Passive Shunt Clipping Diodes

(Driving amplifier shown for clarity. The clipping section is just the far right components, the back-to-back pair of diodes.)

Passive shunt clipping diodes limit the output of a high gain voltage amplifier to the forward bias (or reverse bias for Zeners) voltage of the diodes.

Can be the only method of clipping, or used in series with other circuits.

The above example shows a simple shunt feedback amplifier driving the clipping diodes. The driving amplifier may be any type of voltage amplifier. If low voltage (Schottky or germanium) diodes are used, a passive guitar signal can even be clipped this way. The "Black Ice" module is a commercial version for completely passive use.

The number of each type of diode is variable. A single diode may be used. An uneven pair of diodes may be used (1 of one polarity, 2 of the opposite polarity). LED type diodes are often used as their forward bias voltage is higher than standard rectifier diodes. Transistor may be connected as diodes by tying collectors to bases for BJTs or drains to gates for FETs. MOSFETs used this way are particularly popular in modern boutique overdrives. Zener diodes, germanium diodes - it's all fair game.

Several vintage 1960s-70s "fuzz" pedals were made this way, and it became very popular in the op amp era with effects labelled "Overdrive" and "Distortion."

It would seem the 1966 Astro Amp Astrotone (more often seen as the Sam Ash Fuzzz Boxx) is a contender for the oldest application of passive diode clipping in a guitar effect pedal.

An interesting application of transistor diode clipping can be found in the 1967-72 UK made Vox solid state amps. Called the "MKIII Preamp" in the schematics, these amps feature a distortion that includes germanium transistors connected base to emitter, in sort of a "reverse diode" type connection. These are contained within a neat two transistor amplifier with two transistor switches that activate when the distortion switch is toggled. I think it's a pretty neat circuit, and I like the sound of it too. (Most folks probably won't!)

The final Maestro Fuzz Tone, the FZ-1S, also utilizes transistor diodes. The FZ-1S transistors are silicon and connected in the normal base to emitter manner.

Simple discrete transistor versions:

Astro Amp Astrotone

Sam Ash Fuzzz Boxx

Jordan Boss Tone

Interfax Harmonic Percolator

Maestro Fuzz Tone FZ-1B (version 1)

Univox Square Wave

Used as an element of a more complex discrete transistor unit:

Vox MKIII Preamp Distortion (1967 - germanium transistor reverse diodes)

Maestro Super Fuzz FZ-1S (transistor diodes)

Univox Superfuzz

Fender Blender