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Dynamo Reactive Distortion

Based On
Bixonic Expandora
Effect Type
Envelope-controlled distortion
Build Difficulty
Intermediate
Project Summary
Loosely based on the RAT, but with an optocoupler that dynamically sets the gain based on the input signal level, making for a highly unique and often untamed drive effect.
Dynamo Reactive Distortion printed circuit board

Printed Circuit Board

What's included?
PCB only. Build instructions and parts list can be viewed or downloaded from this page.

$12.50

In stock

Complete Kit

Not yet available.
Kits are developed based on interest, so if you’d like to see one for this project, let us know.
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Project overview

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 is 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.