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1 Project Overview
2 Introduction & Usage
INTRODUCTION
The Dynamo Reactive Distortion is an adaptation of the Bixonic Expandora, a Japanese boutique pedal
first released in 1995. The Expandora was a favorite of Billy Gibbons (ZZ Top) and Stu G (Delirious?),
and has also been seen on the pedalboards of Thom Yorke and St. Vincent.
Looking at the schematic, it's clear that the Expandora circuit originated from an experiment in putting
two RAT Distortion pedals in series, which you can see in the two cascaded LM308 gain stages. The
twist is how the drive level is controlled. A side-chain envelope detector engages an opto-FET (which
in this application is essentially the same as an optocoupler or vactrol) that serves as the op-amp's
gain control, meaning the gain itself varies with the signal strength. The result is a very touch-sensitive
distortion that shares some characteristics with the RAT but is really its own unique creature.
The Expandora had four major versions during its original Bixonic production. The Dynamo is most
similar to the third version (called "2000R"), including the Bass/Guitar switch, but with a few tweaks
from other versions for added versatility. Inspired by V4, the two separate gain toggles have been
condensed into a potentiometer called "Drive" which covers the full range of resistance of the toggles.
"Forbidden" mode its own toggle, disengaging the series resistance in the feedback loop.
The JHS Kilt is the most notable commercial clone of the Expandora, designed in collaboration with Stu
G. We haven't traced one of these, but based on the control layout and product description, the Dynamo
is likely identical except for the potentiometer mod. The Flat/Bass Cut switch is the same as the Bass/
Guitar switch from the 2000R version of the Expandora.
USAGE
The Dynamo has four knobs and two toggles:
• Gain is a standard gain control in the first op-amp stage.
• Drive sets the gain ratio of the envelope-controlled optocoupler.
• Tone is a passive treble-cut control that follows the clipping section.
• Level is an output volume control.
• Bass (toggle) sets the amount of bass at the output. The higher-bass mode is designed for bass
guitar, while the standard mode is equivalent to the earlier guitar-only Expandora.
• Mode (toggle) disconnects the Drive pot and enables "Forbidden Mode", which is an extremely high-
gain untamed fuzz.
DYNAMO REACTIVE DISTORTION
10 Licensing
2