Distortion (guitar)
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Guitar distortion is an effect applied to the electric guitar, the electric bass, and other electronic instruments, giving the instrument's tone more "bite", "grit", "crunch" and sustain. It is used across a wide range of musical genres, from the subtle overdrive used in some forms of the Blues to the hard-edged distortion featured in hard rock, punk rock, and heavy metal.
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[edit] History
In the early days of guitar amplification, amplifiers were rather 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, which would resonate sympathetically with the amplified signal, causing feedback and a resonant sustain in the bass frequencies. The guitar sound coming from these early rigs was therefore very dark and jazzy in comparison to modern rigs. The idea of intentionally using distortion to improve the amplified tone hadn't occurred to amp makers.
Around 1950, solid body electric guitars gained popularity. These guitars afforded higher output signals and more headroom before feedback. Though electric guitars had been around since 1928 and played popularly by Les Paul and Charlie Christian in the 30's and 40's, it wasn't until this time in the early 50's that they really took off.
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. For example, during the recording of "Rocket 88", widely considered to be the first rock and roll song, Ike Turner and the Kings of Rhythm guitarist Willie Kizart used an amplifier that had been damaged in transit, resulting in one of the first recorded examples of guitar distortion. An almost identical story comes from the Johnny Burnette Trio's recording of "The Train Kept A-Rollin'" in which a tube fell out of the amplifier during a live performance, and when a rock reviewer raved about the crazy new sound, he used the same tone in the recording studio.[1]
An early user of tube overdrive was Chuck Berry, who at the start of his career played through small tube 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 "Maybelline", his first hit. 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.[2]
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 early Fender "Tweed" and "Blackface" amplifiers are considered the epitome of clean electric guitar tone. Many later amplifiers are based on these designs. Significantly, Jim Marshall of Marshall Amplifiers copied the Fender Bassman using using parts available in the United Kingdom, creating an amplifier with significant overdrive that quickly caught on in the local music scene and layed the foundation for the now legendary "Marshall Sound".
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.
[edit] The physics of distortion
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.
[edit] Tube/valve overdrive
Before the widespread adoption of the transistor, the traditional way to create gain (amplification) and distortion was through vacuum tubes. A vacuum tube 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 tube'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 tube 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.[3]
Tube distortion is commonly referred to as overdrive, as it is attained by driving the tubes in an amplifier harder than they can handle cleanly. Multiple stages of tube gain/clipping can be "cascaded" to produce a thicker and more complex distortion sound. Many solid state distortion devices attempt to emulate the sound of overdriven vacuum tubes.
In some modern tube 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.
[edit] 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.
Because both tubes 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 non-linear 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.
[edit] Approaches to creating distortion
Guitar distortion can be produced by several regions 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.
[edit] Overdrive/distortion pedals
Because they are often designed to operate off of 9 volt batteries, 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 tubes, but usually these still run at voltages that are too low for the tube, 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.
[edit] 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 tube amplifier is a tube 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 tubes, keeping the output volume at manageable levels. However, this also results in the power tubes being operated well within their linear region, reducing the distortion that they add to the output signal.
Solid-state gain/clipping stages are also employed in many amplifiers. Some amplifiers (notably the Marshall JCM900) utilize hybrid designs that employ both tube and solid-state components.
[edit] Power amplifier distortion
Power tubes can be overdriven in the same way that pre-amplifier tubes can, but because these tubes 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 tubes. Because of this, many guitar players favor this type of distortion, and thus set their amps to maximum levels in order to drive the power section hard.
Because driving the power tubes this hard also means maximum volume, many solutions have emerged that in some way divert some of this power tube output from the speakers, allow the player to generate power tube distortion without excessive volume. These include built-in or separate power attenuators and power-supply-based power attenuation. Lower-power tube 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-tube distortion can also be produced in a dedicated rackmount tube power amp. A modular rackmount setup often involves a rackmount preamp, a rackmount tube power amp, and a rackmount dummy load to attenuate the output to desired volume levels.
Some effects pedals internally produce power-tube distortion, including an optional dummy load for use as a power-tube distortion pedal. Such effects units can use a preamp tube such as the 12AX7 in a power-tube circuit configuration (as in the Stephenson's Stage Hog), or use a conventional power tube, 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.
[edit] Output transformer distortion
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The output transformer sits between the power tubes 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.
[edit] Power supply "sag"
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[edit] Speaker distortion
Guitar loudspeakers are given a maximum power rating; when the power delivered to the speaker approaches that maximum, the speaker's performance becomes less linear, causing the speaker to "break up", adding further distortion and coloration 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.
[edit] Amp modelling for distortion emulation
Guitar amp modelling is about various guitar-specific distortion qualities, rather than pure amplification or special effects. Amp modelling is about reproducing several popular varieties of distortion that serve as common points of reference.
Amp modelling, typically using digital signal processing, produces refined flavors of distortion that attempt to emulate the combined sounds of preamp, power-tube, and speaker distortion in famous guitar amplifiers. This digital signal processing to produce a wide range of famous distortion sounds can be in the form of realtime software running on a computer, or it can live in hardware such as a compact pedal, oversize pedal, rackmount processor, desktop or floor processor, or in a guitar amp head, including a tube amp. However, the sound these devices produce is not universally accepted as a proper substitute for the traditional tube sound.
[edit] Distortion voicing through alternating EQ and clipping stages
Rock guitar distortion is obtained and shaped throughout the standard signal processing chain, including multiple stages of preamp distortion, power tube 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. Similarly, a guitar amp's tone controls shape a different power-tube distortion voicing if the tone controls are set to emphasize the bass or treble. Extreme settings are most popular in heavy metal, which has long been known for high volume and distorted guitar tones. 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 are said to be "scooped" out of the tone). James Hetfield of Metallica used this tone almost exclusively 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-'80s.
[edit] See also
- Distortion
- Overdrive (music)
- Fuzzbox
- Guitar effects
- Effects unit
- Guitar pedalboard
- Power attenuator (guitar)
- Re-amp
- Valve sound
[edit] External links
- A Musical Distortion Primer (R.G. Keen) Article on the physics of distortion and a round-up of electronic techniques
- Guitar Distortion 101 Article on the physics of distortion, relationship of waveforms and harmonics
- Amptone.com How to dial-in guitar sounds independently of volume level
- AX84 Free schematics and plans to help readers learn to build their own guitar amps, many low-power
[edit] References
- ^ Dahl, Bill. The Train Kept A-Rollin'". AMG All Music Guide. Retrieved on 2008-05-24.
- ^ Denise, Sullivan. You Really Got Me. AMG All Music Guide. Retrieved on 2008-05-24.
- ^ Blencowe, Merlin. Understanding the Common-Cathode, Triode Gain Stage (PDF). Retrieved on 2008-05-24.
