic keyboard instruments. Keyboard amplifiers are distinct from other types of amplification systems such as g ...
s also use audio power amplifiers. In some cases, the power amplifier for an instrument amplifier is integrated into a single amplifier "head" which contains a preamplifier, tone controls, and electronic effects. These components may be mounted in a wooden speaker cabinet to create a "combo amplifier". Musicians with unique performance needs and/or a need for very powerful amplification may create a custom setup with separate
rackmount A 19-inch rack is a standardized frame or enclosure for mounting multiple electronic equipment modules. Each module has a front panel that is wide. The 19 inch dimension includes the edges or "ears" that protrude from each side of the equ ...
preamplifiers, equalizers, and a power amplifier mounted in a 19"
road case A road case, ATA case or flight case is a shipping container specifically built to protect musical instruments, motion picture equipment, audio and lighting production equipment, properties, firearms, or other sensitive equipment when it must be ...
. Power amplifiers are available in standalone units, which are used by hi-fi audio enthusiasts and designers of
public address system A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
s (PA systems) and
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sounds ...
s. 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 A subwoofer (or sub) is a loudspeaker designed to reproduce low-pitched audio frequencies known as bass and sub-bass, lower in frequency than those which can be (optimally) generated by a woofer. The typical frequency range for a subwoofer i ...
. The number of power amplifiers used in a sound reinforcement setting depends on the size of the venue. A small
coffeehouse A coffeehouse, coffee shop, or café is an establishment that primarily serves coffee of various types, notably espresso, latte, and cappuccino. Some coffeehouses may serve cold drinks, such as iced coffee and iced tea, as well as other non-ca ...
may have a single power amp driving two PA speakers. A
nightclub A nightclub (music club, discothèque, disco club, or simply club) is an entertainment venue during nighttime comprising a dance floor, lightshow, and a stage for live music or a disc jockey (DJ) who plays recorded music. Nightclubs gener ...
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 Consumer electronics or home electronics are electronic (analog or digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications and recreation. Usually r ...
sound products, such as TVs,
boom box A boombox is a transistorized portable music player featuring one or two cassette tape recorder/players and AM/FM radio, generally with a carrying handle. Beginning in the mid 1980s, a CD player was often included. Sound is delivered through ...
es,
home cinema Home cinema, also called home theaters or theater rooms, are home entertainment audio-visual systems that seek to reproduce a movie theater experience and mood using consumer electronics-grade video and audio equipment that is set up in a room ...
sound systems, Casio and Yamaha
electronic keyboard An electronic keyboard, portable keyboard, or digital keyboard is an electronic musical instrument, an electronic derivative of keyboard instruments. Electronic keyboards include synthesizers, digital pianos, stage pianos, electronic organs an ...
s, "combo" guitar amps and car stereos have power amplifiers integrated inside the chassis of the main product.


See also

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FET amplifier A FET amplifier is an amplifier that uses one or more field-effect transistors (FETs). The most common type of FET amplifier is the MOSFET amplifier, which uses metal–oxide–semiconductor FETs (MOSFETs). The main advantage of a FET used for a ...
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Instrument amplifier An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical ins ...
(amplifiers for musical instruments) *
Push–pull output A push–pull amplifier is a type of electronic circuit that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load. This kind of amplifier can enhance both the load capacity and switching s ...
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Single-ended triode A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted sig ...
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Tone control circuit Tone control is a type of equalization used to make specific pitches or " frequencies" in an audio signal softer or louder. It allows a listener to adjust the tone of the sound produced by an audio system to their liking, for example to compensa ...
s


References

{{Authority control Audio electronics Sound reinforcement system Consumer electronics Electric guitars