Audio power amplifier

Last updated January 23, 2024

The internal view of a Mission Cyrus 1 Hi Fi integrated audio amplifier (1984) MissionCyrus1-2.JPG — The internal view of a Mission Cyrus 1 Hi Fi integrated audio amplifier (1984)

An audio power amplifier (or power amp) amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspeakers 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 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 players, CD players, 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'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, boomboxes and televisions 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. 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 vacuum amplifier was used to make the first AM radio.^[2] 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).

McIntosh MC240 from 1961 with exposed vacuum tubes Mcintosh-MC240-glow.jpg — McIntosh MC240 from 1961 with exposed vacuum tubes

Audio power amplifiers based on transistors 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, invented by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959,^[3] was adapted into a power MOSFET for audio by Jun-ichi Nishizawa at Tohoku University in 1974.^[4] 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.^[4] 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.^[4] Class-D amplifiers became successful in the mid-1980s when low-cost, fast-switching MOSFETs were made available.^[5] Many transistor amps use MOSFET devices in their power sections, because their distortion curve is more tube-like.^[6]

In the 2010s, there are still audio enthusiasts, musicians (particularly electric guitarists, electric bassists, Hammond organ players and Fender Rhodes electric piano players, among others), audio engineers and music producers 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, gain, 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 class AB.

Until the 1970s, most amplifiers used vacuum tubes. 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 markets, such as with home hi-fi enthusiasts, audio engineers and music producers (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 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 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 amplifiers and sound reinforcement system 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.

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),^[7] later also called slew-induced distortion (SID) by others.^[8] TIM distortion was found to occur during very rapid increases in amplifier output voltage.^[9]

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.^[10]^[11]^[12] 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.^[13] 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 systems, theatrical and concert sound reinforcement systems, and domestic systems such as a stereo or home-theatre system. Instrument amplifiers including guitar amplifiers and electric keyboard 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 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 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, boom boxes, home cinema sound systems, Casio and Yamaha electronic keyboards, "combo" guitar amps and car stereos have power amplifiers integrated inside the chassis of the main product.

Related Research Articles

An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is defined as a circuit that has a power gain greater than one.

High fidelity 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 frequency response within the human hearing range.

<span class="mw-page-title-main">Instrument amplifier</span> Amplifier with loudspeaker for use with musical instruments

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 instruments such as an electric guitar, an electric bass, electric organ, electric piano, synthesizers and drum machine to convert the signal from the pickup or other sound source into an electronic signal that has enough power, due to being routed through a power amplifier, capable of driving one or more loudspeaker that can be heard by the performers and audience.

<span class="mw-page-title-main">Guitar amplifier</span> Electronic amplifier for musical instruments

A guitar amplifier 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 are typically housed in a wooden cabinet. A guitar amplifier may be a standalone wood or metal cabinet that contains only the power amplifier circuits, requiring the use of a separate speaker cabinet–or it may be a "combo" amplifier, which contains both the amplifier and one or more speakers in a wooden cabinet. There is a wide range of sizes and power ratings for guitar amplifiers, from small, lightweight "practice amplifiers" with a single 6-inch speaker and a 10-watt amp to heavy combo amps with four 10-inch or four 12-inch speakers and a 100-watt amplifier, which are loud enough to use in a nightclub or bar performance.

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 solid state amplifiers in the 1960s and 1970s. Valve amplifiers can be used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, high quality stereo amplifiers, military applications and very high power radio and UHF television transmitters.

<span class="mw-page-title-main">Preamplifier</span> Electronic amplifier that converts weak signal into strong signal

A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier and a loudspeaker. Without this, the final signal would be noisy or distorted. They are typically used to amplify signals from analog sensors such as microphones and pickups. Because of this, the preamplifier is often placed close to the sensor to reduce the effects of noise and interference.

A DI unit is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high output impedance unbalanced output signal to a low-impedance, microphone level, balanced input, usually via an XLR connector and XLR cable. DIs are frequently used to connect an electric guitar or electric bass to a mixing console's microphone input jack. The DI performs level matching, balancing, and either active buffering or passive impedance matching/impedance bridging. DI units are typically metal boxes with input and output jacks and, for more expensive units, “ground lift” and attenuator switches.

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 speed.

Founded by David Hafler and Ed Laurent in Philadelphia, Pennsylvania in 1955, Dynaco was an American hi-fi audio system manufacturer popular in the 1960s and 1970s for its wide range of affordable, yet high quality audio components.. Its best known product was the ST-70 tube stereo amplifier. They also manufactured other tube and solid state amplifiers, preamplifiers, radio tuners and bookshelf loudspeakers. Dynaco was liquidated in 1980, and the trademark is now owned by Radial Engineering Ltd.

