A guitar amplifier (or guitar amp) is an electronic amplifier designed to amplify the electrical signal of an electric or acoustic guitar so that it will produce sound through a loudspeaker. Most guitar amplifiers can also modify the instruments tone by emphasizing or de-emphasizing certain frequencies and adding electronic effects. Vibrations of the strings are "picked up" by a suitable microphone. For electric guitars, strings are all made of metal, and the pickup works by electro-magnetic induction. Acoustic guitars have a normal microphone, designed to convert acoustic vibrations into an electrical signal, but usually they do so from direct contact with the strings (replacing the guitars bridge) or with the guitars body, rather than having a membrane like general-purpose microphones.
Amplifiers consist of one or more circuit stages which have unique responsibilities in the modification of the input signal. The power amplifier or output stage produces a high current signal to drive a speaker to produce sound. This means that the signal does not really increase in amplitude, but it has a higher energy content. One or more preamplifier stages precede the power amplifier stage. The preamplifier is a voltage amplifier that amplifies the guitar signal to a level that can drive the power stage: the signal is made larger, but without significantly increasing its energy content. There may be one or more tone stages which affect the character of the guitar signal: before the preamp stage (as in the case of guitar pedals), in between the preamp and power stages (as in the cases of effects loop or many dedicated amplifier tone circuits), in between multiple stacked preamp stages, or in feedback loops from a post-preamp signal to an earlier pre-preamp signal (as in the case of presence modifier circuits). The tone stages may also have electronic effects such as equalization, compression, distortion, chorus, or reverb. Amplifiers may use vacuum tubes (in Britain they are called valves), or solid-state (transistor) devices, or both.
The first electric instrument amplifiers were not designed for use with electric guitars. The earliest examples appeared in the early 1930s when the introduction of electrolytic capacitors and rectifier tubes allowed the production of economical built-in power supplies that could be plugged into wall sockets, instead of heavy multiple battery packs, since rechargeable batteries wouldnt be lightweight until later on. While guitar amplifiers from the beginning were used to amplify acoustic guitar, electronic amplification of guitar was first widely popularized by the 1930s and 1940s craze for Hawaiian music, which extensively employed the amplified lap steel Hawaiian guitar.
Tone controls on early guitar amplifiers were very simple and provided a great deal of treble boost, but the limited controls, the loudspeakers used, and the low power of the amplifiers (typically 15 watts or less prior to the mid-1950s) gave poor high treble and bass output. Some models also provided effects such as spring reverb and/or an electronic tremolo unit. Early Fender amps labeled tremolo as "vibrato" and labeled the vibrato arm of the Stratocaster guitar as a "tremolo bar" (see vibrato unit, electric guitar, and tremolo).
In the 1950s, several guitarists experimented with distortion produced by deliberately overdriving their amplifiers, including Goree Carter, Joe Hill Louis, Ike Turner, Willie Johnson, Pat Hare, Guitar Slim, Chuck Berry, Johnny Burnette, and Link Wray. In the early 1960s, surf rock guitarist Dick Dale worked closely with Fender to produce custom made amplifiers, including the first 100-watt guitar amplifier. He pushed the limits of electric amplification technology, helping to develop new equipment that was capable of producing "thick, clearly defined tones" at "previously undreamed-of volumes."
Distortion became more popular from the mid-1960s, when The Kinks guitarist Dave Davies produced distortion effects by connecting the already distorted output of one amplifier into the input of another. Later, most guitar amps were provided with preamplifier distortion controls, and "fuzz boxes" and other effects units were engineered to safely and reliably produce these sounds. In the 2000s, overdrive and distortion has become an integral part of many styles of electric guitar playing, ranging from blues rock to heavy metal and hardcore punk.
Guitar amplifiers were at first used with bass guitars and electronic keyboards, but other instruments produce a wider frequency range and need a suitable amplifier and full-range speaker system. Much more amplifier power is required to reproduce low-frequency sound, especially at high volume. Reproducing low frequencies also requires a suitable woofer or subwoofer speaker and enclosure. Woofer enclosures need to be larger and more sturdily built than cabinets for mid-range or high-frequency (tweeter) speakers.
As said, guitar amplifiers are manufactured in two main forms.
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