The softer clipping of tubes has been a model for effect pedals, sc. stompboxes. The article (Paper INNER, 2010) by Kalev Tiits (Department director at the Sibelius Academy, Centre for Music & Technology) deals with their history, their influence on pop music, their technical construction etc. – Steve Ray Vaughn’s famous pedal, Ibanez Tube Screamer, as a case study. But was the real reason for its sound the legendary JRC 4558D transistor?
Effect devices designed to manipulate the sound of electric instruments have shaped the sound of pop music for over 50 years. Some of the most popular devices have been based on transistor switching to simulate tube distortion. Memory Man, Electric Mistress, French Toast, Cry Baby, Tube Screamer, Dimension IV, Rat are famous effect devices or pedals, stompboxes. These “gas pedals”, “worm boxes” or “cuckoo clocks” are typically used between an electric guitar – but also other instruments – and an instrument amplifier.
The use of effects pedals has gradually grown into the sound of pop(ular) music. It is difficult to pinpoint the exact moment when they first appeared. Guitar Player magazine’s technical editor Art Thompson has told the following story in his book The Stompbox (Thompson 1997: 12-13):
“While recording the bass guitar solo for Marty Robbins’ hit “Don’t Worry” at the Quonset Hut studio in Nashville, a tube in the bass channel of the soundboard got broken, resulting in a very distorted sound. When the recording was listened to, someone present really liked the distorted sound, and it was decided to leave it on the record.” The year was 1960. The song became a chart hit, and the sound attracted wider interest. It wasn’t long before sound engineer Glen Snotty concocted a small transistor circuit that mimicked the sound of the broken mixer channel. The circuit ended up at the Gibson guitar factory of the CMI group (Chicago Musical Instruments), which developed it into the Fuzz-Tone pedal in 1963 (ibid: 84). The Maestro Fuzz-Tone’s popularity skyrocketed in 1965 when guitarist Keith Richards used it on the Rolling Stones’ hit “(I can get no) Satisfaction.”
The rest is history. Electro-Harmonix, MXR, and later Ibanez began developing their own effects pedals.
Tubes and transistors
Simply put, distortion is achieved by overdriving the amplifier stages or by adding components that cause nonlinear behavior to the signal path. If the overdriven component is a transistor, overdriving usually causes a strong clipping of the signal. With tubes, clipping is more gradual. A softer signal function is heard as a softer and fuller sound. In the spectrum of the amplified sound, the difference is visible in that the overdriven semiconductor circuit emphasizes the odd multiples of the fundamental frequency f, 3f, 5f, 7f, … , while the overdriven tube also produces even harmonics.
The more balanced sound of an overdriven tube amplifier has been appreciated to such an extent that the advantages of transistor amplifiers (price, weight, etc.) have not been able to replace tube technology in instrument amplifiers. When designing solid-state distortion pedals, the criterion for success has also been how authentically the soft sound of tube circuitry has been imitated.
However, transistors do not have the exclusive right to high-frequency distortion components. The sound of guitar amplifiers is also significantly affected by the modification of the frequency response caused by the speaker. Electric guitars usually use 10- or 12-inch one-way speakers, which (also) act as a high-frequency filter: frequencies above 4 kHz are only marginally reproduced. In addition, many amplifier housings are open at the back, so the speaker’s rear wave leaks to the front and eats up (short-cuts) the low-frequency response. The directional characteristics are uneven and the off-axis frequency response is colored. When recording, good sound is sought by changing the position of the microphone.
The behavior of transistors, that differs from tubes, causes other differences in design. These, in turn, have multiplier effects. For example, in order to avoid excessive central modulation distortion, circuits based on transistors or similar IC technology, i.e. operational amplifiers, it is usually customary to filter low frequencies heavily. This has been a significant issue in the sound formation of many distortion devices based on transistors or operational amplifiers. When the sounds of electric guitars, bass instruments, Hammond organs and Rhodes pianos have strong low-pass and high-pass filtering, the narrow-band signal reproduction of distortion devices is often an acceptable and even desirable feature.
