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

A guitar amplifier (or guitar amp) is an electronic amplifier that strengthens the weak electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which are typically housed in a wooden cabinet. A guitar amplifier may be a standalone wood or metal cabinet that contains only the power amplifier (and preamplifier) circuits, requiring the use of a separate speaker cabinet–or it may be a "combo" amplifier, which contains both the amplifier and one or more speakers in a wooden cabinet. There is a wide range of sizes and power ratings for guitar amplifiers, from small, lightweight "practice amplifiers" with a single 8" speaker to heavy combo amps with four 10" speakers and a powerful amplifier, which are loud enough to use in a nightclub or bar performance.

Guitar amplifiers can also modify the instrument's tone by emphasizing or de-emphasizing certain frequencies, using equalizer controls, which function the same way as the bass and treble knobs on a home hi-fi stereo, and by adding electronic effects; distortion (also called "overdrive") and reverb are commonly available as built-in features. The input of modern guitar amplifiers is a 1/4" jack, which is fed a signal from an electro-magnetic pickup (from an electric guitar) or a piezoelectric pickup (usually from an acoustic guitar) using a patch cord, or a wireless transmitter. For electric guitar players, their choice of guitar amp and the settings they use on the amplifier are a key part of their signature tone or sound. Some guitar players are longtime users of a specific amp brand or model. Many electric guitar players use external effects pedals to alter the sound of their tone before the signal reaches the guitar amp, such as the wah wah pedal and the chorus pedal.

This article focuses on electric guitar amps. For information on amps for bass guitar, a lower-pitched, similar instrument, see the article on bass amps.

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, which usually include at least bass and treble controls, which function similarly to the equivalent controls on a home hi-fi system. More expensive amplifiers typically have more controls for other frequency ranges, such as one or two "midrange" controls and a "presence" control for very high frequencies. Some guitar amplifiers have a graphic equalizer, which uses vertical fader controls which can control many frequency bands. The first amplifier stage 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 loudspeaker to produce sound that the guitarist and audience can hear.

There may be one or more tone stages that affect the character of the guitar signal:

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 one or more external speaker cabinets. 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", 4×10" or 8x10"). Guitar amplifiers have a wide range in price and quality. Music equipment companies import small, low-powered practice amplifiers for students and beginners 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 to protect the amp during transportation.

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 used with the pre-amp out jack), 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 so that a player can practice along with recorded music. Some amps have a 1/4" jack for connecting a pedal to turn the amp's onboard overdrive and reverb on and off or to switch between channels. The vast majority of guitar amps can only be powered by AC mains power (plugging into a wall outlet); however, a small number of practice amps designed for buskers also have battery power, enabling them to be used for street performances.

A 1940s-era Valvo combo amp. Fender Deluxe 1953

In the 1920s, it was very hard for a musician playing a pickup-equipped guitar to find an amplifier and speaker to make their instrument louder as the only speakers that could be bought were "radio horns of limited frequency range and low acoustic output". The cone speaker, widely used in 2000s-era amp cabinets, was not offered for sale until 1925. The first amplifiers and speakers could only be powered with large batteries, which made them heavy and hard to carry around. When engineers developed the first AC mains-powered amplifiers, they were soon used to make musical instruments louder. Engineers invented the first loud, powerful amplifier and speaker systems for public address systems and movie theaters. These large PA systems and movie theatre sound systems were very large and very expensive, and so they could not be used by most touring musicians. After 1927, smaller, portable AC mains-powered PA systems that could be plugged into a regular wall socket "quickly became popular with musicians"; indeed, "...Leon McAuliffe (with Bob Wills) still used a carbon mic and a portable PA as late as 1935." During the late 1920s to mid-1930s, small portable PA systems and guitar combo amplifiers were fairly similar. These early amps had a "single volume control and one or two input jacks, field coil speakers" and thin wooden cabinets; remarkably, these early amps did not have tone controls or even an on-off switch.[1]

