Op Amp Distortion and Overdrive Topologies

Op Amp with Passive Shunt Diode Clipping

My gut tells me this topology is the most common type of commercial distortion/overdrive pedal, but I've yet to do an actual head count. The op amp circuit pictured above is a series feedback amplifier. A shunt feedback amplifier works the same as far as this diode clipping arrangement is concerned. The series feedback amplifier is non-inverting, and that may explain why the pictured arrangement seems to be preferred over shunt feedback.

While the actual diode clipping isn't really any different than discrete transistor fuzzes that use passive diode clipping, this combo of op amp and diode is so prominent in distortion/overdrive pedals that it seems to deserve its own topology entry.

This type of arrangement is generally favored for "distortion" labelled effects, and it is sometimes referred to as "hard clipping."

The first commercial pedal to use this topology is something of a mystery. At first glance through the histories, the MXR Distortion + appears to be the primary candidate. However, Dan Armstrong made claims that MXR either copied or imitated his Blue Clipper effect. My own dark horse pick is the Gretsch Controfuzz/Expandafuzz, which seems to pop up pretty early as well. These pedals appeared sometime between 1973 and 1975. Seems kind of late, considering the µA741 was released in 1968. I'm still on the hunt for an earlier example.

MXR Distortion + (1974?)

Dan Armstrong Blue Clipper (1974?)

DOD 250 Overdrive Preamp (197?)

Gretsch Controfuzz (197?)

Gretsch Expandafuzz (1973/74?)

Roland AF-60 Bee Gee (197?)

DeArmond Square Wave (197?)

Ibanez Dirt Box (unconfirmed)

Proco RAT Distortion (1978)

Boss DS-1 Distortion (1978)

Boss DS-2 Distortion (1987)

Klon Centaur (1994)

Boss XT-2 Xtortion (1996)

Op Amp Shunt Feedback Clipping

Shunt feedback clipping for guitar distortion effects began with discrete transistors and the Big Muff Pi. This is the op amp equivalent. It isn't nearly as popular as the series feedback arrangement. Like the transistor version, the output is inverted with respect to the input.

Gain is calculated as -(Rfb / Rin). The negative sign indicates that the output is inverted with respect to the input. Note that is Rfb = zero, then the equation is zero divided by something which always equals zero, so this arrangement can be used to both amplify and attenuate. Making Rfb larger than Rin amplifies, and making Rfb smaller than Rin attenuates.

Either resistor can be replaced with a potentiometer for a manual "GAIN" control.

Examples:

Electro Harmonix Big Muff Pi "Version 4/Op Amp Version" (1977)

MXR Distortion II (1980)

Marshall Blues Breaker (1991)

Op Amp Series Feedback Clipping

This is an incredibly popular topology, mostly recognized for its success in the Ibanez Tube Screamer line. The feedback clipping is generally referred to as "soft clipping" and these circuits are often labelled "overdrives."

Shown is a symmetrical diode clipping pair. The diodes can be an odd number to produce asymmetrical clipping. The number and type of diodes is totally variable and is often the way one pedal design will distinguish itself from the others.

The feedback capacitor connected to ground can also be connected to the Vref rail. The primary function of the capacitor is to block the dc path. This lowers the gain of the amplifier to x1 at dc. If this capacitor goes open, ac gain will also drop to x1, and you will get a "low output" symptom. If this capacitor shorts, the dc gain will cause the output pin's voltage to be significantly higher than V_ref. Always check that both inputs and the output are near V_ref with no signal applied.

As a secondary function, the feedback capacitor can be made intentionally smaller so as to attenuate bass frequencies, which allows more bass negative feedback, and thus less bass gain.

Not shown, but often included, is an additional capacitor in parallel with the feedback resistor (between output and inverting input). This will be relatively small in value, typically in the neighborhood of 50-150pF, which serves to restrict high frequencies to unity gain. Normally this is calculated to not effect the upper audio area (about 20kHz), but this cap can be made large on purpose if a fixed low pass filtering effect is desired.

Gain is calculated as 1 + (R1 / R2). The constant term "1" in the equation means you can't get less than unity gain from this arrangement (i.e. you can't attenuate as you can with the shunt feedback arrangement). No matter what R2 is, if R1 is very small or replaced with a direct short, you will get a voltage follower arrangement with a gain of x1. The smaller you make R2, the greater the gain. The larger R1 is made relative the R2, the greater the gain.

Either R1 or R2 can be replaced with a potentiometer for a manual "GAIN" control.

Examples:

Boss OD-1 Overdrive (1977)

Ibanez TS-808 Tube Screamer (1979)

Boss SD-1 Super Overdrive (1981)

Fulltone Full Drive (199?)

PaulC Audio Tim (1997)

PaulC Audio Timmy