Stevie Ray Vaughan and Ibanez Pedals
When talking about electric guitar distortion pedals, especially Ibanez pedals, one cannot ignore the late American blues guitarist Stevie Ray Vaughan (1954-1990). Vaughan played mainly the Fender Stratocaster, and often used Fender tube amplifiers (Twin, Vibroverb, ’59 Bassman), sometimes alongside or instead of them Marshalls (including the JCM800), or several together. (Mike’s Guitar Site – Stevie Ray Vaughan’s Gear).
Vaughan favored thick strings, chunky guitar neck profiles, brisk power handling, and amplifiers with the volume knob turned up. Thanks to his energy, he won over a wider audience of popular music for blues even in the mid-1980s. Due to the rough handling, his guitars required frequent maintenance. The interest in improvement, combined with the fretless playing, led to a lot of modifications to the instruments (Stevie Ray Vaughan’s Guitars). For this reason, the sound of Vaughan’s Stratocaster could differ from the standard Fender Stratocaster.
Vaughan’s use of effect pedals was conservative compared to trends. An essential part of the SRV’s sound has been considered the Ibanez Tube Screamer pedal, the TS-9 version of which was produced in 1982-85. The Tube Screamer is an overdrive/distortion pedal that aims to imitate the sound of an overdriven tube amplifier. The device is at home in the blues and traditional rock genres. Instead of the TS-9, Vaughan also used an earlier version of the TS-808, which many consider to be the ‘original SRV pedal’. Old TS-808s are now very sought after among enthusiasts.
SRV used the Tube Screamer more with an overdrive than with a distortion idea, i.e. he built the sound more by overdriving the amplifier’s preamp and less with the actual distortion connection of the pedal. On the other hand, the Tube Screamer filters out low frequencies considerably, even by the standards of guitar amplifiers, and very high frequencies cannot be passed through the pedal. The sound of an original Tube Screamer cannot be made completely ‘clinical’ either; even with moderate drive settings, it adds its own color to the signal, which is tighter and less airy than an instrument connected directly to a tube amplifier. On the other hand, when amplifiers are turned almost to full volume, distortion is created at every gain level anyway. Due to the distortion of the speakers themselves, it is not easy to distinguish the distortion components added by the pedal from those produced by the amplifier at higher volumes.
In terms of their electronic structure, the Ibanez Tube Screamers represent typical mass production of the 1970s and 80s. After the buffered input, the signal is heavily amplified by an operational amplifier circuit, which produces distortion with a diode clipper placed in the feedback loop, after which it is passed through another operational amplifier circuit with a low gain that acts as a tone control. The last in the chain is the output buffer (The Technology of the Tube Screamer). Different models and years have used different DIL8-encapsulated dual opamps as the amplifier circuit. The input and output buffers each contain one common-collector connected NPN transistor with the necessary peripheral components. This is probably the simplest device suitable for the purpose, which could be manufactured in large series.
Tube Screamers have an active bypass function. This once advanced detail later became a burden. Active bypassing subtly shapes the sound passing through, even in bypass mode, as the pedal becomes an additional load on the high-inductance circuit formed by the guitar microphone and amplifier. In Tube Screamer pedals, however, this load is relatively large, in the order of half a megaohm, so the signal coloring effect is small.
Signal clipping with diodes
Signal peak clipping after signal amplification is usually done with diodes. This takes advantage of the fact that in practice diodes do not switch from reverse to forward at exactly zero voltage, but require a small forward bias. When diodes are connected to the output of an op amp between the signal and ground, they do not affect the signal at low levels, but at levels above the bias voltage they short-circuit, or shunt, it to ground. Shunting produces quite sharp corners in the signal function, and strongly adds odd harmonics to the signal. If the signal to be clipped has been amplified enough, it has an almost square wave shape. This sound corresponds in outline to the general perception of what semiconductor distortion sounds like, compared to tube distortion. The connection method is known as hard clipping. The sharp clipping can be mitigated with peripheral components. (Guitar Amplifiers – Overdrive & Distortion).