In 1928, the Stromberg-Voisinet firm was the first company to sell an electric stringed instrument and amplifier package. However, musicians found that the amps had an "unsatisfactory tone and volume, [and] dependability problems", so the product did not sell well. Even though the Stromberg-Voisinet amp did not sell well, it still launched a new idea: a portable electric instrument amp with a speaker, all in an easily transported wooden cabinet. In 1929, Vega electrics launched a portable banjo amplifier. In 1932, Electro String Instruments and amplifier (this is not the same company asStromberg Electro Instruments) introduced a guitar amp with "high output" and a "string driven magnetic pickup". Electro set out the standard template for combo amps: a wooden cabinet with the electronic amplifier mounted inside, and a convenient carrying handle to facilitate transporting the cabinet. 1n 1933, Vivi-Tone amp set-ups were used for live performances and radio shows. In 1934, Rickenbacker launched a similar combo amp which added the feature of metal corner protectors, which keep the corners in good condition during transportation.[1]

In 1933, Dobro released an electric guitar and amp package. The combo amp had "two 8″ Lansing speakers and a five-tube chassis. Dobro made a two speaker combo amp that was on the market over 12 years before Fender launched its two-speaker "Dual Professional/Super" combo amp. In 1933, Audio-Vox was founded by Paul Tutmarc, the inventor of the first electric bass (Tutmarc's instrument did not achieve market success until Leo Fender's launched the Precision Bass). In 1933, Vega sold a "pickup and amplifier set" which a musician could use with her/his existing guitar. In that same year, the Los Angeles-based Volu-Tone company also sold a pickup/amplifier set. Volu-Tone used "high voltage current" to sense the string vibration, a potentially dangerous approach that did not become popular. In 1934 Dobro released a guitar amp with a vacuum tube rectifier and two power tubes. By 1935, Dobro and National began selling combo amps for Hawaiian guitar. In 1934, Gibson developed prototype combo amps, but these never got produced and sold. By 1935, Electro/Rickenbacher sold "more amps and electric guitars than all the amps and electrified/electric guitars made from ’[19]28 through the end of ’[19]34, combined."[1]

The first electric instrument amplifiers were not designed for use with electric guitars. The earliest examples were portable PA systems, which 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 would not become lightweight until many decades later. 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.[2]

In the 1920s, the earliest combo amplifiers did not have any tone controls. 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,[3]Joe Hill Louis,[4][5]Elmore James,[6]Ike Turner,[7]Willie Johnson,[8]Pat Hare,[9]Guitar Slim,[10]Chuck Berry,[11]Johnny Burnette,[8] and Link Wray.[12] In the early 1960s, surf rock guitarist Dick Dale worked closely with Fender to produce custom made amplifiers,[13] including the first 100-watt guitar amplifier.[14] 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."[13]

U2 guitarist The Edge's 1964 Vox AC30 combo amp.

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 combo amplifiers were at first used with bass guitars and electric pianos, but these instruments produce a wider frequency range and need a 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. As such, in the 1950s, when Ampeg introduced bass amplifier and speaker systems, these began to be used by bass guitarists. Similarly, Hammond organ players used a specialized keyboard combo amplifier, the Leslie speaker cabinet, which contains a woofer for the low frequencies and a horn for the high frequencies. The Leslie horns rotated and a baffle around the woofer rotated as well, producing a chorus effect.

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 one or more speaker(s) in a single wooden speaker enclosure. A separate configuration is available as well, with a separate amplifier (the "head") on top of one or more cabinets, each of which contains one or more speakers. Another alternative device used for guitar are public address amplifiers. Grunge guitarist Kurt Cobain used four 800 watt PA amplifiers for his early guitar set-up.

Besides one or more instrument inputs (typically a 1/4" jack), other jacks may also be provided, such as an auxiliary input jack (sometimes with its own level control, for a drum machine), "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. Some guitar amps have an XLR input so that a microphone can be plugged in for singing. Guitar amps that include a mic input are in effect small, portable PA systems.

A wide range of instrument amplifiers is available at a wide range of price, quality, and performance levels. Some amplifiers are designed for beginners, such as small, low-wattage "practice amplifiers", which typically have a single 8" speaker and about 10 watts, or smaller "combo" amps with relatively low wattage (15 to 20 watts) and a single 10" speaker. Mid- to large-size "combo" amps with 30 to 50 watts and one 12" speaker or four 10" speakers are designed for use in band rehearsals and onstage performances. Some guitar amps are designed for specific instruments or particular genres, such as the Marshall amps, which are widely used in heavy metal music.

