In the first instance you plug in the jack and turn it all on, so what’s to understand? A guitar amplifier (or guitar amp) is an electronic device designed to make the signal from an electric or acoustic guitar louder so that it will make what you play audible through a loudspeaker.

Guitar amplifiers also modify the tone of sounds by emphasizing or de-emphasizing certain frequencies and adding electronic effects. They are the middle man between you and the sound your audience hears, so understanding more about guitar amps is quite important when you are ready to get on stage and start performing.

Understanding Guitar Amplifiers

Guitar Amplifiers consist of one or more circuit stages which have unique responsibilities in the modification of the input signal. The power amplifier or output stage produces a high current signal to drive a speaker to produce sound.

One or more pre amplifier stages precede the power amplifier stage. The pre amplifier is a voltage amplifier that amplifies the guitar signal to a level that can drive the power stage.

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

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

The tone stages may also have electronic effects such as equalization, compression, distortion, chorus, or reverb.

Amplifiers may use vacuum tubes (in Britain they are called valves), or solid state (transistor) devices, or both.

Guitar amplifiers range in price and quality from small, low-powered practice amplifiers, designed for students, which sell for less than US$50, to expensive “boutique” amplifiers which are custom-made for professional musicians and can cost several thousands of dollars.

Two Main Guitar Amplifier Systems

Standalone Guitar Amplifier

(often called a “head” or “amp head”), which does not include a speaker, but rather passes the signal to a speaker cabinet or “cab”.

In the “amp head” form, the amplifier head is separate from the speakers, and joined to them by speaker cables. The separate amplifier is called an amplifier head, and is commonly placed on top of one or more loudspeaker enclosures.

A separate amplifier head placed atop a guitar speaker enclosure or guitar speaker cabinet forms an amplifier “stack” or “amp stack”.

Amp heads may also have the different types of input and output jacks listed above in the combo section. In addition to a 1/4″ input jack, acoustic guitar amplifiers typically have an additional input jack for a microphone, which is easily identified because it will use a three-pin XLR connector.

Phantom power is not often provided on general-use amps, restricting the choice of microphones for use with these inputs. However, for high-end acoustic amplifiers, phantom power is often provided, so that musicians can use condenser microphones.

Amplifiers used with electric guitars may be solid state, which are lighter in weight and less expensive than tube amplifiers. Most guitarists, particularly in the genres of blues and rock, have preferred the sound of vacuum tube amplifiers despite their higher cost, heavier weight, the need to periodically replace tubes and need to re-bias the output tubes (every year or two with moderate use). Some companies design amplifiers that require no biasing as long as properly rated tubes are used. Some more modern amplifiers use a mixture of tube and solid-state technologies.

Since the advent of microprocessors and digital signal processing, “modeling amps” were developed in the late 1990s, these can simulate the sounds of a variety of well-known tube amplifiers without needing to use vacuum tubes.

Amplifiers with processors and software emulate the sound of a classic amp well, but from the player’s point of view the response of these amplifiers may not feel the same as the digital modeling may not accurately model all aspects of a tube amplifier.

Combination Guitar Amplifiers

(combo amps) which include an amplifier and one, two, or four speakers in a wooden cabinet

The “combination” (or “combo”) amplifier contains the amplifier head and guitar speakers in a single unit which is typically housed in a rectangular wooden box. The amplifier head or “amp head” contains the electronic circuitry constituting the preamp, built-in effects processing, and the power amplifier.

Combo amps have at least one 1/4″ input jack where the patch cord from the electric guitar can be plugged in. Other jacks may also be provided, such as an additional input jack, “send” and “return” jacks to create an effects loop (for connecting electronic effects such as compression, reverb, etc.), an extension speaker jack (for connecting an additional speaker cabinet).

Some smaller practice amps have stereo RCA jacks for connecting a CD player, iPod or other sound source and a 1/4″ headphone jack so that the player can practice without disturbing neighbours or family members.

Some amplifiers have a line out jack for connecting the amplifier’s signal to a PA system or recording console or to connect the amplifier to another guitar amp. But in most styles of rock and blues guitar, the line out is not used to connect the guitar amp to a PA system or recording console, because the tonal coloration and overdrive from the amplifier and speaker is considered an important part of the amplifier’s sound.

However, players do use the line out to connect one guitar amplifier to another amplifier, in order to create different tone colors or sound effects.

See Types of Guitar Amplifiers

Amplifier Stacks and Configuration

In the case of electric guitars, an amplifier stack consisting of a head atop one cabinet is commonly called a half stack, while a head atop two cabinets is referred to as a full stack.

The cabinet which the head sits on often has an angled top in front, while the lower cabinet of a full stack has a straight front. The first version of the Marshall stack was an amp head on an 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.

In heavy metal bands, the term “double stack” or “full stack” is sometimes used to refer to two stacks, with a second amplifier head serving as a slave to the first and four speaker cabinets in total.

Typically, a guitar amp’s pre amplifier section provides sufficient gain so that an instrument can be connected directly to its input, and sufficient power to connect loudspeakers directly to its output, both without requiring extra amplification.