Instead of shunting after the opamp, diode clipping can be achieved by placing a signal diode or diodes in the feedback path of the opamp. There, the diodes usually shunt the feedback resistor. Possibly, there is a smaller resistor in series with them, as in the Marshall BluesBreaker, and often a capacitor is seen in parallel with the diodes, as in the Ibanez Tube Screamers and the Boss Super Overdrive SD-1 (ibid.). This connection method is called soft clipping.
The type of diodes has a great importance for the operation of the clipping circuit. Usually, signal diodes made of silicon are used, but germanium-based ones are also used. The forward bias voltage required by a silicon diode is in the order of 0.6 – 0.7 V, while a germanium diode is around 0.3 V. Therefore, these operate differently in distortion circuits. There are variations of the connections; If germanium diodes are connected in series in parallel in pairs, the common bias increases to the order of magnitude of a silicon diode, but the properties are otherwise different, which in turn produces different behavior. The difference in sound between germanium and silicon-based clippers is easily audible. Sometimes the diode is replaced by half a transistor. For example, the second junction of an AC 128-ge transistor has been used successfully.
LEDs have also been used in clipper circuits instead of signal diodes. The use of rectifier diodes instead of signal diodes is also possible.
Naturally, the circuits that clip the different half-cycles of a diode clipper do not have to be identical. Asymmetrical signal clipping is indeed an interesting option if tube electronics are considered as a model. The distortion typical of tube amplifiers is often associated with asymmetrical signal shaping. This is most noticeable in Class A tube circuits. Asymmetrical clippers can be obtained, for example, by cutting only one half-cycle from the signal, by connecting different diodes to the half-cycles, for example silicon to one and germanium to the other, or by connecting two diodes to one half-cycle and one to the other. Not all options are necessarily musically usable, and the smoothness of tube switching with a semiconductor clipper is not necessarily achieved without connecting additional components such as RC filters. The design of a suitable clipper often proceeds by trial and error.
The Legend of JRC 4558D
Compared to diodes, which offer considerable variation, the quality of OP Amps does not seem very interesting. However, the subject is debatable. The effect of an OP Amp could be justified as follows: in distortion circuits, a lot of amplification is taken from OP Amps, typically almost 50 dB or even more. When devices are often built to operate on a 9 V battery, the chip has to operate at the limits of its performance. A strong input signal leads the circuit to a situation where the operating voltage is not sufficient for the signal dynamics and gain, but the amplifier output voltage “bumps” into the limits set by the operating voltage, i.e. +- 4.5 V. This in turn causes distortion, the nature of which can be characteristic of the OP Amp type, especially in transients. Whether this is significant is the core of the discussion on the matter. Other distortion mechanisms may also occur: for example, the clipper may appear to the OP Amp as too small a load, triggering the OP Amp’s own overcurrent protection. Different OP Amps may differ in this respect.
The JRC4558D chip manufactured by Japan Radio Corporation has been tried to be crowned the king of distortion opamps, mainly because the chip is believed to have been common in Ibanez’s original Tube Screamer pedals, used by Stevie Ray Vaughan, among others. The chip was also very common in cheap stereo equipment in the early 1980s.
However, there is varying information about the use of the JRC4558D in Ibanez pedals. Various sources have revealed that other OP Amps were also used, such as the RC4558P Malaysia circuit, the Toshiba TA75558, and the JRC2043. It seems that Ibanez did not originally consider the JRC4558D circuit to be the only suitable circuit for their circuit, but used parts that were cheaply available. Be that as it may, Ibanez has recently released a TS-808 reissue, which, unlike other reissue pedals, claims to have used a NOS (new old stock) JRC4558D.
