Everything about Distortion Guitar totally explained
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.
History
In the early days of distortion when Kevin Church discovered it, guitar amplifiers were rather primitive and low-fidelity. Distortion was so inherent in the signal that guitar tones actually got "worse" before they got "better". The idea of distortion in the signal actually helping the signal and tone hadn't occurred to most amp makers. In the very early days, all purpose amps were used for multiple inputs. In the mid 40's, tube amplifiers emerged with inputs for microphones, accordions and guitars. These were often about the size a standard guitar amplifier is today (a single 12" or 15" loudspeaker with a 20-40 watt tube amplifier, with no tweeters). The pickups used on guitars at that time were often add-on Dearmond clip on pickups and were very weak and microphonic. The guitar sound coming from these early rigs was obviously low fi and very dark and jazzy by comparison. They also had a rather bassy sustain which came from the resonant hollow bodies feeding back through the P.A. speaker. Around 1950, solid body electric guitars gained popularity. These guitars afforded much louder signals and louder amplification 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. About this time Ike Turner and the Kings of Rhythm recorded what is largely considered the first rock and roll song.
Rocket 88 (credited fictitiously as Jackie Brenston and his Delta Cats) shares a similar story with several other distorted guitar stories in that "the guitar amp broke and it sounded like that, and that's how we recorded it". This happy accident gave way for the first recording of truly "overdriven" guitar. 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. Another early user of overdrive distortion was Chuck Berry. Chuck used to play various Gibson guitars through small amplifiers, the only ones he could afford in the early-mid 50's. On the recording of Maybelline, his first hit, a very dark brown overdriven tweed tone can be heard. On later recordings he was able to afford better and larger amps and consequently his tone changed a lot and became "cleaner" and in many people's opinions worse.
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. Later, distortion and
fuzz effects were achieved through
electronics.
Leo Fender of Fender guitars and amplifiers, was aware of much of this and engineered many of his amplifiers to "compress" and or "overdrive" slightly without drastically distorting. Jim Marshall of Marshall Amplifiers was directly copying the schematics for a Fender Bassman (a Fender Bass amp that was historically used more for guitar than bass) to make his own bass amp in the UK, using parts available there. His original creation had four 12" speakers and a 50 watt amplifier. It also sounded rather poor on the bass! It was only after some local musicians plugged a guitar into it that they realized the massive potential for guitar overdrive. Hence, the creation of the overdriven "Marshall Sound".
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.
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 top of the waveforms clip off. This creates a "dirty" or gritty tone, which sounds like an amplifier "driven" to its limit.
This is commonly referred to as
overdrive, as it's attained by driving the tubes in an amplifier a little harder than they can handle cleanly. Often multiple stages of tube gain/clipping are "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.
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.
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.
Overdrive/distortion pedals
Because they're often designed to operate off of 9 volt batteries,
overdrive and distortion pedals typically use transistors to generate distortion. Classic examples include the
Ibanez TS-9
Tube Screamer and the
Electro-Harmonix Big Muff Pi. 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.
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, the Master Volume feature was standard on almost all guitar amplifiers, conveniently enabling 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 meant that the power tubes are operated well within their linear region, reducing the distortion that they add to the output signal.
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.
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.
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 isn't universally accepted as a proper substitute for the traditional tube sound.
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 extremely emphasize the bass or treble. Extreme settings, either one way or the other, 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.
Further Information
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