Another arrangement, often used for public address amplifier systems, is to provide two stages of amplification in separate units. First a pre amplifier or mixer is used to boost the instrument output, normally to line level, and perhaps to mix signals from several instruments. The output from this pre amplifier is then connected to the input of a power amplifier, which powers the loudspeakers.

Performing musicians that use the “two-stage” approach (as opposed to an amplifier with an integrated pre amplifier and power amplifier) often want to custom-design a combination of equipment that best suits their musical or technical needs, and gives them more tonal and technical options.

Some musicians require pre amps that include specific features. Acoustic performers sometimes require pre amps with “notch” filters (to prevent feedback), reverb, an XLR DI output, or parametric equalization.

Hard rock, metal, or punk performers may desire a pre amplifier with a range of distortion effects. As well, some musicians have specific power amplifier requirements, such as low-noise design, very high wattage, the inclusion of limiter features to prevent distortion and speaker damage, or bi-amp-capable operation.

With the “two-stage” approach, the pre amplifier and power amplifier are often mounted together in a rack case. This case may be either free-standing or placed on top of a loudspeaker cabinet. If many rack-mounted effects are used, the rack may be a large unit on wheels.

Some touring players need several racks of effects units to reproduce on stage the sounds they have produced in the studio. At the other extreme, if a small rack case containing both preamp and power amp is placed on top of a guitar speaker cabinet, the distinction between a rack and a traditional amp head begins to blur.

Another variation is to combine the power amplifier into the speaker cabinet, an arrangement called a powered speaker, and use these with a separate preamp, sometimes combined into an effects pedal board or floor pre amp + processor.

Pre amplifiers are also used to connect very low-output or high-impedance instruments to instrument amplifiers. When piezoelectric transducers are used on upright bass or other acoustic instruments, the signal coming directly from the transducer is often too weak and it does not have the correct impedance for direct connection to an instrument amplifier. Small, battery-powered pre amps are often used with acoustic instruments to resolve these problems.

Guitar Amplification Power Output

For electric guitar amplifiers, there is often a distinction between “practice” or “recording studio” guitar amps, which tend to have output power ratings of 20 watts down to a small fraction of a watt, and “performance” amps, which are generally 50 watts or higher.

Traditionally, these have been fixed-power amplifiers, with a few models having a half-power switch to slightly reduce the listening volume while preserving power-tube distortion. The relationship between perceived volume and power output is not immediately obvious.

A 5-watt amplifier is perceived to be half as loud as a 50-watt amplifier (a tenfold increase in power), and a half-watt amplifier is a quarter as loud as a 50-watt amp.

Doubling the power of an amplifier results in a “just noticeable” increase in volume, so a 100-watt amplifier is held to be only just noticeably louder than a 50-watt amplifier. Such generalizations are also subject to the human ear’s tendency to behave as a natural compressor at high volumes.

Power attenuation can be used with either low-power or high-power amplifiers, resulting in variable-power amplifiers. A high-power amplifier with power attenuation can produce power-tube distortion through a wide range of listening volumes.

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.

Guitar Amp Distortion

Distortion is a feature available on many guitar amplifiers that is not typically found on keyboard or bass guitar amplifiers. Vacuum Tube guitar amplifiers can produce distortion through pre-distortion equalization, preamp tube distortion, post-distortion EQ, power-tube distortion, tube rectifier compression, output transformer distortion, guitar speaker distortion, and guitar speaker and cabinet frequency response.

Distortion sound or “texture” from guitar amplifiers is further shaped or processed through the frequency response and distortion factors in the microphones (their response, placement, and multi-microphone comb filtering effects), microphone preamps, mixer channel equalization, and compression.

Additionally, the basic sound produced by the guitar amplifier can be changed and shaped by adding distortion and/or equalization effect pedals before the amp’s input jack, in the effects loop just before the tube power amp, or after the power tubes.

Guitar Amp Volume Control

A variety of labels for volume control or level attenuation potentiometers are used for guitar amplifiers and other equipment. Electric guitars and basses have a volume control to attenuate whichever pickup is selected. There may be two volume controls in parallel to mix the signal levels from the neck and bridge pickups. Rolling back the guitar’s volume control also changes the pickup’s equalization or frequency response, which can provide pre-distortion equalization.

The simplest guitar amplifiers have only a volume control. Most have at least a gain control and a master volume control.

The gain control is equivalent to the distortion control on a distortion pedal, and similarly may have a side-effect of changing the proportion of bass and treble sent to the next stage.

A simple amplifier’s tone controls typically include passive bass and treble controls. In some cases, a midrange control is provided. The amplifier’s master volume control restricts the amount of signal permitted through to the driver stage and the power amplifier.

When using a power attenuator with a tube amplifier, the master volume no longer acts as the master volume control. Instead, the power attenuator’s attenuation control determines 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”.

Musicians often run sound-sources other than guitars through guitar amps. For live performances, synthesizers and drum machines or keyboards are often put through guitar amps to create a richer sound than can be obtained by patching the direct-outs right into the PA system. Guitar amplifiers can add tonal coloration, roll off unwanted high frequencies, and add overdrive or distortion.