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Distortion (guitar)
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Distortion, also known as overdrive or fuzz, is an effect applied to the electric guitar, the electric bass, and other amplified instruments such as the Hammond organ, synthesizers, and even harmonica and vocals. Accomplished by electronically compressing and/or clipping the input signal, this effect adds sustain and additional harmonics and overtones to the signal, creating a richer sound. The most subtle types of distortion add a "warm" thickness and sustain to the original tone; the more extreme types of distortion range from the noisy, buzzy sound of a late 1960s-era fuzztone pedal to the screaming, "bite", "grit",and "crunch" of a late 1980s thrash-style distortion pedal.
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Encyclopedia
Distortion, also known as overdrive or fuzz, is an effect applied to the electric guitar, the electric bass, and other amplified instruments such as the Hammond organ, synthesizers, and even harmonica and vocals. Accomplished by electronically compressing and/or clipping the input signal, this effect adds sustain and additional harmonics and overtones to the signal, creating a richer sound. The most subtle types of distortion add a "warm" thickness and sustain to the original tone; the more extreme types of distortion range from the noisy, buzzy sound of a late 1960s-era fuzztone pedal to the screaming, "bite", "grit",and "crunch" of a late 1980s thrash-style distortion pedal. Distortion is used across a wide range of musical genres, from the subtle overdrive used in traditional blues to the hard-edged distortion featured in noise music, hardcore punk and metal.
Early examples of distortion were often the result of accidents in which the guitar amplifier or its vacuum valves was damaged, or because the amplifiers and speakers were "doctored" by poking holes into their speaker cones. Observing this trend, Leo Fender of the Fender amplifier company designed valve guitar amps that would "overdrive" slightly. In the 1960s, Jimi Hendrix was one of the first guitarists to experiment with fuzzbox effect pedals.
Distortion can be produced by many components of an instrument's signal path, including effects pedals, the pre-amplifier, power amplifier, speakers, or more recently, digital amplifier modelling devices and software. Many players use a combination of these to obtain their "signature" tone.
History
In the early days of guitar amplification, amplifiers were primitive and low-fidelity, and distortion was inherent in the signal chain. Most amplifiers were all-purpose, designed for use with multiple instruments with different output levels, and guitar pickups were often clip-on types that had weak output levels and microphonic properties. The guitars were typically hollow-body instruments, which would resonate sympathetically with the amplified signal, causing unwanted feedback and an excessive resonant sustain in the bass frequencies. Though electric guitars had been around since 1928 and played popularly by Les Paul and Charlie Christian in the 1930s and 1940s, it was not until the early 1950s that they became commercially successful. It was during this period that the first solid-bodied electric guitars became widely available; they did not suffer as badly from feedback as earlier models, hence they could be played at higher output levels.
The idea of intentionally using distortion to improve the amplified tone had not occurred to early amplifier makers. Early examples of distortion were often the result of accidents in which the guitar amplifier was in some way damaged, but the player or producer decided they liked it and recorded it that way. During the recording of "Rocket 88", one of the early rock and roll songs, Ike Turner and the Kings of Rhythm guitarist Willie Kizart used an amplifier that had been damaged in transit, resulting in an early recorded example of guitar distortion. For the recording of "The Train Kept A-Rollin'" by the Johnny Burnette Trio, a valve fell out of the amplifier during a live performance. When a reviewer then raved about the crazy new sound, Burnette used the same tone in the recording studio.
An early user of valve overdrive was Chuck Berry, who at the start of his career played through small valve amplifiers, the only ones he could afford. Because of their low output, they were easy to overdrive, giving Chuck's guitar tone a warm overdriven sound, which can be heard on his recording of his first hit "Maybellene". On later recordings he was able to afford better and larger amps and consequently his tone became cleaner. The earliest uses of intentional distortion that have been recorded were achieved through "doctoring" amplifiers and speakers. Guitarists would use a razor blade, screwdriver or pencil to poke holes into their speaker cones to create a distorted sound.
Leo Fender of Fender guitars and amplifiers observed these trends and engineered many of his amplifiers to "compress" and or "overdrive" slightly without drastically distorting the signal. The early Fender "Tweed" and "Blackface" amplifiers are considered a good example of clean electric guitar tone. Many later amplifiers are based on these designs. Significantly, Jim Marshall of Marshall Amplifiers copied the Fender Bassman using parts available in the United Kingdom, creating an amplifier with significant overdrive that quickly caught on in the local music scene and laid the foundation for the powerful, thick "Marshall Sound" that can be heard on so many early heavy metal albums. Later, distortion and fuzz effects were achieved through electronics. Jimi Hendrix was one of the first guitarists to use outboard effects, many designed or modified by guitar tech Roger Mayer.
