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{{Short description|Audio amplifier with power output sufficient to drive a loudspeaker}}
{{Redirect-distinguish|Power amplifier|RF power amplifier}}
{{more citations needed|date=April 2019}}
[[File:Mcintosh MC-2300 Front.jpg|thumb|right|250px|Audio stereo power amplifier made by [[McIntosh Laboratory|McIntosh]]]]
[[File:MissionCyrus1-2.JPG|thumb|right|250px| The internal view of a Mission Cyrus One hi-fi [[integrated amplifier]] (1984)<ref>{{cite web|url=https://bestforacar.com/integrated-amplifier-all-versions/ |title=1 – Integrated Amplifier (All Versions) |access-date=2011-01-16 |url-status=dead |archive-url=https://web.archive.org/web/20110424050529/http://www.cyrusaudio.com/product-archive/amps/1-integrated-amplifier-all-versions |archive-date=2011-04-24 }} Cyrus Audio: Product Archive: Cyrus One</ref>]]

An '''audio power amplifier''' (or '''power amp''') [[electronic amplifier|amplifies]] low-power electronic [[audio signal]]s, such as the signal from a [[radio receiver]] or an electric guitar [[pickup (music technology)|pickup]], to a level that is high enough for driving [[loudspeaker]]s or [[headphones]]. Audio power amplifiers are found in all manner of sound systems including [[sound reinforcement]], [[public address]], [[home audio]] systems and musical [[instrument amplifier]]s like [[guitar amplifier]]s.  It is the final electronic stage in a typical audio playback [[signal chain|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|pre-amplification]] of the signal, [[equalization (audio)|equalization]], [[mixing console|mixing different input signals]]. The inputs can also be any number of audio sources like [[record player]]s, [[CD player]]s, [[digital audio player]]s and [[cassette player]]s. 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|microwatts]], its output may be a few watts for small consumer electronics devices, such as [[clock radio]]s, 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 (radio)|receiver]], clock radios, [[boombox]]es and [[television]]s have both a preamplifier and a power amplifier contained in a single chassis.

== History ==
[[File:First Audion amplifier 1914.jpg|thumb|left|upright=0.6|De Forest's prototype audio amplifier of 1914]]

The audio amplifier was invented around 1912 by [[Lee de Forest]]. This was made possible by his invention of the first practical amplifying electrical component, the [[triode]] [[vacuum tube]] (or "valve" in British English) in 1907. The [[triode]] was a three-terminal device with a control grid that can modulate the flow of electrons from the filament to the plate. The triode [[Valve amplifier|vacuum amplifier]] was used to make the first [[AM radio]].<ref>[https://web.archive.org/web/20080705150057/http://nobelprize.org/educational_games/physics/transistor/history/ The Transistor in a Century of Electronics]. nobelprize.org</ref> 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).
[[File:Mcintosh-MC240-glow.jpg|thumb|right|300px|[[McIntosh Labs|McIntosh]] MC240 from 1961, with exposed vacuum tubes]]

Audio power amplifiers based on [[transistor]]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 electronics|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 amplifier]]s.