If the OP Amp type has had an effect on the sound, it has not been the only component-based factor affecting the sound. The observed differences in sound between individual devices have probably also been significantly influenced by the fact that the resistors used at the time of the TS-808 and original TS-9 pedals were not metal film resistors, but carbon mass resistors, which have higher tolerance limits.
Digital modeling has advanced, processor power has increased, but digital technology has not replaced analog in distortion devices. A good electric guitar sound is lively and variable, and difficult to model. The sound depends, among other things, on the interaction between the instrument and the amplifier, the nonlinearity of the amplifier’s output transformer and the loudspeaker elements. All of this may not be known with sufficient accuracy for modeling. In addition, since analog effect pedals are practical in many ways, these small, low-tech cuckoo clocks will probably hold their ground well for the time being.
Boost, overdrive, distortion, fuzz
Boosters, or preamps, were originally designed to add more gain to the signal path, which can be used to overdrive the input level of the actual instrument amplifier. On the other hand, boosters have been used to increase the brightness of the sound in situations where the sound of the actual instrument amplifier has been found to be too soft (treble booster). Normally, a booster mainly amplifies the sound, without coloring it as strongly as overdrive or fuzz, and the additional color is created when the actual instrument amplifier’s (tube) preamp is overdriven and produces distortion. In this way, a booster brings out the natural distortion of a tube amplifier more strongly. The sound is not created solely in the effect box, but depends on the interaction between the booster and the actual guitar amplifier.
Overdrive is a more aggressive version of a booster. Overdrive creates distortion components that are usually tailored to resemble the distortion behavior of a tube amplifier. Most overdrive pedals also amplify the signal level so that a booster-like behavior, i.e. the pre-stage distortion of an actual instrument amplifier, is created. With small distortion settings (usually distortion, gain or drive), these devices are often quite clean-sounding preamplifiers. Settings where the pedal produces its own distortion are intended to make the distortion components of the pedal sound and the amplifier sound mix together evenly and ‘imperceptibly’. The resulting sound is often described in terms such as thick, solid, mellow or warm. Overdrive is typical of roots-inspired music: rhythm’n’blues and rock classics.
Distortion is the most widely used term for distortion. Sometimes it refers to all types of distortion – i.e. a distortion device or the distortion channel of a guitar amplifier in general, sometimes the term is used to refer to a distortion device that is more aggressive – more strongly coloring – than an overdrive but ‘milder’ than a fuzz. The term distortion is heard used both for a blues-oriented singing sound and for the sound that comes from pedals aimed at heavy metal music.
The term fuzz came into use in the early 1960s with the Maestro Fuzz-Tone. Soon, various fuzz devices were manufactured under many different names and with somewhat different connections. One early device, the Arbiter Fuzz Face, became famous, among other things, because it was part of Jimi Hendrix’s toolkit for a long time. The fuzz sound is usually characterized by the term buzzy, buzzing, when compared to overdrive or distortion machines. Originally, fuzz was also intended to replace the distortion technique used in the 1950s – damaging speaker drivers by punching holes in cone cardboard. Fuzz refers to a heavily processed or aggressive distorted sound.
Literature:
Patoski, Joe Nick & Bill Crawford (1993). Stevie Ray Vaughan. Caught in the Crossfire. Boston: Little, Brown & Company.
Pittman, Aspen (2003). The Tube Amp Book. 4th edition. London: Backbeat UK.
Thompson, Art (1997). The Stompbox. San Francisco, CA: Miller Freeman.
Internet:
Guitar Amplifiers – Overdrive & Distortion.
Ibanez TS-9 Pedal Page.
Ibanez Tube Screamer History.
Mike’s Guitar Site – Stevie Ray Vaughan’s Gear.
Stevie Ray Vaughan’s Guitars.
The Technology of the Tube Screamer.