Physics of clipping
Literally, the word distortion refers to any aberration of the waveform of an electronic circuit's output signal from its input signal. In the context of musical instrument amplification, it refers to various forms of clipping, which is the truncation of the part of an input signal that exceeds certain voltage limits. Because both valves and transistors behave linearly within a certain voltage region, distortion circuits are finely tuned so that the average signal peak just barely pushes the circuit into the clipping region, resulting in the softest clip and the least harsh distortion. Because of this, as the guitar strings are plucked harder, the amount of distortion and the resulting volume both increase, and lighter plucking cleans-up the sound.
Valve overdrive
Before the widespread adoption of the transistor, the traditional way to create gain (amplification) and distortion was through vacuum valves (called "tubes" in the US). A vacuum valve has a maximum input voltage determined by its bias and a minimum input voltage determined by its supply voltage. When any part of the input waveform approaches these limits, the valve's amplification becomes less linear, meaning that smaller voltages get amplified more than the large ones. This causes the peaks of the output waveform to be compressed, resulting in a waveform that looks "squashed". This is known as "soft clipping", and generates even-order harmonics that add to the warmth and richness of the guitar's tone. If the valve is driven harder, the compression becomes more extreme and the peaks of the waveforms are clipped. This adds additional odd-order harmonics, creating a "dirty" or "gritty" tone.
Valve distortion is commonly referred to as overdrive, as it is attained by driving the valves in an amplifier at a higher level than can be handled cleanly. Multiple stages of valve gain/clipping can be "cascaded" to produce a thicker and more complex distortion sound. In some modern valve effects, the "dirty" or "gritty" tone is actually achieved not by high voltage, but by running the circuit at voltages that are too low for the circuit components, resulting in greater non-linearity and distortion. These designs are referred to as "starved plate" configurations, and result in an "amp death" sound.
Transistor clipping
Transistor clipping stages, on the other hand, behave far more linearly within their operating regions, and thus faithfully amplify the instrument's signal until the input voltage falls outside its operating region, at which point the signal is clipped without compression, known as "hard clipping" or limiting. This type of distortion tends to produce more odd-order harmonics. Electronically, this is usually achieved by either amplifying the signal to a point where it must be clipped to the supply rails, or by clipping the signal across diodes. Many solid state distortion devices attempt to emulate the sound of overdriven vacuum valves.
Approaches
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.
Overdrive/distortion pedals
Because they are often designed to operate off of low voltages such as a 9 volt battery, overdrive and distortion pedals typically use transistors to generate distortion. Classic examples include the Ibanez Tube Screamer and the Electro-Harmonix Big Muff. A few more modern effects pedals incorporate valves, but usually these still run at voltages that are too low for the valve, resulting in a "starved plate" configuration that generates harsh and buzzy distortion. Distortion pedals usually also provide signal gain, which can be used to drive the input stage of the pre-amplifier harder, resulting in further distortion and, in some cases, higher volume.
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, many amps featured a "master volume" control, essentially an adjustable attenuator between the preamp section and the power amp that conveniently enables the generation of high distortion levels in the guitar amp's preamp section while diverting most of the resulting signal away from the power valves, keeping the output volume at manageable levels. However, this also results in the power valves being operated well within their linear region, reducing the distortion that they add to the output signal. These "master volume" amps, when run with the master volume control on full and the preamp volume low--while creating much clean headroom--can result in very high volumes indeed if paired with an overdrive effect such as the Ibanez Tube Screamer.
Solid-state gain/clipping stages are also employed in many amplifiers. Some amplifiers (notably the Marshall JCM900) utilize hybrid designs that employ both valve and solid-state components.
Power amplifier distortion
Power valves 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. Because driving the power valves this hard also means maximum volume, many solutions have emerged that in some way divert some of this power valve output from the speakers, 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. Lower-power valve amps (such as a quarter-watt or less), speaker isolation boxes, and low-efficiency guitar speakers are also used to tame the volume.
Although traditional amplifiers were complete circuits including both preamp and power amp, 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.