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

Vacuum tubes (called "valves" in British English) were by far the dominant active electronic components in most instrument amplifier applications until the 1970s, when solid-state semiconductors (transistors) started taking over. Transistor amplifiers are less expensive to build and maintain, reduce the weight and heat of an amplifier, and tend to be more reliable and more shock-resistant. Tubes are fragile and they need to be replaced and maintained periodically. As well, serious problems with the tubes can render an amplifier inoperable until the issue is resolved. In the 2000s, high-end tube instrument amplifiers (along with a small number of hi-fi power amplifiers used by audiophiles and high-end studio microphone preamplifiers) survive as the few exceptions, because of their perceived sound quality. Tube enthusiasts believe that tube amps produce a "warmer" sound and a more natural "overdrive" sound. 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.[15]

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.

Since the 1980s, most inexpensive and mid-priced guitar amplifiers are based on semiconductor (solid-state) circuits. 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.[15] 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.[15][16][17] Solid-state amplifiers vary in output power, functionality, size, price, and sound quality in a wide range, from practice amplifiers to combos suitable for gigging to professional models intended for session musicians who do studio recording work.

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.[15]

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 in guitar amps to create numerous different sounds and tones that simulate the sound of a range of tube amplifiers and different sized speaker cabinets, all using the same amplifier and speaker. These are known as modeling amplifiers, and can be programmed with simulated characteristic tones of different existing amplifier models (and speaker cabinets—even microphone type or 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 or laptop.[15]Line 6 is generally credited with bringing modeling amplification to the market.[18][19] Modeling amplifiers and stompbox pedals, rackmount units and software programs which provide amplifier, speaker cabinet and mic modeling can give a guitarist with the single modeling amp or unit access to a large number of sounds and tones, such as the simulated sound of tube amplifiers, vintage combo amplifiers, huge 8x10" stacks, without having to bring all of this heavy equipment to the studio or stage.

The use of "Full Range Flat Response" (FRFR) amplification systems by electric guitarists has received an extra impetus from modeling amplifiers. Before the widespread availability of modeling amplifiers and units, an electric guitarist would not be able to plug her/his electric guitar straight into a PA system or powered speaker, because in most rock genres, the tonal coloration that a regular guitar amp's preamplifier, equalization filters, power amp, guitar speaker and cabinet design (e.g., 8x10" cabinet) are an essential part of a guitarist's unique tone. The basic concept of FRFR is that the tone is shaped by sound processors placed in the signal chain before the amplifier/speaker stage. However, instead of a guitar amplifier, with its particular sound-shaping characteristics (e.g., many amps roll off high treble frequencies and many amplifiers add frequency coloration), a flat-frequency response amplification systems can be used, such as amplified speakers or a PA system (full-range speakers),[20] or dedicated combo-style amplifiers with a broad frequency range.[21] Such processors can be traditional guitar effects, a modeling amplifier (without power amplifier), or a computer running tone-shaping software.[20] With an amp modeling pedal or unit, a guitarist could even use a flat-response keyboard amplifier combo amp to produce overdriven, vintage tones and sounds.

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 condenser 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.[22]

Metal guitarist Klaus Eichstadt in front of his Marshall stack. 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 the cabinets to be more transportable. Some touring metal and rock bands have used a large array of guitar speaker cabinets for their impressive appearance. Some of these arrangements include only the fronts of speaker cabinets mounted on a large frame.[23]

There are many varieties of speaker combinations used in guitar speaker cabinets, including one 12" speaker, one 15" speaker (this is more common for bass amplifiers than for electric guitar cabinets), two 10" speakers, four 10" speakers, four 12" speakers, or eight 10" speakers. Less commonly, guitar cabinets may contain different sizes of speaker in the same cabinet. Cabinets with eight 10" speakers are large and heavy, and they are often equipped with wheels and a "towel bar"-style handle for transport. Some cabinets use mixed speaker types, such as one 15" speaker and two 10" speakers.

