Guitar distortion can be produced by many components of the guitar's signal path, including effects pedals, the pre-amplifier, power amplifier, and speakers. Many players use a combination of these to obtain their "signature" tone.
Pre-amplifier distortion The
pre-amplifier section of a guitar amplifier serves to amplify a weak instrument signal to a level that can drive the power amplifier. It often also contains circuitry to shape the tone of the instrument, including
equalization and
gain controls. Often multiple cascading gain/clipping stages are employed to generate distortion. Because the first component in a valve amplifier is a valve gain stage, the output level of the preceding elements of the signal chain has a strong influence on the distortion created by that stage. The output level of the guitar's pickups, the setting of the guitar's volume knob, how hard the strings are plucked, and the use of volume-boosting effects pedals can drive this stage harder and create more distortion. During the 1980s and 1990s, most valve amps featured a "master volume" control, an adjustable
attenuator between the preamp section, and the power amp. When the preamp volume is set high to generate high distortion levels, the master volume is lowered, keeping the output volume at manageable levels.
Effects pedals Effects pedals are among the earliest and most widely used tools for producing intentional distortion in electric guitar performance. Compared to valve amplifiers, effects pedals offer a more affordable, portable, and versatile means of achieving a wide range of overdrive, distortion, and fuzz tones. Analog effects pedals work on similar principles to preamplifier distortion. Because most effects pedals are designed to operate from battery voltages, using vacuum tubes to generate distortion and overdrive is impractical; instead, most pedals use solid-state transistors,
op-amps, and diodes. Generally, overdrive pedals aim to simulate the sound of a tube amplifier pushed into saturation, providing a dynamic response to picking intensity. The first effects pedals strictly marketed as "overdrive" were aimed at players who wanted the characteristic "amp breakup" tone without having to play at very high volume levels or risk damaging their equipment. Distortion pedals produce a coloured, more compressed and heavily saturated tone, less dependent on playing dynamics; as such, they are often paired with relatively clean amplifiers so that the pedal provides the bulk of the distortion. Fuzz pedals, which date back to the 1960s, use extreme clipping to generate a buzzy, harmonically rich sound that can resemble a square wave. Classic examples of overdrive pedals include the
Ibanez Tube Screamer and the
Boss OD series. Distortion pedals include the
Pro Co RAT and
Boss DS-1, which both found use in heavier guitar based music like grunge and alternative rock, while fuzz pedals like the
Electro-Harmonix Big Muff Pi create a sustaining, harmonically rich sound. Most distortion effects pedals can be used in two ways: a pedal can be used as a "boost" with an already overdriven amplifier to drive it further into saturation and "color" the tone, or it can be used with a completely clean amplifier to generate the whole overdrive/distortion effect. With care and appropriately chosen pedals, it is possible to "stack" multiple overdrive/distortion pedals together, allowing one pedal to act as a "boost" for another. Fuzz boxes and other heavy distortions can produce unwanted dissonances when playing chords. To get around this, guitarists (and keyboardists) using these effects may restrict their playing to single notes and simple "
power chords" (root, fifth, and octave). Indeed, with the most extreme fuzz pedals, players may choose to play mostly single notes, because the fuzz can make even single notes sound very thick and heavy. Heavy distortion also tends to limit the player's control of dynamics (loudness and softness)—similar to the limitations imposed on a
Hammond organ player (Hammond organs do not produce louder or softer sounds depending on how hard or soft the performer plays the keys; however, the performer can still control the volume with drawbars and the expression pedal). Heavy metal music has evolved around these restrictions, using complex rhythms and timing for expression and excitement. Lighter distortions and overdrives can be used with triadic chords and seventh chords; additionally, lighter overdrive allows more control of dynamics.