Output transformer distortion
The output transformer sits between the power valves and the speaker, serving to match impedance and voltage. When a transformer's ferromagnetic core becomes electromagnetically saturated, it will also clip, adding additional distortion to the signal delivered to the speakers.
Power supply "sag"
Early valve amplifiers usually used unregulated power supplies. This was due to the high cost associated with high-quality high-voltage power supplies. The typical anode supply was simply a rectifier, an inductor and a filter 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. In a Class A amplifier, current draw is constant, so sag does not occur.
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. In theory, these keep the supply voltage constant, but in reality there is still some small variation, largely due to resistive losses in the cabling from the power supply to the gain stage.
Speaker distortion
Guitar loudspeakers are designed differently than high fidelity stereo speakers or PA system speakers. While hi-fi and PA speakers are designed to reproduce the sound with as little distortion as possible, guitar speakers are usually designed so that they will shape or colour the tone of the guitar, either by enhancing some frequencies or attenuating unwanted frequencies. As well, when the power delivered to a guitar speaker approaches its maximum rated power, the speaker's performance becomes less linear, causing the speaker to "break up", adding further distortion and colouration to the signal. Some speakers are designed to have lots of clean headroom, while others are designed to break up early to deliver grit and growl.
Amp modelling for distortion emulation
Guitar amp modelling devices and software can reproduce various guitar-specific distortion qualities that are associated with a range of popular "stomp box" pedals and amplifiers. Amp modelling 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 modelling pedal could simulate the sound of plugging their electric guitar into a heavy vintage valve amplifier and an 8 X 10" stack of speaker cabinets. Some modelling devices allow even more detailed simulation, such as the different tonal effect that would occur from mic'ing the speaker cabinets with a cariodid microphone or a ribbon mic.
Amplifier and distortion modelling can be accomplished by using different methods: realtime software running on a computer: hardware such as a compact pedal, oversize pedal, rackmount processor, desktop or floor processor; guitar amp heads, including hybrid valve amps that use both analogue and digital technology. As sound of the highest-end modelling devices can be very convincing, these processors are widely used for live performances, because it reduces the amount of heavy, vintage amplifiers that guitarists have to transport. Some smaller independent recording studios use modelling software or processors, because they allow performers to produce widely-used "classic tones" without having to purchase or rent expensive vintage equipment or record at high volume levels. Digital modelling devices may not be able to recreate all the subtle aspects of the sound of vintage, overdriven valve amplifiers, because this sound is the product of a range of non-linear and random factors, ranging from the heat of the vacuum valves to the age and condition of the speakers. As a result, professional musicians tend to use actual vintage valve amps for recordings, because the recorded sound will have to stand up to greater scrutiny from listeners and critics.
Voicing with equalization
Rock guitar distortion is obtained and shaped at various points in the signal processing chain, including multiple stages of preamp distortion, power valve distortion, power transformer distortion, and guitar speaker distortion. Much of the distortion character or voicing is controlled by the frequency response curve before and after each distortion stage. This dependency of distortion voicing on frequency response can be heard in the effect that a wah pedal has on the subsequent distortion stage, or by using an EQ pedal to favor the bass or treble components of the guitar pickup signal prior to the first distortion stage. Some guitarists place an equalizer pedal after the distortion effect, to emphasize or de-emphasize different frequencies and create different tonal coloration.
A guitar amplifier's tone controls shape a different power-valve distortion voicing if the tone controls are set to emphasize the bass or treble. Extreme settings are most popular in heavy metal. Increasing the bass and treble while reducing or eliminating the centre midrange (750Hz) results in what is popularly known as a "scooped" sound (since the midrange frequencies "scooped" out). James Hetfield of Metallica used this tone on many songs on Metallica's first four studio albums. Conversely, decreasing the bass while increasing the midrange and treble creates a harsher sound. Kerry King and Jeff Hanneman of Slayer have both used midrange-heavy tones since the mid-1980s.
See also
External links
- (R.G. Keen) Article on the physics of distortion and a round-up of electronic techniques
- Article on the physics of distortion, relationship of waveforms and harmonics
- - Website on overdriven guitar amplifier and effects, covering: tone settings, distortion voicing, simulation and modeling, processors, speakers, power-supply modifications, switching and signal routing gear, software and recording, and DIY projects.
- - Cooperative, non-profit website offering free schematics and plans to help readers learn to build their own guitar amps.
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