Guitar amplifier

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Mesa-Boogie "Mark IV", a guitar combo amplifier

A guitar amplifier (or guitar amp) is an electronic amplifier that amplifies the electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through a loudspeaker, which is typically housed in a wooden cabinet. A guitar amplifier may be a standalone wood or metal cabinet that contains only the amplifier (and preamplifier) circuits—or it may be a "combo" amplifier, which contains both the amplifier and one or more speakers in a wooden cabinet. 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).

This article focuses on electric guitar amps For information on amps for bass guitar, see the article on bass amps.

Structure

A Fender Bassman amp head with a 15" speaker cabinet.

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 that affect the character of the guitar signal:

  • Before the preamp stage (as in the case of guitar pedals)
  • Between the preamp and power stages (as in the cases of effects loop or many dedicated amplifier tone circuits)
  • Between multiple stacked preamp stages
  • In feedback loops from a post-preamp signal to an earlier pre-preamp signal (as in the case of presence modifier circuits)

Tone stages may also provide electronic effects—such as equalization, compression, distortion, chorus, or reverb. Amplifiers may use vacuum tubes (called valves in Britain), 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 passes the amplified signal via a speaker cable to a speaker cabinet. A wide range of speaker configurations are available in guitar cabinets, ranging from cabinets with a single speaker (e.g., 1×10" or 1×12") or multiple speakers (e.g., 2×10" or 4×10"). Guitar amplifiers range in price and quality. Music equipment companies import small, low-powered practice amplifiers for students that sell for less than $50 USD. Other companies produce expensive custom-made amplifiers for professional musicians, which can cost thousands of dollars. Most combo amplifiers have a carrying handle, and many combo amplifiers and cabinets have metal or plastic-reinforced corners.

Control knobs are typically mounted on the front of the cabinet or chassis, though in some cases, the knobs are on a recessed panel at the back of the top of the amplifier. The most basic amps only have a few knobs, which typically control volume, bass and treble. More expensive amps may have a number of knobs that control pre-amp volume (or "gain"), distortion or overdrive, volume, bass, mid and treble, and reverb. Some older amps (and their re-issued versions) have a knob that controls a vibrato effect. The 1/4" input jack is typically mounted on the front of the amplifier. In the simplest, least expensive amplifiers, this 1/4" jack is the only jack on the amplifier. More expensive amplifiers may have a patch bay for multiple inputs and outputs, such as a pre-amp out (for sending to another guitar amplifier), an in jack to create an effects loop (when use with the pre-amp out), an external speaker output (for powering an additional speaker cabinet), and stereo RCA jacks or an 1/8" jack, for connecting a CD player or MP3 player. Some amps have a 1/4" jack for connecting a pedal to turn the amp's onboard overdrive and reverb on and off.

History

Fender Deluxe 1953

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.[1]

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.

Gibson Lancer GA-35 (mid-1960s) guitar amplifier

In the 1950s, several guitarists experimented with distortion produced by deliberately overdriving their amplifiers, including Goree Carter,[2] Joe Hill Louis,[3][4] Elmore James,[5] Ike Turner,[6] Willie Johnson,[7] Pat Hare,[8] Guitar Slim,[9] Chuck Berry,[10] Johnny Burnette,[7] and Link Wray.[11] In the early 1960s, surf rock guitarist Dick Dale worked closely with Fender to produce custom made amplifiers,[12] including the first 100-watt guitar amplifier.[13] 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."[12]

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 must be larger and more sturdily built than cabinets for mid-range or high-frequency (tweeter) speakers.

Types

Kustom 200 bass amp – amp head and speakers, 100 watts RMS, two channels, two 15" speakers, 1971

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. Another alternative device often used for guitar in this fashion are public address amplifiers, which provide similar amplification, though not specifically designed for instrumental use.

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

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Main article: Valve amplifier
The glow from four "Electro Harmonix KT88" brand power tubes lights up the inside of a Traynor YBA-200 bass guitar amplifier

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. Transistor amplifiers are less expensive to build and maintain, reduce weight and heat, and may be more reliable. High-end tube instrument amplifiers survive as one of few exceptions, because of their sound quality. Typically, tube amps use one or more dual triodes in the preamplifier section to provide sufficient voltage gain to offset tone control losses and drive the power amplifier section. While tube technology is, in many ways, outdated, tube amps remain popular since many guitarists prefer their sound.[14]

Rear view of a tube (valve) combo guitar amplifier. Visible are two glass output tubes, six smaller preamp tubes in their metal tube retainers, and both the power transformer and the output transformer.

Solid-state 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.[14]

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.[14][15][16] Solid-state amplifiers vary in output power, functionality, size, price, and sound quality in a wide range, from practice amplifiers to professional models.

Hybrid amplifiers

A hybrid amplifier involves one of two combinations 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 easily achieves modern power levels.[14]

Modeling amplifiers

A modeling amplifier, shown from above. Note the various amplifier and speaker emulations selectable via the rotary knob on the left.

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.[14] Line 6 is generally credited with bringing modeling amplification to the market.[17][18]

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,[19] or dedicated combo-style amplifiers with a broad frequency range.[20] Such processors can be traditional guitar effects, a modeling amplifier (without power amplifier), or a computer running tone-shaping software.[19]

Acoustic guitar amplifiers

These amplifiers are intended for 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 are usually designed so that neither power amplifier nor speakers add 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 provide 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.[21]

Amplifier configuration

A 3×6 stack of mock Marshall guitar cabinets for Jeff Hanneman of Slayer

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 that 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 8×12 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 4×12 cabinets, meaning four 12" speakers, to enable transporting the amp rig.

Some touring metal and rock bands have used a large array of guitar amplifiers for the impressive appearance. Some of these arrangements include only the fronts of speaker cabinets mounted on a large frame.[22]

Distortion, power, and volume

Power output

The FBT electronica Super Guitar 125 guitar amplifier

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 30 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. The human ear perceives a 5-watt amplifier as 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 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.[23]

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

Marshall is a widely-used amp in metal and hard rock. Pictured is the MG15DFX guitar amplifier.

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. Because there are so many factors beyond preamp distortion that create a guitarist's "signature sound", in recording and sound reinforcement applications, the sound of the guitar amp is almost always miked, rather than using the guitar amp's pre-amp out signal.

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

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.

Volume controls

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 that attenuates selected pickups. 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".

See also

References

  1. ↑ Timothy Miller, "Hawaiian Guitar", The Grove Dictionary of American Music, 2nd editio
  2. ↑ Robert Palmer, "Church of the Sonic Guitar", pp. 13-38 in Anthony DeCurtis, Present Tense, Duke University Press, 1992, p. 19. ISBN 0-8223-1265-4.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ John Morthland (2013), How Elmore James Invented Metal, Wondering Sound, eMusic
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  8. ↑ Robert Palmer, "Church of the Sonic Guitar", pp. 13-38 in Anthony DeCurtis, Present Tense, Duke University Press, 1992, pp. 24-27. ISBN 0-8223-1265-4.
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  13. ↑ History, Dick Dale official website
  14. ↑ 14.0 14.1 14.2 14.3 14.4 Lua error in package.lua at line 80: module 'strict' not found.
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  19. ↑ 19.0 19.1 Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ 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.)
  22. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  23. ↑ Guitar Player Magazine, March 2004, page 179</a>

[1]

Further reading

External links

  1. ↑ Timothy Miller, "Hawaiian Guitar", The Grove Dictionary of American Music, 2nd edition
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