The MOSFET was invented at [[Bell Labs]] between 1955 and 1960.<ref>{{Cite patent|number=US2802760A|title=Oxidation of semiconductive surfaces for controlled diffusion|gdate=1957-08-13|invent1=Lincoln|invent2=Frosch|inventor1-first=Derick|inventor2-first=Carl J.|url=https://patents.google.com/patent/US2802760A}}</ref><ref>{{Cite journal |last1=Frosch |first1=C. J. |last2=Derick |first2=L |date=1957 |title=Surface Protection and Selective Masking during Diffusion in Silicon |url=https://iopscience.iop.org/article/10.1149/1.2428650 |journal=Journal of the Electrochemical Society |language=en |volume=104 |issue=9 |pages=547 |doi=10.1149/1.2428650|url-access=subscription }}</ref><ref name="Lojek1202">{{cite book |last1=Lojek |first1=Bo |title=History of Semiconductor Engineering |date=2007 |publisher=[[Springer Science & Business Media]] |isbn=9783540342588 |page=120}}</ref><ref>{{Cite journal |last=KAHNG |first=D. |date=1961 |title=Silicon-Silicon Dioxide Surface Device |url=https://doi.org/10.1142/9789814503464_0076 |journal=Technical Memorandum of Bell Laboratories|pages=583–596 |doi=10.1142/9789814503464_0076 |isbn=978-981-02-0209-5 |url-access=subscription }}</ref><ref>{{Cite book |last=Lojek |first=Bo |title=History of Semiconductor Engineering |date=2007 |publisher=Springer-Verlag Berlin Heidelberg |isbn=978-3-540-34258-8 |location=Berlin, Heidelberg |page=321}}</ref><ref>{{Cite journal |last1=Ligenza |first1=J.R. |last2=Spitzer |first2=W.G. |date=1960 |title=The mechanisms for silicon oxidation in steam and oxygen |url=https://linkinghub.elsevier.com/retrieve/pii/0022369760902195 |journal=Journal of Physics and Chemistry of Solids |language=en |volume=14 |pages=131–136 |doi=10.1016/0022-3697(60)90219-5|bibcode=1960JPCS...14..131L |url-access=subscription }}</ref> was adapted into a [[power MOSFET]] for audio by [[Jun-ichi Nishizawa]] at [[Tohoku University]] in 1974.<ref name="Duncan177">{{cite book |last1=Duncan |first1=Ben |title=High Performance Audio Power Amplifiers |date=1996 |publisher=[[Elsevier]] |isbn=9780080508047 |pages=[https://cucdaycongsuat.com.vn/ 177–8, 406] |url=https://cucdaycongsuat.com.vn/ }}</ref> Power MOSFETs were soon manufactured by [[Yamaha Corporation|Yamaha]] for their [[hi-fi]] audio amplifiers. [[JVC]], [[Pioneer Corporation]], [[Sony]] and [[Toshiba]] also began manufacturing amplifiers with power MOSFETs in 1974.<ref name="Duncan177"/> 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 system]]s.<ref name="Duncan177"/> [[Class-D amplifier]]s became successful in the mid-1980s when low-cost, fast-switching MOSFETs were made available.<ref>{{cite book |last=Duncan |first=Ben |date=1996 |title=High Performance Audio Power Amplifiers |publisher=Newnes |pages=147–148 |isbn=9780750626293 }}</ref> Many transistor amps use MOSFET devices in their [[power electronics|power]] sections, because their [[distortion (music)|distortion]] curve is more [[tube sound|tube-like]].<ref>{{cite book |last1=Fliegler |first1=Ritchie |last2=Eiche |first2=Jon F. |title=Amps! The Other Half of Rock 'n' Roll |date=1993 |publisher=[[Hal Leonard Corporation]] |isbn=9780793524112 |url=https://books.google.com/books?id=NgG8bmBayLwC&pg=PA19}}</ref>

In the 2010s, there are still audio enthusiasts, musicians (particularly [[electric guitar]]ists, [[electric bass]]ists, [[Hammond organ]] players and [[Fender Rhodes]] [[electric piano]] players, among others), audio engineers and [[music producer]]s who prefer tube-based amplifiers, and what is perceived as a "warmer" [[tube sound]].
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== Design parameters ==
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[[File:Professional PA power amps from Crest Audio and QSC with control modules in rack.jpg|thumb|upright=0.8|Five rack-mounted audio power amplifiers used in a [[sound reinforcement system]]]]
Key design parameters for audio power amplifiers are [[frequency response]], [[gain (electronics)|gain]], [[noise (electronics)|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 distortion. Most audio amplifiers are linear amplifiers operating in [[Power amplifier classes|class AB]].

Until the 1970s, most amplifiers used [[vacuum tube]]s. 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 [[niche market]]s, such as with home [[hi-fi enthusiasts]], [[audio engineer]]s and [[music producer]]s (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 [[effects unit]]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]], [[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, [[bass amplifier]]s and [[sound reinforcement system]] 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.{{dubious|Restructing of the Entire Article is needed|date=October 2025}} 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==
[[File:Micro amplifier (booster) FIIO Fujiyama for smartphone etc. headphones.jpg|thumb|A micro audio amplifier for boosting the output of [[line level]] sources to [[headphone]]s or small speakers. Edge length 4{{nbsp}}cm, mass {{val|16|ul=g}}, power output about 100{{nbsp}}mW into a 32{{nbsp}}Ω load.]] 

