Audio power amplifier
An audio power amplifier (or power amp) is an electronic amplifier that amplifies or strengthens very low-power electronic audio signals, such as the signal from radio receiver (signals composed primarily of frequencies between 20 - 20,000 Hz, the human range of hearing) to a level that is strong enough for driving (or powering) loudspeakers. Power amplifiers make the signal--whether it is recorded music, a live speech, or an electric guitar--audible to listeners. 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 (this is particularly associated with record turntable signals, microphone signals and electric instruments, 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. The inputs can 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 to adhere to line levels.
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 and sound reinforcement system professionals, most consumer electronics sound products, such as televisions and car stereos, have power amplifiers integrated inside the chassis of the main product.
The audio amplifier was invented in 1909 by Lee De Forest when he invented the triode vacuum tube (or "valve" in British English). 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.
Early audio power amplifiers were based on vacuum tubes and some of these achieved notably high quality (e.g., the Williamson amplifier of 1947-9). Most modern audio amplifiers are based on solid state devices (transistors such as BJTs, FETs and MOSFETs), but there are still some who prefer tube-based amplifiers, and the valve sound. Audio power amplifiers based on transistors became practical with the wide availability of inexpensive transistors in the late 1960s.
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.
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 amplifiuer, 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.
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. 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 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 "monoblock" single channel 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, electronic keyboards, "combo" guitar amps and car stereos have power amplifiers integrated inside the chassis of the main product.
- "Archived copy". Archived from the original on 2011-04-24. Retrieved 2011-01-16. Cyrus Audio: Product Archive: Cyrus One
- http://nobelprize.org/educational_games/physics/transistor/history/ The Transistor in a Century of Electronics
- Otala, M. (1970). "Transient distortion in transistorized audio power amplifiers". IEEE Transactions on Audio and Electroacoustics. 18 (3): 234. doi:10.1109/TAU.1970.1162117.
- Walter G. Jung, Mark L. Stephens, and Craig C. Todd (June 1979), "An overview of SID and TIM", Audio
- "Circuit Design Modifications for Minimizing Transient Intermodulation Distortion in Audio Amplifiers", Matti Otala, Journal of Audio Engineering Society, Vol 20 # 5, June 1972
- Distribution of the Phonograph Signal Rate of Change, Lammasniemi, Jorma; Nieminen, Kari, Journal of Audio Engineering Society, Vol. 28 # 5, May 1980.
- "Psychoacoustic Detection Threshold of Transient Intermodulation Distortion", Petri-Larmi, M.; Otala, M.; Lammasniemi, J. Journal of Audio Engineering Society, Vol 28 # 3, March 1980
- 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 chapter 9 in John Linsley Hood's 'The Art of Linear Electronics' (Butterworth-Heinemann, Oxford, 1993).
- "Audio power amplifier design", Peter Baxandall. Wireless World magazine, February 1979
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