Combo guitar amplifier cabinets and guitar speaker cabinets use several different designs, including the "open back" cabinet, the closed back cabinet (a sealed box), and, less commonly, bass reflex designs which use a closed back along with a vent or port cut into the cabinet.[24] With guitar amps, most "open back" amp cabinets are not fully open; in fact, part of the back is enclosed with panels. Combo guitar amp cabinets and standalone speaker cabinets are often made of plywood; "[d]ecent...cabs have even been constructed from MDF and particle board—although these are typically considered low-budget options."[24] In a cabinet, "...the size and depth of the cabinet, the type of wood used to build it, the way that wood is fixed together, the type and thickness of the baffle material (the sheet of wood to which the speaker is mounted), and the way this baffle is mounted to the cab" all affect the sound and tone of the cabinet.[24]

When two or more speakers are used in the same cabinet, or when two cabinets are used together, the speakers can be wired in parallel or in series, or in a combination of the two (e.g., two 2x10" cabinets, with the two speakers wired in series, can be connected together in parallel). Whether speakers are wired in parallel or in series affects the impedance of the system. Two 8 ohm speakers wired in parallel will have a 4 ohm impedance. Guitarists who are connecting multiple cabinets to their amplifier need to consider the minimum impedance of their amplifier. Whether speakers are wired in parallel or in series also affects the tone and sound. When speakers are wired in parallel, this "...dampen[s] and restrain[s]" the speakers, giving a "tighter response, and a smoother breakup"; speakers that are wired in "...series (usually no more than two) run a little looser, giving a slightly more raw, open and edgy sound."[24]

A Marshall JCM 900's knobs for equalization, gain, reverb 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 less than one watt to 20 watts and "performance" or "stage" amps, which are generally 30 watts or higher. Traditionally, these have been fixed-power amplifiers, with some 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 in watts is not immediately obvious. While beginners sometimes assume that there is a linear relationship between perceived volume and wattage (e.g., they assume that a 5-watt amp will be much quieter at its maximum output than a 50-watt amp), in fact 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.[25]

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.

Marshall is a popular amplifier manufacturer for 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 recorded with a microphone placed in front of the guitar speaker, rather than using the guitar amp's pre-amp out signal (even though the guitar sound is typically recorded with a mic in front of the amp's speaker, for recordings, the sound engineer or music producer may ask for the DI out signal from the pickups to be recorded onto a separate track at the same time, so that the engineer or producer can re-amp the signal through various different amps at a later time). In contrast, it is fairly common to use a DI box with electric bass.

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.

Even in the 2010s, the vintage Fender Bandmaster remains a sought-after amp by guitarists. Note the four inputs, two for regular sound and two which are run through the onboard vibrato effect unit. The amp pictured is a 1968 model.

A variety of labels are used for level attenuation potentiometers (knobs) in a guitar amplifier and other guitar equipment. Electric guitars and basses have a volume control on the instrument that attenuates the signal from selected pickups. There may be two volume controls on an electric guitar or bass, wired 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, such as some vintage amps and modern practice amps, have only a single volume control. Most have two volume controls: a first volume control called "preamplifier" or "gain" and a master volume control. The preamp or gain control is designed differently on different types of guitar amps. On an amp designed for an acoustic guitar, turning up the preamp knob will preamplify the signal, but even at its maximum setting, the preamp control is unlikely to produce much overdrive. However, with amps designed for electric guitarists playing electric blues, hard rock and heavy metal music, turning up the preamp or gain knob will usually produce overdrive or distortion. Some electric guitar amps have three controls in the volume section: preamplifier, distortion and master control. Turning up the preamp and distortion knobs in varying combinations can create a range of overdrive tones, from a gentle, warm growling overdrive suitable for a traditional blues show or a rockabilly band to the extreme distortion used in hardcore punk and death metal. On some electric guitar amps, the "gain" knob 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.

The patch bay at the rear panel of this Line 6 Flextone guitar amp provides a number of additional inputs and outputs, including stereo XLR DI unit outputs.

A simple, inexpensive amplifier may have only two tone controls, a passive bass and treble control. In some better quality amps, one or more midrange controls are provided. On the most expensive amps, there may be shelving equalizers for bass and treble, a number of mid-range controls (e.g., low mid, mid and high mid), and a graphic equalizer or parametric equalizer. 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".

