Boss distortion pedal |
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| Notes | A well-used "Turbo Distortion" guitar effect pedal made by Boss |
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Distortion effects create "warm", "dirty" and "fuzzy" sounds by compressing the peaks of a musical instrument's sound wave and adding overtones. The three principal types of distortion effects are overdrive, distortion and fuzz. Distortion effects are sometimes called “gain” effects, as distorted guitar sounds were first achieved by over-driving tube amplifiers.[1][2][3] Distortion has long been integral to the sound of rock and roll music, and is important to other music genres such as electric blues and jazz.[4][5]
The terms “distortion”, “overdrive” and “fuzz” are often used interchangeably, but they have subtle differences in meaning. Overdrive effects are the mildest of the three, producing "warm" overtones at quieter volumes and harsher distortion as gain is increased. A "distortion" effect produces approximately the same amount of distortion at any volume, and its sound alterations are much more pronounced and intense.[6][7] A fuzzbox (or “fuzz box”) alters an audio signal until it is nearly a square wave and adds complex overtones by way of a frequency multiplier.[8]
Distortion can be produced by effects pedals and rackmounts, pre-amplifiers, power amplifiers, speakers and more recently, digital amplifier modeling devices and software.[2][3]
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The first amplifiers built for electric guitar were relatively low-fidelity, and would often produce distortion when their volume (gain) was increased beyond their design limit or if they sustained minor damage.[9] One of the earliest recorded examples of distortion in rock music is the 1951 Ike Turner and the Kings of Rhythm song "Rocket 88", on which guitarist Willie Kizart used an amplifier that had been slightly damaged in transport.[10] In the early 1950s, pioneering rock guitarist Willie Johnson of Howlin' Wolf began deliberately increasing gain beyond its intended levels to produce "warm" distorted sounds.[9] Chuck Berry's 1955 classic "Maybellene" features a guitar solo with warm overtones created by his small valve amplifier.[11]
By the mid 1950s rock guitarists began intentionally "doctoring" amplifiers and speakers in order to create even harsher distortion.[12] In 1956 guitarist Paul Burlison of the Johnny Burnette Trio deliberately dislodged a vacuum tube in his amplifier to record "The Train Kept A-Rollin” after a reviewer raved about the sound Burlison’s damaged amplifier produced during a live performance.[13] Guitarist Link Wray began intentionally manipulating his amplifiers' vacuum tubes to create a “noisy" and “dirty” sound for his solos after a similarly accidental discovery. Wray also poked holes in his speaker cones with pencils to further distort his tone. The resultant sound can be heard on his highly influential 1958 instrumental, "Rumble".[14]
In 1961, the American instrumental rock band The Ventures asked their friend session musician and electronics enthusiast Orville "Red" Rhodes for help recreating the “fuzz” sound caused by a faulty preamplifier on Grady Martin’s guitar track for the Marty Robbins song "Don't Worry".[15] Rhodes offered The Ventures a fuzzbox he had made, which they used to record "2000 Pound Bee" in 1962.[16] The first purpose-designed commercial distortion circuit was the Maestro "Fuzz Tone" Model FZ-1, released in 1962.[17]
Distortion gained widespread popularity after guitarist Dave Davies of The Kinks used a razor blade to slash his speaker cones for the 1964 single "You Really Got Me". The song was a number one hit and the first rock and roll track to feature a distorted power chord riff.[18] In 1966, Jim Marshall of the British company Marshall Amplification began modifying the electronic circuitry of his amplifiers so as to achieve a "brighter, louder" sound and fuller distortion capabilities.[19][20]
In May 1965 Keith Richards used a Gibson Maestro Fuzz-Tone to record "(I Can't Get No) Satisfaction".[21] The song's success greatly boosted sales of the device, and all available stock sold out by the end of 1965.[22] Early fuzzboxes include the Mosrite FuzzRITE and Arbiter Group Fuzz Face used by Jimi Hendrix,[23] the Electro-Harmonix Big Muff Pi used by Hendrix and Carlos Santana,[24] and the Vox Tone Bender used by Paul McCartney on "Think for Yourself" and other Beatles recordings.[25]
In the late 1960s and early 1970s hard rock bands such as Deep Purple, Led Zeppelin and Black Sabbath forged what would eventually become the heavy metal sound through a combined use of high volumes and heavy distortion.[26]
The word distortion literally refers to any modification or addition to the wave form of a signal by any kind of equipment.[27] In the context of music amplification distortion is equated with clipping.[28]
Clipping is a non-linear process that produces frequencies not originally present in the audio signal. These frequencies can either be "harmonic", meaning they are whole number multiples of the signal's original frequencies, or "inharmonic", meaning dissonant odd-order overtones.[29][30] Harmonic distortion produces harmonically related overtones while intermodulation distortion produces inharmonic overtones.[31]
