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A guitar amplifier (or guitar amp) is an electronic amplifier designed to amplify the electrical signal of an electric guitar, bass guitar, or acoustic guitar so that it will produce sound through a loudspeaker. Most guitar amplifiers can also modify the instrument's tone by emphasizing or de-emphasizing certain frequencies and adding electronic effects, typically distortion and reverb. The input of modern guitar amplifiers is a 1/4" jack, which is fed a signal from a piezoelectric pickup (usually from an acoustic guitar) or an electro-magnetic one (from an electric guitar).
- 1 Structure
- 2 History
- 3 Types
- 4 Amplifier configuration
- 5 Distortion, power, and volume
- 6 See also
- 7 References
- 8 Further reading
- 9 External links
Typically, guitar amplifiers have two amplifying circuit stages, and in addition frequently have tone-shaping electric circuits. The first is a preamplifier stage (there may be more than one), which amplifies the guitar signal to a level that can drive the power stage. The power amplifier or output stage produces a high current signal to drive a speaker to produce sound.
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.
There are two configurations of guitar amplifiers: combination ("combo") amplifiers, which include an amplifier and one or more speakers in a wooden cabinet; and the standalone amplifier (often called a "head" or "amp head"), which does not include a speaker, but rather passes the signal to a speaker cabinet or "cab". Guitar amplifiers range in price and quality from small, low-powered practice amplifiers, designed for students, which sell for less than $50 USD, to expensive amplifiers which are custom-made for professional musicians and can cost thousands of dollars.
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 wouldn't 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 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). Some later models included an onboard spring reverb effect, one of the first being the Ampeg Reverberocket amp.
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.
Guitar amplifiers are manufactured in two main forms: a "combo" contains the amplifier and speaker(s) in a single unit. A separate configuration is available as well, with a separate amplifier (the "head") on top of one or more cabinets.
Besides an instrument input (typically a 1/4" jack), other jacks may also be provided, such as an additional input jack, "send" and "return" jacks to create an effects loop, an extension speaker jack. Practice amps may have stereo RCA or mini jacks for connecting a CD player, portable media player or other sound source and a 1/4" headphone jack.
A wide range of instrument amplifiers is available, some for general purposes and others designed for specific instruments or particular sounds.
Vacuum tube amplifiers
Vacuum tubes (valves) were by far the dominant active electronic components in most instrument amplifier applications until the 1970s, when semiconductors (transistors) started taking over for performance and economic reasons, including heat and weight reduction, and improved reliability. High-end tube instrument amplifiers have survived as one of few exceptions, because of the sound quality. Typically, one or more dual triodes are used in the preamplifier section in order to provide sufficient voltage gain to offset losses by tone controls and to drive the power amplifier section. While tube technology is in many ways outdated, they remain popular since many guitarists prefer the sound of tube amplifiers.
Most inexpensive guitar amplifiers are based on semiconductor (solid-state) circuits, and some designs incorporate tubes in the preamp stage for their subjectively warmer overdrive sound—see "Hybrid amplifiers", below. Solid-state amplifiers are much cheaper to produce and more reliable, and they are usually much lighter than tube amplifiers.
High-end solid-state amplifiers are less common, since many professional guitarists tend to favor vacuum tubes. Some jazz guitarists, however, tend to favor the "cleaner" sound of solid-state amplifiers; only a few solid-state amps have enduring attraction, such as the Roland Jazz Chorus. Solid-state amplifiers vary in output power, functionality, size, price, and sound quality in a wide range, from practice amplifiers to professional models.
A hybrid amplifier can involve one of two combination of tube and solid-state amplification. It may have a tube power amp fed by a solid-state pre-amp circuit, as in most of the original MusicMan Amps, later amplifier models from Alamo Electronics, the Fender Super Champ XD, and the Roland Bolt amplifier. Randall Amplifiers V2 and T2 use hybrid amp technology. Alternatively, a tube pre-amp can feed a solid-state output stage, as in models from Kustom, Hartke, SWR and Vox. This approach dispenses with the need for an output transformer and allow modern power levels to be easily achieved.
Microprocessor technology allows the use of digital onboard effects to create numerous different sounds all within the same amplifier. These are known as modeling amplifiers, and can be programmed with simulated characteristic tones of different existing amplifier models (and speaker cabinets—even microphone placement), or dialed in to the user's taste. Many amps of this type are also programmable by way of USB connection to a home computer. Line 6 is generally credited with bringing modeling amplification to the market.
What writers have called "Full Range Flat Response" (FRFR) amplification has received an extra impetus from modeling amplifiers. The basic concept of FRFR is that the tone is shaped by sound processors placed in the signal chain before the amplifier, but instead of a guitar amplifier, with its particular sound characteristics, a flat-frequency response amplification systems is used, such as amplified speakers or a PA system. Such processors can be traditional guitar effects, a modeling amplifier (without power amplifier), or a computer running tone-shaping software.
Acoustic guitar amplifiers
These amplifiers are designed to be used with acoustic guitars, especially for the way these instruments are used in relatively quiet genres such as folk and bluegrass. They are similar in many ways to keyboard amplifiers, in that they have a relatively flat frequency response, and they are usually designed so that neither the power amplifier nor the speakers will introduce additional coloration.
To produce this relatively "clean" sound, these amplifiers often have very powerful amplifiers (providing up to 800 watts RMS), to provide additional "headroom" and prevent unwanted distortion. Since an 800 watt amplifier built with standard Class AB technology would be very heavy, some acoustic amplifier manufacturers use lightweight Class D amplifiers, which are also called "switching amplifiers."