 

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

A guitar amplifier (or guitar amp) is an electronic amplifier that strengthens the weak electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which are typically housed in a wooden cabinet. A guitar amplifier may be a standalone wood or metal cabinet that contains only the power amplifier (and preamplifier) circuits, requiring the use of a separate speaker cabinet–or it may be a "combo" amplifier, which contains both the amplifier and one or more speakers in a wooden cabinet. There is a wide range of sizes and power ratings for guitar amplifiers, from small, lightweight "practice amplifiers" with a single 8" speaker to heavy combo amps with four 10" speakers and a powerful amplifier, which are loud enough to use in a nightclub or bar performance.

Guitar amplifiers can also modify the instrument's tone by emphasizing or de-emphasizing certain frequencies, using equalizer controls, which function the same way as the bass and treble knobs on a home hi-fi stereo, and by adding electronic effects; distortion (also called "overdrive") and reverb are commonly available as built-in features. The input of modern guitar amplifiers is a 1/4" jack, which is fed a signal from an electro-magnetic pickup (from an electric guitar) or a piezoelectric pickup (usually from an acoustic guitar) using a patch cord, or a wireless transmitter. For electric guitar players, their choice of guitar amp and the settings they use on the amplifier are a key part of their signature tone or sound. Some guitar players are longtime users of a specific amp brand or model. Many electric guitar players use external effects pedals to alter the sound of their tone before the signal reaches the guitar amp, such as the wah wah pedal and the chorus pedal.

This article focuses on electric guitar amps. For information on amps for bass guitar, a lower-pitched, similar instrument, see the article on bass amps.

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, which usually include at least bass and treble controls, which function similarly to the equivalent controls on a home hi-fi system. More expensive amplifiers typically have more controls for other frequency ranges, such as one or two "midrange" controls and a "presence" control for very high frequencies. Some guitar amplifiers have a graphic equalizer, which uses vertical fader controls which can control many frequency bands. The first amplifier stage 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 loudspeaker to produce sound that the guitarist and audience can hear.

There may be one or more tone stages that affect the character of the guitar signal:

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 one or more external speaker cabinets. 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", 4×10" or 8x10"). Guitar amplifiers have a wide range in price and quality. Music equipment companies import small, low-powered practice amplifiers for students and beginners 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 to protect the amp during transportation.

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 used with the pre-amp out jack), 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 so that a player can practice along with recorded music. Some amps have a 1/4" jack for connecting a pedal to turn the amp's onboard overdrive and reverb on and off or to switch between channels. The vast majority of guitar amps can only be powered by AC mains power (plugging into a wall outlet); however, a small number of practice amps designed for buskers also have battery power, enabling them to be used for street performances.

A 1940s-era Valvo combo amp. Fender Deluxe 1953

In the 1920s, it was very hard for a musician playing a pickup-equipped guitar to find an amplifier and speaker to make their instrument louder as the only speakers that could be bought were "radio horns of limited frequency range and low acoustic output". The cone speaker, widely used in 2000s-era amp cabinets, was not offered for sale until 1925. The first amplifiers and speakers could only be powered with large batteries, which made them heavy and hard to carry around. When engineers developed the first AC mains-powered amplifiers, they were soon used to make musical instruments louder. Engineers invented the first loud, powerful amplifier and speaker systems for public address systems and movie theaters. These large PA systems and movie theatre sound systems were very large and very expensive, and so they could not be used by most touring musicians. After 1927, smaller, portable AC mains-powered PA systems that could be plugged into a regular wall socket "quickly became popular with musicians"; indeed, "...Leon McAuliffe (with Bob Wills) still used a carbon mic and a portable PA as late as 1935." During the late 1920s to mid-1930s, small portable PA systems and guitar combo amplifiers were fairly similar. These early amps had a "single volume control and one or two input jacks, field coil speakers" and thin wooden cabinets; remarkably, these early amps did not have tone controls or even an on-off switch.[1]