Power amplifier distortion power valves, often used in American-made amplifiers Power valves (tubes) can be overdriven in the same way that pre-amplifier valves can, but because these valves are designed to output more power, the distortion and character they add to the guitar's tone is unique. During the 1960s to early 1970s, distortion was primarily created by overdriving the power valves. Because they have become accustomed to this sound, many guitar players favour this type of distortion, and thus set their amps to maximum levels in order to drive the power section hard. Many valve-based amplifiers in common use have a
push-pull output configuration in their power section, with matched pairs of tubes driving the
output transformer. Power amplifier distortion is normally entirely symmetric, generating predominantly odd-order harmonics. Because driving the power valves this hard also means maximum volume, which can be difficult to manage in a small recording or rehearsal space, many solutions have emerged that divert some of this power valve output from the speakers, and allow the player to generate power valve distortion without excessive volume. These include built-in or separate
power attenuators and power-supply-based power attenuation, such as a VVR, or Variable Voltage Regulator to drop the voltage on the valves' plates, to increase distortion whilst lowering volume. Guitarists such as Eddie Van Halen have been known to use variacs before VVR technology was invented. Lower-power valve amps (such as a quarter-watt or less), speaker
isolation cabinets, and low-efficiency guitar speakers are also used to tame the volume. Power-valve distortion can also be produced in a dedicated rackmount valve power amp. A modular rackmount setup often involves a
rackmount preamp, a rackmount valve power amp, and a rackmount
dummy load to attenuate the output to desired volume levels. Some effects pedals internally produce power-valve distortion, including an optional dummy load for use as a power-valve distortion pedal. Such effects units can use a preamp valve such as the
12AX7 in a power-valve circuit configuration (as in the Stephenson's Stage Hog), or use a conventional power valve, such as the
EL84 (as in the
H&K Crunch Master compact tabletop unit). However, because these are usually placed before the pre-amplifier in the signal chain, they contribute to the overall tone in a different way. Power amplifier distortion may damage speakers. A
Direct Inject signal can capture the power-tube distortion sound without the direct coloration of a guitar speaker and microphone. This DI signal can be blended with a miked guitar speaker, with the DI providing a more present, immediate, bright sound, and the miked guitar speaker providing a colored, remote, darker sound. The DI signal can be obtained from a DI jack on the guitar amp, or from the Line Out jack of a power attenuator.
Output transformer distortion The output
transformer sits between the power valves and the speaker, serving to match
impedance. When a transformer's
ferromagnetic core becomes electromagnetically saturated, a loss of inductance takes place, since the back E.M.F. is reliant on a change in flux in the core. As the core reaches saturation, the flux levels off and cannot increase any further. With no change in flux, there is no back E.M.F. and hence no reflected impedance. The transformer and valve combination then generate large 3rd order harmonics. So long as the core does not go into saturation, the valves will clip naturally as they drop the available voltage across them. In single ended systems, the output harmonics will be largely even ordered due to the valve's relatively non linear characteristics at large signal swings. This is only true, however, if the magnetic core does not saturate.
Power supply "sag" Early valve amplifiers used unregulated
power supplies. This was due to the high cost associated with high-quality
high-voltage power supplies. The typical
anode (plate) supply was simply a
rectifier, an inductor and a
capacitor. When the valve amplifier was operated at high volume, the power supply
voltage would dip, reducing power output and causing signal attenuation and compression. This dipping effect is known as "sag", and is sought after by some electric guitarists. Sag only occurs in
class-AB amplifiers. This is because, technically, sag results from more current being drawn from the power supply, causing a greater voltage drop over the rectifier valve. Class AB amplifiers draw the most power at both the maximum and minimum point of the signal, putting more stress on the power supply than class A, which only draws maximum power at the peak of the signal. As this effect is more pronounced with higher input signals, the harder "attack" of a note will be compressed more heavily than the lower-voltage "decay", making the latter seem louder and thereby improving
sustain. Additionally, because the level of compression is affected by input volume, the player can control it via their playing intensity: playing harder results in more compression or "sag". In contrast, modern amplifiers often use high-quality, well-regulated power supplies.
Speaker distortion Guitar
loudspeakers are designed differently from
high fidelity stereo speakers or public address system speakers. While hi-fi and public address speakers are designed to reproduce the sound with as little distortion as possible, guitar speakers are usually designed so that they will shape or color the tone of the guitar, either by enhancing some frequencies or attenuating unwanted frequencies. When the power delivered to a guitar speaker approaches its maximum rated power, the speaker's performance degrades, causing the speaker to "break up", adding further distortion and colouration to the signal. Some speakers are designed to have much clean
headroom, while others are designed to break up early to deliver grit and growl.
Amp modeling for distortion emulation Guitar amp modeling devices and software can reproduce various guitar-specific distortion qualities that are associated with a range of popular "stomp box" pedals and amplifiers. Amp modeling devices typically use
digital signal processing to recreate the sound of plugging into analogue pedals and overdriven valve amplifiers. The most sophisticated devices allow the user to customize the simulated results of using different preamp, power-tube, speaker distortion, speaker cabinet, and microphone placement combinations. For example, a guitarist using a small amp modeling pedal could simulate the sound of plugging their electric guitar into a heavy vintage valve amplifier and a stack of 8 × 10" speaker cabinets. ==Voicing with equalization==