Guitar amplifier

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Combo amplifier from the British brand VOX, model Valvetronix AD60VT . Before that, an electric guitar from Rickenbacker

A guitar amplifier is an electronic device used to amplify the sound of a guitar or an electric guitar . In a broader sense, amplifiers for bass guitars ( electric bass ) are also referred to as guitar amplifiers, even if they have some different design features adapted to low sound frequencies.

The signal from one or more guitar pickups is transmitted to the amplifier via a special instrument cable or via a radio link . In contrast to most devices for amplifying acoustic guitars, amplifiers for electric guitars and basses are generally not supposed to amplify the signal of the musical instruments in a sound-neutral manner, but rather contribute significantly to the formation of the sound. Guitarists often regard guitar amplifiers as part of the instrument and use the devices not only to generate volume, but also to give sound expression to their playing.


Guitar amplifiers can be classified into the following categories:

  • According to the design (combo - open or closed -, top part, pre- and power amplifier separately, rack system),
  • According to the intended use (for electric guitar , electric bass , acoustic guitar , practice, studio and stage amplifier),
  • After using the reinforcing components ( electron tube , semiconductor or both ("hybrid")),
  • According to the predominant sound characteristics ("clean", distorted, "British" or "American").

Practically all categories of guitar amplifiers available on the market can be combined with one another, with an emphasis on one or the other. This explains the great variety of the offer.

Structure of a guitar amplifier

Block diagram of a typical tube-type guitar amplifier. The signal flow and the arrangement of the reinforcing components as well as the arrangement of the control elements are illustrated.

The individual components of a guitar amplifier are the input (signal input), mostly realized by a connection with a 6.35 mm mono jack plug , preamplification with volume control ("Gain"), tone control ( equalizer ) for raising or lowering individual frequency ranges ( bass , Mids , treble ) ( linear distortion ), power amplifier stage, speaker system, power supply ( power supply unit ).

The loudspeaker system usually consists of 8 to 15 inch loudspeakers, for example a typical Marshall box is equipped with 4 loudspeakers of 12 ″ each and can handle 100 to over 400 watts RMS power .

In many devices a so-called reverberation spiral ( spring reverb ) is built into the generation of artificial reverberation. Occasionally, additional effects such as chorus and tremolo are integrated.

Guitar amplifiers are available in different power levels from around 2 watts (Roland Microcube) to over 350 watts (Marshall Mode Four) output power. As a rule, even higher output powers do not make sense, since in larger stage installations the instruments are transmitted via the sound system .

Nonlinear Distortion

A non-linear distortion of the signal is desirable in certain types of pop music , especially in rock music, and is achieved by targeted overdriving of the guitar amplifier or individual amplifier stages within the guitar amplifier. Overdrive of the preamp is called distortion by the manufacturers , overdrive of the output stage is called overdrive . These can take place in combination and also influence each other in the signal chain. The distortion can also take place outside the guitar amplifier in upstream effects devices , usually through a distortion .

The distortion in the preamp compresses the signal more strongly (like e.g. used in heavy metal ), while with power amplifier distortion more dynamics, similar to the clean sound, remains (like e.g. in blues / rock ).

The distorted operation of all stages of the guitar amplifier leads to operation at very high volume without special measures. If this is not desired, but the entire amplifier should still be distorted, a resistor network can be connected upstream of the loudspeaker , which converts most of the output signal into heat and only supplies a fraction of the output power to the loudspeaker ( power soak ). Alternatively, a power reduction in the output stage (e.g. by reducing the operating voltage of the tubes) leads to the desired result. In all cases, however, it must be taken into account that if the overall volume is reduced, the non-linear distortion of the loudspeaker, which also contributes to the sound, as does the distortion of the power amplifier, inevitably decrease.

Guitar amplifiers usually have several signal paths ("channels") with different sound properties. A so-called clean channel has only one volume adjuster, sometimes combined with a tone setting. A channel with more distortion usually has a so-called gain control, which influences the degree of distortion, coupled with a volume control for the output volume. The channels can be selected by pressing a button on the amplifier or with a foot switch.



Warm output stage of a self-made guitar amplifier with two EL34 tubes

In tube amplifiers , electron tubes are used to amplify the signal . Although tubes are rarely used in electronics nowadays (due to their size, weight, heat generation, long-term stability and complex power supply), this is still the case with guitar amplifiers due to their special transmission properties.

On the dynamics out continuously the behavior under overload (has in tubes English "overdrive" ) of the amplifier particular importance: This is a condition produced in the so strong signals, the amplifier is no longer able to reproduce it faithfully. The result is a distortion of the signal.

RFT ECC83 as a preamp tube in a guitar amplifier

Due to the quasi-modular structure of modern tube amplifiers (one or two "pre-stages" and one "power stage" connected in series) - different distortions can be generated. The preliminary stages are usually implemented with triode systems (double triodes ECC81, ECC82, ECC83 ), the output stages with power pentodes ( 6L6 , 6V6, EL84 , EL34, etc.). If you overdrive the preamp, you already get a distorted signal at the input of the output stage and by turning down the output stage, this distorted signal can be sent to the loudspeakers with a relatively low voltage. The low overall volume is an advantage, but the sound is not the same compared to that of a distorting power amplifier.

