Power amplifier
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An audio power amplifier (or power amp) 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 signal (signal processing), digital signals. Audio signals have frequencies i ...
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. ...
or an electric guitar pickup, to a level that is high enough for driving
loudspeaker A loudspeaker (commonly referred to as a speaker or, more fully, a speaker system) is a combination of one or more speaker drivers, an enclosure, and electrical connections (possibly including a crossover network). The speaker driver is an ...
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 ...
. Audio power amplifiers are found in all manner of sound systems including sound reinforcement,
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 ...
, home audio systems and musical
instrument amplifier An instrument amplifier is an electronic amplifier 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 music ...
s like guitar amplifiers. It is the final electronic stage in a typical audio playback 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 pre-amplification of the signal, equalization, mixing different input signals. The inputs can also be any number of audio sources like
record player A phonograph, later called a gramophone, and since the 1940s a record player, or more recently a turntable, is a device for the mechanical and analogue reproduction of sound. The sound vibration Waveform, waveforms are recorded as correspond ...
s,
CD player A CD player is an electronic device that plays audio compact discs, which are a digital audio, digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such a ...
s, digital audio players and cassette players. 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 microwatts, its output may be a few watts for small consumer electronics devices, such as clock radios, tens or hundreds of watts for a home stereo system, several thousand watts for a
nightclub A nightclub or dance club is a club that is open at night, usually for drinking, dancing and other entertainment. Nightclubs often have a Bar (establishment), bar and discotheque (usually simply known as disco) with a dance floor, laser lighti ...
'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 High fidelity (hi-fi or, rarely, HiFi) is the high-quality reproduction of sound. It is popular with audiophiles and home audio enthusiasts. Ideally, high-fidelity equipment has inaudible noise and distortion, and a flat (neutral, uncolored) ...
audiophile market (a niche market) of audio enthusiasts and sound reinforcement system professionals, many
consumer electronics Consumer electronics, also known as home electronics, are electronic devices intended for everyday household use. Consumer electronics include those used for entertainment, Communication, communications, and recreation. Historically, these prod ...
audio products such as an integrated amplifier, a receiver, clock radios, boomboxes and
television Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...
s have both a preamplifier and a power amplifier contained in a single chassis.


History

The audio amplifier was invented around 1912 by
Lee de Forest #REDIRECT Lee de Forest {{redirect category shell, {{R from move{{R from other capitalisation ...
. This was made possible by his invention of the first practical amplifying electrical component, the
triode A triode is an electronic amplifier, amplifying vacuum tube (or ''thermionic valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated Electrical filament, filament or cathode, a control grid, grid ...
vacuum tube A vacuum tube, electron tube, thermionic 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. It ...
(or "valve" in British English) in 1907. The
triode A triode is an electronic amplifier, amplifying vacuum tube (or ''thermionic valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated Electrical filament, filament or cathode, a control grid, grid ...
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") transmi ...
. Early audio power amplifiers were based on vacuum tubes and some of these achieved notably high audio quality (e.g., the Williamson amplifier of 1947–9). Audio power amplifiers based on
transistor A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...
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 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 tube amplifiers. The MOSFET was invented at
Bell Labs Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, Murray Hill, New Jersey, the compa ...
between 1955 and 1960. 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 High fidelity (hi-fi or, rarely, HiFi) is the high-quality reproduction of sound. It is popular with audiophiles and home audio enthusiasts. Ideally, high-fidelity equipment has inaudible noise and distortion, and a flat (neutral, uncolored) ...
audio amplifiers. JVC, Pioneer Corporation,
Sony is a Japanese multinational conglomerate (company), conglomerate headquartered at Sony City in Minato, Tokyo, Japan. The Sony Group encompasses various businesses, including Sony Corporation (electronics), Sony Semiconductor Solutions (i ...
and
Toshiba is a Japanese multinational electronics company headquartered in Minato, Tokyo. Its diversified products and services include power, industrial and social infrastructure systems, elevators and escalators, electronic components, semiconductors ...
also began manufacturing amplifiers with power MOSFETs in 1974. In 1977,
Hitachi () is a Japanese Multinational corporation, multinational Conglomerate (company), conglomerate founded in 1910 and headquartered in Chiyoda, Tokyo. The company is active in various industries, including digital systems, power and renewable ener ...
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 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 electric Guitar amplifier, sound amplification in order to be heard at typical performance volumes, unlike a standard acoustic guitar. It uses one or more pickup (music technology), pickups ...
ists, electric bassists,
Hammond organ The Hammond organ is an electric organ invented by Laurens Hammond and John M. Hanert, first manufactured in 1935. Multiple models have been produced, most of which use sliding #Drawbars, drawbars to vary sounds. Until 1975, sound was created ...
players and Fender Rhodes electric piano players, among others), audio engineers and
music producer A record producer or music producer is a music creating project's overall supervisor whose responsibilities can involve a range of creative and technical leadership roles. Typically the job involves hands-on oversight of recording sessions; ensu ...
s who prefer tube-based amplifiers, and what is perceived as a "warmer" tube sound.