In 1928, the Stromberg-Voisinet firm was the first company to sell an electric stringed instrument and amplifier package. However, musicians found that the amps had an "unsatisfactory tone and volume, [and] dependability problems", so the product did not sell well. Even though the Stromberg-Voisinet amp did not sell well, it still launched a new idea: a portable electric instrument amp with a speaker, all in an easily transported wooden cabinet. In 1929, Vega electrics launched a portable banjo amplifier. In 1932, Electro String Instruments and amplifier (this is not the same company asStromberg Electro Instruments) introduced a guitar amp with "high output" and a "string driven magnetic pickup". Electro set out the standard template for combo amps: a wooden cabinet with the electronic amplifier mounted inside, and a convenient carrying handle to facilitate transporting the cabinet. 1n 1933, Vivi-Tone amp set-ups were used for live performances and radio shows. In 1934, Rickenbacker launched a similar combo amp which added the feature of metal corner protectors, which keep the corners in good condition during transportation.[1]

In 1933, Dobro released an electric guitar and amp package. The combo amp had "two 8″ Lansing speakers and a five-tube chassis. Dobro made a two speaker combo amp that was on the market over 12 years before Fender launched its two-speaker "Dual Professional/Super" combo amp. In 1933, Audio-Vox was founded by Paul Tutmarc, the inventor of the first electric bass (Tutmarc's instrument did not achieve market success until Leo Fender's launched the Precision Bass). In 1933, Vega sold a "pickup and amplifier set" which a musician could use with her/his existing guitar. In that same year, the Los Angeles-based Volu-Tone company also sold a pickup/amplifier set. Volu-Tone used "high voltage current" to sense the string vibration, a potentially dangerous approach that did not become popular. In 1934 Dobro released a guitar amp with a vacuum tube rectifier and two power tubes. By 1935, Dobro and National began selling combo amps for Hawaiian guitar. In 1934, Gibson developed prototype combo amps, but these never got produced and sold. By 1935, Electro/Rickenbacher sold "more amps and electric guitars than all the amps and electrified/electric guitars made from ’[19]28 through the end of ’[19]34, combined."[1]

The first electric instrument amplifiers were not designed for use with electric guitars. The earliest examples were portable PA systems, which 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 would not become lightweight until many decades later. 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.[2]

In the 1920s, the earliest combo amplifiers did not have any tone controls. 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,[3]Joe Hill Louis,[4][5]Elmore James,[6]Ike Turner,[7]Willie Johnson,[8]Pat Hare,[9]Guitar Slim,[10]Chuck Berry,[11]Johnny Burnette,[8] and Link Wray.[12] In the early 1960s, surf rock guitarist Dick Dale worked closely with Fender to produce custom made amplifiers,[13] including the first 100-watt guitar amplifier.[14] 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."[13]

U2 guitarist The Edge's 1964 Vox AC30 combo amp.

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 combo amplifiers were at first used with bass guitars and electric pianos, but these instruments produce a wider frequency range and need a 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. As such, in the 1950s, when Ampeg introduced bass amplifier and speaker systems, these began to be used by bass guitarists. Similarly, Hammond organ players used a specialized keyboard combo amplifier, the Leslie speaker cabinet, which contains a woofer for the low frequencies and a horn for the high frequencies. The Leslie horns rotated and a baffle around the woofer rotated as well, producing a chorus effect.

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 one or more speaker(s) in a single wooden speaker enclosure. A separate configuration is available as well, with a separate amplifier (the "head") on top of one or more cabinets, each of which contains one or more speakers. Another alternative device used for guitar are public address amplifiers. Grunge guitarist Kurt Cobain used four 800 watt PA amplifiers for his early guitar set-up.

Besides one or more instrument inputs (typically a 1/4" jack), other jacks may also be provided, such as an auxiliary input jack (sometimes with its own level control, for a drum machine), "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. Some guitar amps have an XLR input so that a microphone can be plugged in for singing. Guitar amps that include a mic input are in effect small, portable PA systems.

A wide range of instrument amplifiers is available at a wide range of price, quality, and performance levels. Some amplifiers are designed for beginners, such as small, low-wattage "practice amplifiers", which typically have a single 8" speaker and about 10 watts, or smaller "combo" amps with relatively low wattage (15 to 20 watts) and a single 10" speaker. Mid- to large-size "combo" amps with 30 to 50 watts and one 12" speaker or four 10" speakers are designed for use in band rehearsals and onstage performances. Some guitar amps are designed for specific instruments or particular genres, such as the Marshall amps, which are widely used in heavy metal music.

