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Anaudio power amplifier (orpower amp)amplifies low-power electronicaudio signals, such as the signal from aradio receiver or an electric guitarpickup, to a level that is high enough for drivingloudspeakers orheadphones. Audio power amplifiers are found in all manner of sound systems includingsound reinforcement,public address,home audio systems and musicalinstrument amplifiers likeguitar amplifiers. It is the final electronic stage in a typical audio playbackchain before the signal is sent to the loudspeakers.
The preceding stages in such a chain are low-power audio amplifiers which perform tasks likepre-amplification of the signal,equalization,mixing different input signals. The inputs can also be any number of audio sources likerecord players,CD players,digital audio players andcassette players. Most audio power amplifiers require these low-level inputs, which areline level.
While the input signal to an audio power amplifier, such as the signal from an electric guitar, may measure only a few hundredmicrowatts, its output may be a few watts for small consumer electronics devices, such asclock radios, tens or hundreds of watts for ahome stereo system, several thousand watts for anightclub'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 thehi-fi audiophile market (a niche market) of audio enthusiasts and sound reinforcement system professionals, manyconsumer electronics audio products such as anintegrated amplifier, areceiver, clock radios,boomboxes andtelevisions have both a preamplifier and a power amplifier contained in a single chassis.
The audio amplifier was invented around 1912 byLee de Forest. This was made possible by his invention of the first practical amplifying electrical component, thetriodevacuum tube (or "valve" in British English) in 1907. Thetriode was a three-terminal device with a control grid that can modulate the flow of electrons from the filament to the plate. The triodevacuum amplifier was used to make the firstAM radio.[2] Early audio power amplifiers were based on vacuum tubes and some of these achieved notably high audio quality (e.g., theWilliamson amplifier of 1947–9).
Audio power amplifiers based ontransistors became practical with the wide availability of inexpensive transistors in the late 1960s. Since the 1970s, most modern audio amplifiers are based onsolid-state transistors, especially thebipolar junction transistor (BJT) and themetal–oxide–semiconductor field-effect transistor (MOSFET). Transistor-based amplifiers are lighter in weight, more reliable and require less maintenance thantube amplifiers.
The MOSFET was invented atBell Labs between 1955 and 1960.[3][4][5][6][7][8] was adapted into apower MOSFET for audio byJun-ichi Nishizawa atTohoku University in 1974.[9] Power MOSFETs were soon manufactured byYamaha for theirhi-fi audio amplifiers.JVC,Pioneer Corporation,Sony andToshiba also began manufacturing amplifiers with power MOSFETs in 1974.[9] In 1977,Hitachi introduced theLDMOS (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 asHH Electronics (V-series) andAshly Audio, and were used for music andpublic address systems.[9]Class-D amplifiers became successful in the mid-1980s when low-cost, fast-switching MOSFETs were made available.[10] Many transistor amps use MOSFET devices in theirpower sections, because theirdistortion curve is moretube-like.[11]
In the 2010s, there are still audio enthusiasts, musicians (particularlyelectric guitarists,electric bassists,Hammond organ players andFender Rhodeselectric piano players, among others), audio engineers andmusic producers who prefer tube-based amplifiers, and what is perceived as a "warmer"tube sound.
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Key design parameters for audio power amplifiers arefrequency response,gain,noise, anddistortion. These are interdependent; increasing gain often leads to undesirable increases in noise and distortion. Whilenegative feedback actually reduces the gain, it also reduces distortion. Most audio amplifiers are linear amplifiers operating inclass AB.
Until the 1970s, most amplifiers usedvacuum tubes. During the 1970s, tube amps were increasingly replaced withtransistor-based amplifiers, which were lighter in weight, more reliable, and lower maintenance. Nevertheless, tube preamplifiers are still sold inniche markets, such as with homehi-fi enthusiasts,audio engineers andmusic producers (who use tube preamplifiers in studio recordings to "warm up" microphone signals) and electric guitarists, electric bassists andHammond organ players, of whom a minority continue to use tube preamps, tube power amps and tubeeffects units. 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 asblues,rock music andheavy metal music, among others, use tube amplifiers because they like the natural overdrive that tube amps produce when pushed hard.
Theclass-D amplifier, which is much more efficient than class-AB amplifiers, is now widely used inconsumer electronics audio products,bass amplifiers andsound reinforcement system gear, as class-D amplifiers are much lighter in weight and produce much less heat, but may introduce some noise.
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 –discuss] One alternative to a separate preamp is to simply use passive volume and switching controls, sometimes integrated into a power amplifier to form anintegrated amplifier.
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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 insingle-ended triode amplifiers) or devices (forpush-pull output stages), such as theclass 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 atransformerless output stage, the devices are essentially in series with the power supply and outputload (such as a loudspeaker), possibly via some large capacitor and/or small resistances.
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 (seevalve audio amplifier). This ledaudiophiles to believe that "tube sound" orvalve 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: transientintermodulation distortion (TIM),[12] later also calledslew-induced distortion (SID) by others.[13] TIM distortion was found to occur during very rapid increases in amplifier output voltage.[14]
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 increasingslew rate, decreasing preamp frequency bandwidth, and the insertion of a lag compensation circuit in the input stage of the amplifier.[15][16][17] 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.[18] 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.
Important applications includepublic address systems, theatrical and concertsound reinforcement systems, and domestic systems such as astereo orhome-theatre system.Instrument amplifiers includingguitar amplifiers and electrickeyboard amplifiers 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 separaterackmount 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 ofpublic address systems (PA systems) andsound 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 asubwoofer. The number of power amplifiers used in a sound reinforcement setting depends on the size of the venue. A smallcoffeehouse may have a single power amp driving two PA speakers. Anightclub may have several power amps for the main speakers, one or more power amps for themonitor 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. Mostconsumer electronics sound products, such asTVs,boom boxes,home cinema sound systems, Casio and Yamahaelectronic keyboards,"combo" guitar amps and car stereos have power amplifiers integrated inside the chassis of the main product.
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