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 (waves), phase of the output as a function of input frequency. The frequency response is widely used in the design and ...
, gain,
noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...
, 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 ...
actually reduces the gain, it also reduces distortion. Most audio amplifiers are linear amplifiers operating in class AB. Until the 1970s, most amplifiers used
vacuum tube A vacuum tube, electron tube, thermionic 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. It ...
s. During the 1970s, tube amps were increasingly replaced with
transistor A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...
-based amplifiers, which were lighter in weight, more reliable, and lower maintenance. Nevertheless, tube preamplifiers are still sold in niche markets, such as with home
hi-fi High fidelity (hi-fi or, rarely, HiFi) is the high-quality reproduction of sound. It is popular with audiophiles and home audio enthusiasts. Ideally, high-fidelity equipment has inaudible noise and distortion, and a flat (neutral, uncolored) ...
enthusiasts, audio engineers and
music producer A record producer or music producer is a music creating project's overall supervisor whose responsibilities can involve a range of creative and technical leadership roles. Typically the job involves hands-on oversight of recording sessions; ensu ...
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, first manufactured in 1935. Multiple models have been produced, most of which use sliding #Drawbars, drawbars to vary sounds. Until 1975, sound was created ...
players, of whom a minority continue to use tube preamps, tube power amps and tube
effects unit An effects unit, effects processor, 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 (music), distortion/overdrive, ...
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 that originated among African Americans in the Deep South of the United States around the 1860s. Blues has incorporated spiritual (music), spirituals, work songs, field hollers, Ring shout, shouts, cha ...
,
rock music Rock is a Music genre, genre of popular music that originated in the United States as "rock and roll" in the late 1940s and early 1950s, developing into a range of styles from the mid-1960s, primarily in the United States and the United Kingdo ...
and
heavy metal music Heavy metal (or simply metal) is a Music genre, 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 band ...
, 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 Consumer electronics, also known as home electronics, are electronic devices intended for everyday household use. Consumer electronics include those used for entertainment, Communication, communications, and recreation. Historically, these prod ...
audio products, bass amplifiers and
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in Loudspeaker enclosure, enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also ...
gear, as class-D amplifiers are much lighter in weight and produce much less heat, but may introduce some noise.


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.


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 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 (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). This led audiophiles to believe that "tube sound" or valve sound 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 (TIM), later also called slew-induced distortion (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, 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 Loudspeaker enclosure, enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also ...
s, and domestic systems such as a stereo or home-theatre system.
Instrument amplifier An instrument amplifier is an electronic amplifier 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 music ...
s including guitar amplifiers and electric
keyboard amplifier A keyboard amplifier is a powered instrument amplifier, electronic amplifier and loudspeaker in a speaker enclosure, speaker cabinet used for the amplification of electronic keyboard instruments. Keyboard amplifiers are distinct from other type ...
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 preamplifiers, equalizers, and a power amplifier mounted in a 19" road case. Power amplifiers are available in standalone units, which are used by
hi-fi High fidelity (hi-fi or, rarely, HiFi) is the high-quality reproduction of sound. It is popular with audiophiles and home audio enthusiasts. Ideally, high-fidelity equipment has inaudible noise and distortion, and a flat (neutral, uncolored) ...
audio enthusiasts and designers of public address systems (PA systems) and
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in Loudspeaker enclosure, enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also ...
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. 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 A nightclub or dance club is a club that is open at night, usually for drinking, dancing and other entertainment. Nightclubs often have a Bar (establishment), bar and discotheque (usually simply known as disco) with a dance floor, laser lighti ...
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, also known as home electronics, are electronic devices intended for everyday household use. Consumer electronics include those used for entertainment, Communication, communications, and recreation. Historically, these prod ...
sound products, such as TVs, boom boxes,
home cinema A home cinema, also called home theater, is a home entertainment audio-visual system that seeks to reproduce a movie theater experience and mood using consumer grade electronic video and audio equipment and is set up in a private home. In ...
sound systems, Casio and Yamaha electronic keyboards, "combo" guitar amps and car stereos have power amplifiers integrated inside the chassis of the main product.


See also

* FET amplifier * * OCL amplifier * Push–pull output * Single-ended triode * Tone control circuits


References

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