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

Vacuum tubes (called "valves" in British English) were by far the dominant active electronic components in most instrument amplifier applications until the 1970s, when solid-state semiconductors (transistors) started taking over. Transistor amplifiers are less expensive to build and maintain, reduce the weight and heat of an amplifier, and tend to be more reliable and more shock-resistant. Tubes are fragile and they need to be replaced and maintained periodically. As well, serious problems with the tubes can render an amplifier inoperable until the issue is resolved. In the 2000s, high-end tube instrument amplifiers (along with a small number of hi-fi power amplifiers used by audiophiles and high-end studio microphone preamplifiers) survive as the few exceptions, because of their perceived sound quality. Tube enthusiasts believe that tube amps produce a "warmer" sound and a more natural "overdrive" sound. 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.[15]

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.

Since the 1980s, most inexpensive and mid-priced guitar amplifiers are based on semiconductor (solid-state) circuits. 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.[15] 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.[15][16][17] Solid-state amplifiers vary in output power, functionality, size, price, and sound quality in a wide range, from practice amplifiers to combos suitable for gigging to professional models intended for session musicians who do studio recording work.

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.[15]

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 in guitar amps to create numerous different sounds and tones that simulate the sound of a range of tube amplifiers and different sized speaker cabinets, all using the same amplifier and speaker. These are known as modeling amplifiers, and can be programmed with simulated characteristic tones of different existing amplifier models (and speaker cabinets—even microphone type or 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 or laptop.[15]Line 6 is generally credited with bringing modeling amplification to the market.[18][19] Modeling amplifiers and stompbox pedals, rackmount units and software programs which provide amplifier, speaker cabinet and mic modeling can give a guitarist with the single modeling amp or unit access to a large number of sounds and tones, such as the simulated sound of tube amplifiers, vintage combo amplifiers, huge 8x10" stacks, without having to bring all of this heavy equipment to the studio or stage.

The use of "Full Range Flat Response" (FRFR) amplification systems by electric guitarists has received an extra impetus from modeling amplifiers. Before the widespread availability of modeling amplifiers and units, an electric guitarist would not be able to plug her/his electric guitar straight into a PA system or powered speaker, because in most rock genres, the tonal coloration that a regular guitar amp's preamplifier, equalization filters, power amp, guitar speaker and cabinet design (e.g., 8x10" cabinet) are an essential part of a guitarist's unique tone. The basic concept of FRFR is that the tone is shaped by sound processors placed in the signal chain before the amplifier/speaker stage. However, instead of a guitar amplifier, with its particular sound-shaping characteristics (e.g., many amps roll off high treble frequencies and many amplifiers add frequency coloration), a flat-frequency response amplification systems can be used, such as amplified speakers or a PA system (full-range speakers),[20] or dedicated combo-style amplifiers with a broad frequency range.[21] Such processors can be traditional guitar effects, a modeling amplifier (without power amplifier), or a computer running tone-shaping software.[20] With an amp modeling pedal or unit, a guitarist could even use a flat-response keyboard amplifier combo amp to produce overdriven, vintage tones and sounds.

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 condenser 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.[22]

Metal guitarist Klaus Eichstadt in front of his Marshall stack. 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 the cabinets to be more transportable. Some touring metal and rock bands have used a large array of guitar speaker cabinets for their impressive appearance. Some of these arrangements include only the fronts of speaker cabinets mounted on a large frame.[23]

There are many varieties of speaker combinations used in guitar speaker cabinets, including one 12" speaker, one 15" speaker (this is more common for bass amplifiers than for electric guitar cabinets), two 10" speakers, four 10" speakers, four 12" speakers, or eight 10" speakers. Less commonly, guitar cabinets may contain different sizes of speaker in the same cabinet. Cabinets with eight 10" speakers are large and heavy, and they are often equipped with wheels and a "towel bar"-style handle for transport. Some cabinets use mixed speaker types, such as one 15" speaker and two 10" speakers.