"Soft clipping" gradually flattens the peaks of a signal and de-emphasizes higher odd harmonics. "Hard clipping" flattens peaks abruptly, resulting in harsh-sounding, high amplitude odd harmonics.[32]
Distortion circuits distort a signal before it reaches the main amplifier. Overdrive circuits do not create distortion themselves but rather boost signals to levels that cause distortion to occur at the main amplifier's front end stage.[33]
Vacuum tube or “valve” distortion is achieved by “overdriving” the valves in an amplifier.[34]Valve amplifiers--particularly those using triodes--tend to produce asymmetric soft clipping that creates "warm"-sounding even-order harmonics.[32][35]
A basic triode valve contains a cathode, a plate and a grid. When a positive voltage is applied to the plate, a current of negatively charged electrons flows to it from the heated cathode through the grid. This increases the voltage of the audio signal, amplifying its volume. The grid regulates the extent to which plate voltage is increased. A small negative voltage applied to the grid causes a large decrease in plate voltage.[36]
Valve amplification is more or less linear--meaning the parameters (amplitude, frequency, phase) of the amplified signal are proportional to the input signal--so long as the voltage of the input signal does not exceed the valve's "linear region of operation". The linear region falls between 1. the saturation region: the voltages at which plate current stops responding to positive increases in grid voltage and 2. the cutoff region: the voltages at which the charge of the grid is too negative for electrons to flow to the plate. If a valve is biased within the linear region and the input signal's voltage exceeds this region, overdrive and non-linear clipping will occur.[37][34]
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.
Solid-state amplifiers incorporate discrete transistors and/or op amps and are prone to produce hard clipping and symmetrical distortion when overdriven. This adds additional high-amplitude odd harmonics, creating a "dirty" or "gritty" tone.[32] 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 using additional solid-state circuitry. Some amplifiers (notably the Marshall JCM 900) utilize hybrid designs that employ both valve and solid-state components.
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.
Because they are often designed to operate with low voltages (such as 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.
To reduce unwanted dissonance, single notes and simple power chords (root, fifth, and octave) are often used when using fuzzboxes, rather than triads (root, third, and fifth) or four-note chords (root, third, fifth, and seventh).
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, an adjustable attenuator between the preamp section and the power amp. When the preamp is set up to generate high distortion levels the master volume can be used to divert most of the resulting signal away from the power valves, keeping the output volume at manageable levels.
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. 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 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 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 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.
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.
The output transformer sits between the power valves and the speaker, serving to match impedance. When a transformer's ferromagnetic core becomes electromagnetically saturated, it will clip symmetrically. This adds additional odd-order distortion to the signal delivered to the speakers.
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 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.[38] 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.
Guitar loudspeakers are designed differently from 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.
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 lots of clean headroom, while others are designed to break up early to deliver grit and growl.
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 X 10" speaker cabinets.
Guitar distortion is obtained and shaped at various points in the signal processing chain, including multiple stages of preamp distortion, power valve distortion, output and power transformer distortion, and guitar speaker distortion. Much of the distortion character or voicing is controlled by the frequency response 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 tone controls built in to the guitar, the preamp or 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 in the distorted signal.
Increasing the bass and treble while reducing or eliminating the centre midrange (750 Hz) results in what is popularly known as a "scooped" sound (since the midrange frequencies are "scooped" out). Conversely, decreasing the bass while increasing the midrange and treble creates a punchy, harsher sound.
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