An audio amplifier is an electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 hertz to 20,000 hertz, the human range of hearing) to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.
The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification, equalization, tone control, mixing/effects, or audio sources like record players, CD players, and cassette players. Most audio amplifiers require these low-level inputs to adhere to line levels.
While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens, hundreds, or thousands of watts.
Early audio amplifiers were based on vacuum tubes (also known as "valves"). Most modern audio amplifiers are based on solid state devices like transistors, FETs and MOSFETs, but there are still aficionados who prefer tube-based amplifiers, due to a perceived 'warmer' valve sound. Audio amplifiers based on transistors became practical with the wide availability of inexpensive transistors in the late 1960s.
Key design parameters for audio 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 noise, and distortion. Most audio amplifiers are linear amplifiers operating in class AB.
Historically, the majority of commercial audio preamplifiers made had complex filter circuits for equalization and tone adjustment, due to the far from ideal quality of recordings, playback technology, and speakers of the day.
Using today's high quality (often digital) source material and speakers etc, such filter circuits are usually not needed. Audiophiles generally agree that filter circuits are to be avoided wherever possible. Today's audiophile amplifiers do not have tone controls or filters.
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 volume control. One alternative to a separate preamp is to simply use passive volume and switching controls, sometimes integrated into a power amp to form an "integrated" amplifier.
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 valve sound had an intrinsic quality due to the vacuum tube technology itself. In 1972, Matti Otala demonstrated the origin of a previously unobserved form of distortion: Transitory Intermodulation Distortion (TIM), also called "slew rate distortion". 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 1972. 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. However, TIM distortion is still present in most low price home quality amplifiers.
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 audio amplifier. This new idea helped audio design engineers to better evaluate the distortion processes within an audio amplifier.
In 1980, a further improvement by Oscar Bonello at the University of Buenos Aires reduced amplifier distortion by employing "Double Loop Feedback" circuitry. This technology led to solid state amplifier designs which could achieve far better distortion measurements than valve amplifiers, at low cost and with high power. At the same time, Bonello proposed using poles and zeros at the feedback network to get a 9 dB/octave slope instead of the traditional 6 dB/octave. This allowed an audio amplifier to be designed without any perceived distortion in the treble spectrum.
Amplifiers often include operational amplifiers and filters. Key to designing linear amplifiers is the examination and evaluation of the distortion introduced by the Distortion Multiplication Factor (Kd). Optimizing the behavior of this type of operational amplifier is important to achieving low distortion amplifiers and audio consoles for sound recording and reproduction.
Since the mid-1950s, LP phonograph records have been mastered using RIAA equalization, in which the dynamics of the recording have been altered so that the amplitude of the signal that has been cut into the record increases with increasing frequency. Equalization helps to mask the high frequency noise ("hiss") that is generated as the pickup's stylus rubs against the groove walls. The RIAA curve also attenuates the bass, which reduces the maximum excursions of the stylus to a practical level during loud passages. This has the desirable effect of reducing distortion, as well as making the grooves narrower and increasing the potential maximum recording time per record side. Also, with less excursion, less stress is applied to the stylus, which helps to reduce record wear.
During playback, the RIAA curve is reversed by preamplification, resulting in nearly flat frequency response. It should also be noted that the preamplifier is employed to boost the weak signal emitted by a magnetic pickup. Piezoelectric pickups generally produce much higher output voltages and seldom require preamplification.
Prior to the adoption of the RIAA curve, a number of competing and partially incompatible equalization schemes were utilized during record mastering. Early high fidelity systems often had an equalization selector switch to match playback characteristics to the recording curve of the particular label being played. The development and acceptance of the RIAA curve eliminated this requirement.
Important applications include public address systems, theatrical and concert sound reinforcement, and domestic sound systems. The sound card in a personal computer contains several audio amplifiers (depending on number of channels), as does every stereo or home-theatre system.