Acoustic amplifiers are designed to produce a "clean", transparent, "acoustic" sound when used with acoustic instruments with built-in transducer pickups and/or microphones. The amplifiers often come with a simple mixer, so that the signals from a pickup and microphone can be blended. Since the early 2000s, it has become increasingly common for acoustic amplifiers to be provided with a range of digital effects, such as reverb and compression. As well, these amplifiers often contain feedback-suppressing devices, such as notch filters or parametric equalizers.
An amplifier stack consists of an amplifier head atop a speaker cabinet—a head on top of one cabinet is commonly called a half stack, a head atop two cabinets a full stack. The cabinet which the head sits on often has an angled top in front, while the lower cabinet of a full stack has a straight front. The first version of the Marshall stack was an amp head on an 8x12 cabinet, meaning a single speaker cabinet containing eight 12" guitar speakers. After six of these cabinets were made, the cabinet arrangement was changed to an amp head on two 4x12 cabinets, meaning four 12" speakers, to enable transporting the amp rig.
Some touring bands have used the appearance of a large array of guitar amplifiers for aesthetic reasons. Some of these arrangements include only the fronts of amplifiers mounted on a large frame.
Distortion, power, and volume
For electric guitar amplifiers, there is often a distinction between "practice" or "recording studio" guitar amps, which tend to have output power ratings of 20 watts down to a small fraction of a watt, and "performance" amps, which are generally 50 watts or higher. Traditionally, these have been fixed-power amplifiers, with a few models having a half-power switch to slightly reduce the listening volume while preserving power-tube distortion.
The relationship between perceived volume and power output is not immediately obvious. A 5-watt amplifier is perceived to be half as loud as a 50-watt amplifier (a tenfold increase in power), and a half-watt amplifier is a quarter as loud as a 50-watt amp. Doubling the power of an amplifier results in a "just noticeable" increase in volume, so a 100-watt amplifier is held to be only just noticeably louder than a 50-watt amplifier. Such generalizations are also subject to the human ear's tendency to behave as a natural compressor at high volumes.
Power attenuation can be used with either low-power or high-power amplifiers, resulting in variable-power amplifiers. A high-power amplifier with power attenuation can produce power-tube distortion through a range of listening volumes, but with a decrease in high power distortion. Other technologies, such as dual rectifiers and the Sag Circuit—which should not be confused with attenuation—allow high power amplifiers to produce low power volume while preserving high power distortion.
Speaker efficiency is also a major factor affecting a tube amplifier's maximum volume. For bass instruments, higher-power amplifiers are needed to reproduce low-frequency sounds. While an electric guitarist would be able to play at a small club with a 50-watt amplifier, a bass player performing in the same venue would probably need an amplifier with 200 or more watts.
Distortion and volume
Distortion is a feature available on many guitar amplifiers that is not typically found on keyboard or bass guitar amplifiers. Tube guitar amplifiers can produce distortion through pre-distortion equalization, preamp tube distortion, post-distortion EQ, power-tube distortion, tube rectifier compression, output transformer distortion, guitar speaker distortion, and guitar speaker and cabinet frequency response.
Distortion sound or "texture" from guitar amplifiers is further shaped or processed through the frequency response and distortion factors in the microphones (their response, placement, and multi-microphone comb filtering effects), microphone preamps, mixer channel equalization, and compression. Additionally, the basic sound produced by the guitar amplifier can be changed and shaped by adding distortion and/or equalization effect pedals before the amp's input jack, in the effects loop just before the tube power amp, or after the power tubes.
Power-tube distortion is required for amp sounds in some genres. In a standard master-volume guitar amp, as the amp's final or master volume is increased beyond the full power of the amplifier, power tube distortion is produced. The "power soak" approach places the attenuation between the power tubes and the guitar speaker. In the re-amped or "dummy load" approach, the tube power amp drives a mostly resistive dummy load while an additional low power amp drives the guitar speaker. In the isolation box approach, the guitar amplifier is used with a guitar speaker in a separate cabinet. A soundproofed isolation cabinet, isolation box, isolation booth, or isolation room can be used.
A variety of labels are used for level attenuation potentiometers in a guitar amplifier and other guitar equipment. Electric guitars and basses have a volume control to attenuate whichever pickup is selected. There may be two volume controls in parallel to mix the signal levels from the neck and bridge pickups. Rolling back the guitar's volume control also changes the pickup's equalization or frequency response, which can provide pre-distortion equalization.
The simplest guitar amplifiers have only a volume control. Most have at least a gain control and a master volume control. The gain control is equivalent to the distortion control on a distortion pedal, and similarly may have a side-effect of changing the proportion of bass and treble sent to the next stage.
A simple amplifier's tone controls typically include passive bass and treble controls. In some cases, a midrange control is provided. The amplifier's master volume control restricts the amount of signal permitted through to the driver stage and the power amplifier. When using a power attenuator with a tube amplifier, the master volume no longer acts as the master volume control. Instead, the power attenuator's attenuation control controls the power delivered to the speaker, and the amplifier's master volume control determines the amount of power-tube distortion. Power-supply based power reduction is controlled by a knob on the tube power amp, variously labeled "Wattage", "Power", "Scale", "Power Scale", or "Power Dampening".
- List of guitar amplifier manufacturers
- Vintage musical equipment
- Tube sound
- Bass instrument amplification
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- Note: This style of amplifiers should not be confused with the brand of guitar and bass amplifiers called Acoustic, still available in second-hand music stores.)
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