With the first tube amplifiers from the 1950s, you could hardly overdrive the preamps, but the distortion was achieved by turning the amplifier as loud as possible. This means that the power amp tubes were overdriven and the transformer ( transformer for impedance matching between tubes and speakers) went into saturation , which made the sound of guitarists such as Jimi Hendrix or Alvin Lee ( Ten Years After ).

The oscilloscope shows a
sine that has been clipped

Such a signal as a curve on an oscilloscope is shown opposite. You can see that the signal peaks are flattened. This effect becomes stronger with increasing power.

In terms of communications technology, single-ended tube amplifiers increasingly add even harmonics ( overtones ) to the signal and the signal is increasingly softly limited ( soft clipping ). Transistor amplifiers, on the other hand, work linearly up to maximum power; if they are overdriven, the limitation (" clipping ") starts suddenly. The approximately rectangular shape of the signal results in very high frequency components ( Fourier analysis ) and, due to the push - pull circuits used, odd harmonics occur.

The acoustic impression of the distorting tube amplifier is described as "denser", "louder", "rougher", "groovier" than when the setting is not overdriven. This sound is important in all branches of rock music and as a typical " electric guitar sound" is part of z. B. the music styles hard rock and heavy metal .

Presumably due to the benevolent overdriving behavior and the more pronounced natural resonance of the loudspeakers due to the higher internal resistance of the tube amplifier, the following rule of thumb is common: Same output power → tube sounds twice as loud as the corresponding transistor power. It should be noted, however, that the volume impression ( loudness ) can only be described subjectively, only increases roughly logarithmically with increasing output power and also depends on the frequency.

A 100 watt amplifier is not perceived as twice as loud as a 50 watt amplifier.

  • Compared to 50 watts:
    • 40 watts is 94% as loud as 50 watts
    • 25 watts is 81% as loud as 50 watts
    • 15 watts is 70% as loud as 50 watts
    • 5 watts is 50% as loud as 50 watts
    • 1 watt is 31% as loud as 50 watts

The final volume that can be achieved with an amplifier also depends on other factors. A decisive factor is the efficiency of the connected loudspeaker and the construction of the box. An increase in efficiency of 6 dB achieves a higher sound pressure level than doubling the amplifier output and can also significantly influence the dynamics and the sound character by changing the frequency curve. A loudspeaker with 10 dB more efficiency roughly doubles the perceived volume.

Guitar loudspeaker boxes usually have pronounced natural resonances, the loudspeakers are hung up hard and are less attenuated by the high source resistance of the tube power amplifiers. In contrast, HIFI boxes should not convey a sound of their own to the music signal. The resonance behavior of the wooden cabinet, which is often open at the back, is another reason for the lower damping and the "warm" sound with pronounced mid-range boost.

The output stages of tube guitar amplifiers mostly work in AB mode , i. In other words, there is a tube to amplify the positive or negative half-wave (push-pull). This leads to minor transfer distortions that do not occur when the output stage is in A mode . Here the full signal is amplified by a single power amplifier tube. In the idle state, half the maximum current flows through the tube, which is then modulated by the guitar signal. The AB operation is mostly used for higher outputs, while the A operation requires fewer components. In small guitar amplifiers you can z. For example, one EL84 can achieve an output power of around 5W, while two EL84s in push-pull operation typically achieve around 15W. Often several tubes are connected in parallel to increase the total output. So z. B. with several EL34 parallel in push-pull operation, outputs of 100 W or more can be achieved. On the other hand, there are amplifiers that combine the sound properties of A mode by connecting power amplifier tubes in parallel with higher power. A disadvantage of A mode is the constant, relatively high power consumption of the output stage tubes.

Replacing the power amp tube changes not only the performance of the amplifier, but also its sound behavior. Often the power amp tubes are not re-measured. There may also be a difference in sound when tubes from different manufacturers are used. Their nominal data are the same, but they often produce a different sound. The reason for this are manufacturing tolerances.


Rocktron transistor amplifier R120 DSP

In transistor amplifiers today mainly come MOSFET circuits used.


Another variant is the hybrid amplifier. The variant in which tubes are used in the preliminary stage while the power stage is operated with transistors has been widespread since the 1980s.

Conversely, hybrid amplifiers can also combine transistor preamp and tube output stage. This construction method was especially popular in the 1970s, when the robustness of the tried and tested tube output stages was still difficult to achieve with transistor output stages, but on the other hand, transistors enabled greater flexibility in the input and tone control stages. However, tube output stages are more complex than transistor output stages because of the necessary transformers, but with the same output.

Modeling / simulation

For the first time, the Roland company successfully simulated complete guitar amplifiers of various types and companies that were acoustically picked up by microphones using integrated computer models . The company Line 6 made this method popular. This technology uses DSPs (digital signal processors) to simulate the behavior of tube amplifiers using mathematical models. They are also known as modeling amps.