A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices operate as electronic switches, and not as linear gain devices as in other amplifiers. They operate by rapidly switching back and forth between the supply rails, using pulse-width modulation, pulse-density modulation, or related techniques to produce a pulse train output. A simple low-pass filter may be used to attenuate their high-frequency content to provide analog output current and voltage. Little energy is dissipated in the amplifying transistors because they are always either fully on or fully off, so efficiency can exceed 90%.

A pentode is an electronic device having five electrodes. The term most commonly applies to a three-grid amplifying vacuum tube or thermionic valve that was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode was developed from the screen-grid tube or shield-grid tube by the addition of a grid between the screen grid and the plate. The screen-grid tube was limited in performance as an amplifier due to secondary emission of electrons from the plate. The additional grid is called the suppressor grid. The suppressor grid is usually operated at or near the potential of the cathode and prevents secondary emission electrons from the plate from reaching the screen grid. The addition of the suppressor grid permits much greater output signal amplitude to be obtained from the plate of the pentode in amplifier operation than from the plate of the screen-grid tube at the same plate supply voltage. Pentodes were widely manufactured and used in electronic equipment until the 1960s to 1970s, during which time transistors replaced tubes in new designs. During the first quarter of the 21st century, a few pentode tubes have been in production for high power radio frequency applications, musical instrument amplifiers, home audio and niche markets.

A valve audio amplifier (UK) or vacuum tube audio amplifier (US) is a valve amplifier used for sound reinforcement, sound recording and reproduction.

A bass amplifier 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 audience. Bass amps typically consist of a preamplifier, tone controls, a power amplifier and one or more loudspeakers ("drivers") in a cabinet.

A radio-frequency power amplifier is a type of electronic amplifier that converts a low-power radio-frequency (RF) signal into a higher-power signal. Typically, RF power amplifiers are used in the final stage of a radio transmitter, their output driving the antenna. Design goals often include gain, power output, bandwidth, power efficiency, linearity, input and output impedance matching, and heat dissipation.

Distortion and overdrive are forms of audio signal processing used to alter the sound of amplified electric musical instruments, usually by increasing their gain, producing a "fuzzy", "growling", or "gritty" tone. Distortion is most commonly used with the electric guitar, but may also be used with other electric instruments such as electric bass, electric piano, synthesizer and Hammond organ. Guitarists playing electric blues originally obtained an overdriven sound by turning up their vacuum tube-powered guitar amplifiers to high volumes, which caused the signal to distort. While overdriven tube amps are still used to obtain overdrive, especially in genres like blues and rockabilly, a number of other ways to produce distortion have been developed since the 1960s, such as distortion effect pedals. The growling tone of a distorted electric guitar is a key part of many genres, including blues and many rock music genres, notably hard rock, punk rock, hardcore punk, acid rock, and heavy metal music, while the use of distorted bass has been essential in a genre of hip hop music and alternative hip hop known as "SoundCloud rap".

Technical specifications and detailed information on the valve audio amplifier, including its development history.

<span class="mw-page-title-main">Tube sound</span> Characteristic quality of sounds from vacuum tube amplifiers

Tube sound is the characteristic sound associated with a vacuum tube amplifier, a vacuum tube-based audio amplifier. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. However, solid state amplifiers have been developed to be flawless and the sound is later regarded neutral compared to tube amplifiers. Thus the tube sound now means 'euphonic distortion.' The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.

Slew-induced distortion is caused when an amplifier or transducer is required to change output, i.e. slew rate, faster than it is able to do so without error. At such times any other signals may suffer considerable gain distortion, leading to intermodulation distortion. Transient intermodulation distortion may involve some degree of SID and/or distortion due to peak compression.

The NAD 3020 is a stereo integrated amplifier by NAD Electronics, considered to be one of the most important components in the history of high fidelity audio. Launched in 1978, this highly affordable product delivered a good quality sound, which acquired a reputation as an audiophile amplifier of exceptional value. By 1998, the NAD 3020 had become the most well known and best-selling audio amplifier in history.

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 first three classes are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. This metric is known as conduction angle (θ). A class A amplifier is conducting through all the period of the signal (θ=360°); Class B only for one-half the input period (θ=180°), class C for much less than half the input period (θ<180°). Class D amplifiers operate their output device in a switching manner; the fraction of the time that the device is conducting may be adjusted so a pulse-width modulation output can be obtained from the stage.

