aion DYNAMO User manual

DYNAMO REACTIVE DISTORTION 1
PROJECT NAME
DYNAMO
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
Bixonic Expandora
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.
Envelope-controlled distortion 1.0.0 (2023-07-04)
BUILD DIFFICULTY
Intermediate
Actual size is 2.3” x 2.61” (main board) and 1.78” x 0.90” (bypass board).

DYNAMO REACTIVE DISTORTION 2
TABLE OF CONTENTS
1Project Overview 7Drill Template
2Introduction & Usage 8Enclosure Layout
3-4 Parts List 9Wiring Diagram
5Build Notes 10 Licensing
6Schematic 10 Document Revisions
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 3
PARTS LIST
This parts list is also available in a spreadsheet format which can be imported directly into Mouser for
easy parts ordering. Mouser doesn’t carry all the parts—notably potentiometers—so the second tab lists
all the non-Mouser parts as well as sources for each.
View parts list spreadsheet →
PART VALUE TYPE NOTES
R1 100k Metal film resistor, 1/4W
R2 2M2 Metal film resistor, 1/4W
R3 2M2 Metal film resistor, 1/4W
R4 43k Metal film resistor, 1/4W
R5 240R Metal film resistor, 1/4W
R6 47R Metal film resistor, 1/4W
R7 560R Metal film resistor, 1/4W
R8 1k1 Metal film resistor, 1/4W
R9 1k5 Metal film resistor, 1/4W
R10 1M Metal film resistor, 1/4W
R11 11k Metal film resistor, 1/4W
R12 100k Metal film resistor, 1/4W
R13 51k Metal film resistor, 1/4W
R14 820k Metal film resistor, 1/4W
R15 4k7 Metal film resistor, 1/4W
R16 47k Metal film resistor, 1/4W
R17 4k7 Metal film resistor, 1/4W
R18 47R Metal film resistor, 1/4W
R19 10k Metal film resistor, 1/4W
R20 10k Metal film resistor, 1/4W
RPD 2M2 Metal film resistor, 1/4W Input pull-down resistor.
LEDR 10k Metal film resistor, 1/4W LED current-limiting resistor. Adjust value to change LED brightness.
C1 100n Film capacitor, 7.2 x 2.5mm
C2 OMIT IC1 compensation capacitor, only used for LM308.
C3 1uF Film capacitor, 7.2 x 3.5mm
C4 22n Film capacitor, 7.2 x 2.5mm
C5 1n Film capacitor, 7.2 x 2.5mm
C6 100pF MLCC capacitor, NP0/C0G
C7 OMIT IC2 compensation capacitor, only used for LM308.
C8 4.7uF Electrolytic capacitor, 4mm
C9 2.2uF Electrolytic capacitor, 4mm
C10 4.7uF Electrolytic capacitor, 4mm

DYNAMO REACTIVE DISTORTION 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
C11 3n3 Film capacitor, 7.2 x 2.5mm
C12 100n Film capacitor, 7.2 x 2.5mm
C13 100n Film capacitor, 7.2 x 2.5mm
C14 10uF Electrolytic capacitor, 5mm
C15 100n Film capacitor, 7.2 x 2.5mm
C16 100uF Electrolytic capacitor, 6.3mm Reference voltage filter capacitor.
C17 220uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C18 100n MLCC capacitor, X7R Power supply filter capacitor.
D1 1N5817 Schottky diode, DO-41
D2 1N914 Fast-switching diode, DO-35
D3 1N914 Fast-switching diode, DO-35
D4 1N914 Fast-switching diode, DO-35
Q1 2N3906 BJT transistor, PNP, TO-92
OPTO H11F1 Optocoupler, photoFET, DIP6 See build notes for information on optocoupler selection.
OPTO-S DIP-6 socket IC socket, DIP-6
IC1 OP07 Operational amplifier, single, DIP8
IC1-S DIP-8 socket IC socket, DIP-8
IC2 OP07 Operational amplifier, single, DIP8
IC2-S DIP-8 socket IC socket, DIP-8
IC3 JRC4558D Operational amplifier, DIP8
IC3-S DIP-8 socket IC socket, DIP-8
GAIN 1MB 16mm right-angle PCB mount pot
DRIVE 1kB 16mm right-angle PCB mount pot
TONE 100kC 16mm right-angle PCB mount pot
LEVEL 1MB 16mm right-angle PCB mount pot
BASS SPDT Toggle switch, SPDT on-on
MODE SPDT Toggle switch, SPDT on-on
LED 5mm LED, 5mm, red diffused
IN 1/4" mono 1/4" phone jack, closed frame Switchcraft 111X or equivalent.
OUT 1/4" mono 1/4" phone jack, closed frame Switchcraft 111X or equivalent.
DC 2.1mm DC jack, 2.1mm panel mount Mouser 163-4302-E or equivalent.
FSW 3PDT Stomp switch, 3PDT
ENC 125B Enclosure, die-cast aluminum Can also use a Hammond 1590N1.