Combo guitar amplifier cabinets and guitar speaker cabinets use several different designs, including the "open back" cabinet, the closed back cabinet (a sealed box), and, less commonly, bass reflex designs which use a closed back along with a vent or port cut into the cabinet.[24] With guitar amps, most "open back" amp cabinets are not fully open; in fact, part of the back is enclosed with panels. Combo guitar amp cabinets and standalone speaker cabinets are often made of plywood; "[d]ecent...cabs have even been constructed from MDF and particle board—although these are typically considered low-budget options."[24] In a cabinet, "...the size and depth of the cabinet, the type of wood used to build it, the way that wood is fixed together, the type and thickness of the baffle material (the sheet of wood to which the speaker is mounted), and the way this baffle is mounted to the cab" all affect the sound and tone of the cabinet.[24]

When two or more speakers are used in the same cabinet, or when two cabinets are used together, the speakers can be wired in parallel or in series, or in a combination of the two (e.g., two 2x10" cabinets, with the two speakers wired in series, can be connected together in parallel). Whether speakers are wired in parallel or in series affects the impedance of the system. Two 8 ohm speakers wired in parallel will have a 4 ohm impedance. Guitarists who are connecting multiple cabinets to their amplifier need to consider the minimum impedance of their amplifier. Whether speakers are wired in parallel or in series also affects the tone and sound. When speakers are wired in parallel, this "...dampen[s] and restrain[s]" the speakers, giving a "tighter response, and a smoother breakup"; speakers that are wired in "...series (usually no more than two) run a little looser, giving a slightly more raw, open and edgy sound."[24]

A Marshall JCM 900's knobs for equalization, gain, reverb 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 less than one watt to 20 watts and "performance" or "stage" amps, which are generally 30 watts or higher. Traditionally, these have been fixed-power amplifiers, with some 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 in watts is not immediately obvious. While beginners sometimes assume that there is a linear relationship between perceived volume and wattage (e.g., they assume that a 5-watt amp will be much quieter at its maximum output than a 50-watt amp), in fact 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.[25]

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.

Marshall is a popular amplifier manufacturer for 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 recorded with a microphone placed in front of the guitar speaker, rather than using the guitar amp's pre-amp out signal (even though the guitar sound is typically recorded with a mic in front of the amp's speaker, for recordings, the sound engineer or music producer may ask for the DI out signal from the pickups to be recorded onto a separate track at the same time, so that the engineer or producer can re-amp the signal through various different amps at a later time). In contrast, it is fairly common to use a DI box with electric bass.

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.

Even in the 2010s, the vintage Fender Bandmaster remains a sought-after amp by guitarists. Note the four inputs, two for regular sound and two which are run through the onboard vibrato effect unit. The amp pictured is a 1968 model.

A variety of labels are used for level attenuation potentiometers (knobs) in a guitar amplifier and other guitar equipment. Electric guitars and basses have a volume control on the instrument that attenuates the signal from selected pickups. There may be two volume controls on an electric guitar or bass, wired 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, such as some vintage amps and modern practice amps, have only a single volume control. Most have two volume controls: a first volume control called "preamplifier" or "gain" and a master volume control. The preamp or gain control is designed differently on different types of guitar amps. On an amp designed for an acoustic guitar, turning up the preamp knob will preamplify the signal, but even at its maximum setting, the preamp control is unlikely to produce much overdrive. However, with amps designed for electric guitarists playing electric blues, hard rock and heavy metal music, turning up the preamp or gain knob will usually produce overdrive or distortion. Some electric guitar amps have three controls in the volume section: preamplifier, distortion and master control. Turning up the preamp and distortion knobs in varying combinations can create a range of overdrive tones, from a gentle, warm growling overdrive suitable for a traditional blues show or a rockabilly band to the extreme distortion used in hardcore punk and death metal. On some electric guitar amps, the "gain" knob 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.

The patch bay at the rear panel of this Line 6 Flextone guitar amp provides a number of additional inputs and outputs, including stereo XLR DI unit outputs.

A simple, inexpensive amplifier may have only two tone controls, a passive bass and treble control. In some better quality amps, one or more midrange controls are provided. On the most expensive amps, there may be shelving equalizers for bass and treble, a number of mid-range controls (e.g., low mid, mid and high mid), and a graphic equalizer or parametric equalizer. 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".

 

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