References

↑ "1 – Integrated Amplifier (All Versions)". Archived from the original on 2011-04-24. Retrieved 2011-01-16. Cyrus Audio: Product Archive: Cyrus One
↑ The Transistor in a Century of Electronics. nobelprize.org
↑ "Rethink Power Density with GaN". Electronic Design . 21 April 2017. Retrieved 23 July 2019.
1 2 3 Duncan, Ben (1996). High Performance Audio Power Amplifiers. Elsevier. pp. 177–8, 406. ISBN 9780080508047.
↑ Duncan, Ben (1996). High Performance Audio Power Amplifiers. Newnes. pp. 147–148. ISBN 9780750626293.
↑ Fliegler, Ritchie; Eiche, Jon F. (1993). Amps! The Other Half of Rock 'n' Roll. Hal Leonard Corporation. ISBN 9780793524112.
↑ Otala, M. (1970). "Transient distortion in transistorized audio power amplifiers". IEEE Transactions on Audio and Electroacoustics. 18 (3): 234–239. doi:10.1109/TAU.1970.1162117. S2CID 13952562.
↑ Jung, Walter G.; Stephens, Mark L. and Todd, Craig C. (June 1979). "An overview of SID and TIM". Audio.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Otala, Matti (June 1972). "Circuit Design Modifications for Minimizing Transient Intermodulation Distortion in Audio Amplifiers". Journal of the Audio Engineering Society . 20 (5).
↑ Lammasniemi, Jorma; Nieminen, Kari (May 1980). "Distribution of the Phonograph Signal Rate of Change". Journal of the Audio Engineering Society . 28 (5).{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Petri-Larmi, M.; Otala, M.; Lammasniemi, J. (March 1980). "Psychoacoustic Detection Threshold of Transient Intermodulation Distortion". Journal of the Audio Engineering Society . 28 (3).
↑ 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 Hood, John Linsley (1993). "Ch. 9". The Art of Linear Electronics. Oxford: Butterworth-Heinemann. doi:10.1016/B978-0-7506-0868-8.50013-8. ISBN 978-0-7506-0868-8.
↑ Baxandall, Peter (February 1979) "Audio power amplifier design" Archived 2022-06-09 at the Wayback Machine , Wireless World magazine

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[1] "1 – Integrated Amplifier (All Versions)". Archived from the original on 2011-04-24. Retrieved 2011-01-16. Cyrus Audio: Product Archive: Cyrus One

[2] The Transistor in a Century of Electronics. nobelprize.org

[3] "Rethink Power Density with GaN". Electronic Design . 21 April 2017. Retrieved 23 July 2019.

[Duncan177-4] 1 2 3 Duncan, Ben (1996). High Performance Audio Power Amplifiers. Elsevier. pp. 177–8, 406. ISBN 9780080508047.

[5] Duncan, Ben (1996). High Performance Audio Power Amplifiers. Newnes. pp. 147–148. ISBN 9780750626293.

[6] Fliegler, Ritchie; Eiche, Jon F. (1993). Amps! The Other Half of Rock 'n' Roll. Hal Leonard Corporation. ISBN 9780793524112.

[7] Otala, M. (1970). "Transient distortion in transistorized audio power amplifiers". IEEE Transactions on Audio and Electroacoustics. 18 (3): 234–239. doi:10.1109/TAU.1970.1162117. S2CID 13952562.

[8] Jung, Walter G.; Stephens, Mark L. and Todd, Craig C. (June 1979). "An overview of SID and TIM". Audio.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[9] Otala, Matti (June 1972). "Circuit Design Modifications for Minimizing Transient Intermodulation Distortion in Audio Amplifiers". Journal of the Audio Engineering Society . 20 (5).

[10] Lammasniemi, Jorma; Nieminen, Kari (May 1980). "Distribution of the Phonograph Signal Rate of Change". Journal of the Audio Engineering Society . 28 (5).{{cite journal}}: CS1 maint: multiple names: authors list (link)

[11] Petri-Larmi, M.; Otala, M.; Lammasniemi, J. (March 1980). "Psychoacoustic Detection Threshold of Transient Intermodulation Distortion". Journal of the Audio Engineering Society . 28 (3).

[12] 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 Hood, John Linsley (1993). "Ch. 9". The Art of Linear Electronics. Oxford: Butterworth-Heinemann. doi:10.1016/B978-0-7506-0868-8.50013-8. ISBN 978-0-7506-0868-8.

[13] Baxandall, Peter (February 1979) "Audio power amplifier design" Archived 2022-06-09 at the Wayback Machine , Wireless World magazine

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