Audio amplifier

Mission Cyrus 1 Hi Fi integrated audio amplifier

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.

History

Three audio amplifiers in a smaller scale application

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.

Design parameters

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.

Filters and preamplifiers

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.

Further developments in amplifier design

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.^[1] 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.^[2][3] 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.^{[citation needed]}

The next step in advanced design was the Baxandall Theorem, created by Peter Baxandall in England.^[4] 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.^[5] 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).^[6] Optimizing the behavior of this type of operational amplifier is important to achieving low distortion amplifiers and audio consoles for sound recording and reproduction.

Phono (vinyl record) "equalization"

Phono pickups provide such a weak signal that preamps are necessary. The poor noise margin of the vinyl record has also resulted in the use of "equalization", where the treble and bass are recorded with different gains. It is therefore necessary to both boost and also correct this frequency response ("equalize") of a phono signal prior to feeding into the rest of the (flat) replay chain. In recent times the RIAA equalization has been standardized, but 78's and other earlier records used a large number of other equalizations.

Applications

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.

References

See also

Wikimedia Commons has media related to:

Audio amplifier

• Valve audio amplifier
• Valve sound
• Audiophile
• Single-ended triode
• Tone control circuits

External links

• Audio Design Guide