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|push-pull]] output stages), such as the [[Amplifier#Power amplifier classes|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 [[Output transformerless|transformerless output stage]], the devices are essentially in series with the power supply and output [[Electrical load|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 [[audiophile]]s 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),<ref>{{Cite journal | doi = 10.1109/TAU.1970.1162117| title = Transient distortion in transistorized audio power amplifiers| journal = IEEE Transactions on Audio and Electroacoustics| volume = 18| issue = 3| pages = 234–239| year = 1970| last1 = Otala | first1 = M.| bibcode = 1970ITAuE..18..234O| s2cid = 13952562}}</ref> later also called [[slew-induced distortion]] (SID) by others.<ref>{{cite journal|author=Jung, Walter G.; Stephens, Mark L. and Todd, Craig C. |date=June 1979|title=An overview of SID and TIM|journal=Audio}}</ref> TIM distortion was found to occur during very rapid increases in amplifier output voltage.<ref>{{cite journal|title=Circuit Design Modifications for Minimizing Transient Intermodulation Distortion in Audio Amplifiers|url=http://www.aes.org/e-lib/browse.cfm?elib=2065|author=Otala, Matti |journal=[[Journal of the Audio Engineering Society]]|volume=20 |issue= 5|date= June 1972}}</ref>

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.<ref>{{cite journal|title=Distribution of the Phonograph Signal Rate of Change|url=https://www.aes.org/e-lib/online/browse.cfm?elib=2906|author=Lammasniemi, Jorma; Nieminen, Kari |journal=[[Journal of the Audio Engineering Society]]|volume= 28 |issue= 5|date= May 1980}}</ref><ref>{{cite journal|title=Psychoacoustic Detection Threshold of Transient Intermodulation Distortion|url=https://www.aes.org/e-lib/browse.cfm?elib=4002|author1=Petri-Larmi, M. |author2=Otala, M. |author3=Lammasniemi, J. |journal=[[Journal of the Audio Engineering Society]]|volume=28|issue=3|date=March 1980}}</ref><ref>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 {{cite book|chapter=Ch. 9|author=Hood, John Linsley |title=The Art of Linear Electronics|isbn=978-0-7506-0868-8|doi=10.1016/B978-0-7506-0868-8.50013-8|publisher=Butterworth-Heinemann|place= Oxford|year= 1993}}</ref> In high-quality modern amplifiers the open loop response is at least 20&nbsp;kHz, canceling TIM distortion.

The next step in advanced design was the Baxandall Theorem, created by Peter Baxandall in England.<ref>Baxandall, Peter (February 1979) [http://hifisonix.com/audio-power-amplifier-design-peter-j-baxandall/baxandall_audio-power-amplifier-design/ "Audio power amplifier design"] {{Webarchive|url=https://web.archive.org/web/20220609031924/http://hifisonix.com/audio-power-amplifier-design-peter-j-baxandall/baxandall_audio-power-amplifier-design/ |date=2022-06-09 }}, ''Wireless World magazine''</ref> 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 ==
[[File:Rear panel of mixing console and amplifiers at concert of band 'Dead Can Dance' in Barcelona, May 2022.jpg|thumb|Rear panel of a medium-sized [[sound reinforcement system]] located at one side of the stage at a pop concert. The setup includes the [[mixing console]] for the [[sound engineer]] (standing behind) and the power amplifiers which are partly stacked in a 19-inch rack on the right.]]

Important applications include [[public address]] systems, theatrical and concert [[sound reinforcement system]]s, and domestic systems such as a [[High fidelity|stereo]] or [[Home cinema|home-theatre system]]. [[Instrument amplifier]]s including [[guitar amplifier]]s and electric [[keyboard amplifier]]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 audio enthusiasts and designers of [[public address system]]s (PA systems) and [[sound reinforcement system]]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]] may have several power amps for the main speakers, one or more power amps for the [[Foldback (sound engineering)|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 [[TV]]s, [[boom box]]es, [[home cinema]] sound systems, Casio and Yamaha [[electronic keyboard]]s, [[guitar amplifier|"combo" guitar amps]] and car stereos have power amplifiers integrated inside the chassis of the main product.

== See also ==
* [[FET amplifier]]
* {{anl|Instrument amplifier}}
* [[OCL amplifier]]
* [[Push–pull output]]
* [[Single-ended triode]]

==References==
{{Reflist}}

{{Commons category|Audio power amplifiers}}

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[[Category:Audio amplifiers| ]]
[[Category:Audio electronics]]
[[Category:Sound reinforcement system]]
[[Category:Consumer electronics]]
[[Category:Electric guitars]]