There are numerous guitar amplifiers today that use this method of sound generation . Due to the greater flexibility and variety of sounds, these amplifiers are increasingly replacing the classic guitar amplifiers. So far, however, modeling amps on stage have not yet been able to completely replace the classic tube amplifiers. Tech21 offers an alternative result, which in turn uses a computer to convert the amp calculation models into a real circuit made of electronic components in order to generate very real sounds.

  • Examples:
    • Roland VGA-7, Cube
    • Hughes & Kettner Zentera
    • Line 6 Vetta, Spider, Helix
    • Fender G-DEC, Mustang
    • Marshall Code
    • VOX AD-120 VTH
    • Yamaha THR
    • Fractal Audio AxeFx

In October 2007, Line 6, in cooperation with Reinhold Bogner, presented the so-called Spider Valve 112/212 for the first time, an all-tube-based modeling combo that is supposed to simulate the sound of classic and high-quality tube amps almost identically.

Since 2006, the US start-up "FractalAudio", under the direction of Cliff Chase, has been offering the "AxeFx", in their own words, a new generation of guitar amp simulators. High-quality analog-digital converters and processors with high computing power are used. The dynamics of classic guitar amps are simulated with the help of fractal algorithms.


Combo amplifier

Combo amplifier

With the combo amplifier (short: combo) the electronics and one or more loudspeakers are mounted in a common housing. The first guitar amplifiers from the late 1920s to the 1950s were all of this type. The housing can be open or closed at the back, which affects the sound characteristics of the loudspeaker. Smaller combos are easier to transport than other types. Larger devices, such as the Fender Twin Reverb, are more difficult to transport because all components have to be moved in one piece. The output power is usually in the range of 5 to 100 watts. Combos with even lower output power can in part also be supplied with voltage from batteries. They are suitable for practice and for venues without a mains power supply.

Top part & box

Marshall full stack

If the amplifier and box are separate, one speaks of top part (English head ) and box (English cabinet ). If the mentioned parts are stacked on top of each other, one speaks of a tower (English stack ). Here one subdivides again into a half-stack (amplifier with one box) and a full-stack (amplifier stands on 2 boxes). The boxes of the so-called stacks are usually units with 4 x 12 ″ guitar speakers each. However, there are also 1 × 12 ″, 2 × 12 ″ or 4 × 10 ″ boxes.

Rack (pre & power stage)

In traditional amplifiers, the preamp and power amplifier form a unit. But there are also numerous variants in which the pre- and final stage form their own units. These are mostly built into 19 ″ rack systems.

  • Examples (preliminary stage)
    • Marshall JMP 1st
    • Mesa / Boogie Triaxis
  • Examples (final stage)
    • Mesa / Boogie Strategy 500
    • Peavey Classic 50/50

Headphone amplifier

Rockman headphone amplifier from the 1980s

One of the first amplifiers that could only be used with headphones was the Rockman , which was developed by Tom Scholz, founder of the rock group Boston , in the 1980s.

Battery operated guitar amplifiers

Battery-powered amplifiers are increasingly being offered, either with a permanently installed, rechargeable battery or with replaceable batteries. Smaller models are intended for home use, while larger models with speakers up to 12 inches in diameter are also suitable for street musicians . Some of the smallest battery powered amplifiers can be attached to the musician's belt.


Well-known manufacturers of guitar amplifiers and speakers are among others (in alphabetical order):

Ampeg | Bogner | Behringer | Blackstar | Diezel | Dynacord | EBS | Echolette | Engl | Fender | Bell sound | Hiwatt | Hughes & Kettner | Ibanez | Laney | Line 6 | Marshall | Mesa / Boogie | Orange | Peavey | Roland | Trace Elliot | VOX | Warwick | Yamaha


  • Wolfgang Teder: Guitar amplifier in transistor technology. Aachen 1987, Elektor Verlag, ISBN 3-921608-52-X .
  • Helmuth Lemme: Guitar amplifier sound. Pflaum-Verlag Munich 1994, ISBN 3-7905-0717-2 .
  • Rainer zur Linde: tube amplifiers for guitars and hi-fi. Aachen 1986, Elektor Verlag, ISBN 3-921608-41-4 .
  • Manfred Zollner: Physics of the electric guitar (research documentation, including about guitar amplifiers, PDF download possible).
  • Aspen Pittman: The Tube Amp Book (en.), Backbeat Books 2003, ISBN 0-87930-767-6 (information and circuit diagrams of historical tube guitar amplifiers).

See also

Wikibooks: Guitar  - Learning and Teaching Materials
Commons : Guitar Amp  - Collection of Images, Videos, and Audio Files

Web links

Individual evidence

  1. ^ Carlo May: Vintage guitars and their stories . In it: Chapter The transistor looks into the tube - A look into the history of guitar amplifiers . MM-Musik-Media-Verlag, Augsburg 1994. ISBN 3-927954-10-1 . P. 96 ff.