DYNAMO REACTIVE DISTORTION 5
BUILD NOTES
LM308 and OP07
The Expandora was based on the Pro Co RAT, and like the RAT, it originally used the LM308—an early
op-amp with a very low slew rate compared to modern alternatives. The slew rate is critical to the
unique sound of the RAT, and likely the Expandora as well by extension.
In the mid-1990s, the LM308 supply began to dry up, so after some research & experimentation, Pro Co
changed over to the OP07. Bixonic followed suit in later versions of the Expandora.
The RAT is plagued with decades of heated debates about whether the LM308 sounds better than the
OP07. However, it’s been shown on more than one occasion (both by the DIY community and more
thoroughly by JHS) that the OP07 performs and sounds identical when used in the same circuit with the
same settings.
The LM308 has been obsolete for a long time, and most of what’s available today on eBay and
component distributors in Asia are relabeled fakes, so it’s not recommended to risk using one. Let the
article from JHS convince you the LM308 is not special, and use the OP07 with a clear conscience!
C2 and C7 compensation capacitors
The OP07 has an internal slew-rate compensation capacitor while the LM308 does not. C2 and C7 (both
33pF) should be omitted when using the OP07, but they’re required for the LM308 to work properly.
Optocoupler selection
The original Expandora used a Sharp PC419 photo-FET optocoupler. They’re now obsolete, but not
terribly hard to find (e.g. from Small Bear Electronics). However, they were only available in SMD format.
The H11F1M is an exact substitute with the same current-to-resistance curves, and it’s available in
through-hole format, so it’s a better choice all around. Note that it’s a DIP-6 package, so if you can’t
source a 6-pin socket, you’ll need to cut down an 8-pin socket or use two SIP snap-apart strips.
The JHS Kilt uses the H11F3, which is similar to the F1 but with slightly higher resistance at the same
currents. We haven’t traced a Kilt to know whether there are any other circuit changes, but if you want
to experiment, you can try this optocoupler as an alternate to see if you like it any better.
Drive knob
The original Expandora had a set of DIP switches that selected between three different fixed resistances
in parallel with the optocoupler, plus a fourth to engage “Forbidden mode” by removing the parallel
resistors and using the optocoupler to control the gain directly.
In version 4, this was converted into a potentiometer called “Drive” along with a minimum-value
resistor. This is a significant improvement in usability, so we’ve back-ported this change to our version of
the circuit, which is otherwise largely based on V3.
Using 240R for R5 (the minimum-value resistor) and a 1kB pot for the Drive control, the original three
settings can be found at approximately 13% rotation (371 ohms), 32% rotation (560 ohms), and 86%
rotation (1100 ohms). It can go slightly lower and higher than the stock range, as well as having all the
in-between settings that were otherwise unavailable.

SCHEMATIC
DYNAMO REACTIVE DISTORTION 6
IN
OUT
1N5817
220uF
1kB
1MB
100kC
+9V
100n
GND
2M2
GND
100n
100k
OP07CP
VA
10k
10k
VB
GND GND
VB
VA
GND OMIT
1uF
2M2
22n
2M2
VB
GND
43k
1n
GND
OP07CP
GND
VA
100pF
OMIT
240R
47R
560R
4.7uF2.2uF
GND GND
H11F3
VA
4k7
1k1
4.7uF
1N914
GND
1k5
3n3
100n
GND
JRC4558D
JRC4558D
VA
GND
VB
1M
1MB
100n
11k
GND
1N914
100n
100k
VB VB
51k
VB
820k
4k7
47k
2N3906
GND
100uF
10uF
D1
C17
DRIVE
1
2
3
GAIN
1
2
3
TONE
1
2
3
C18
RPD
C1
R1
1
2
3
8
6
74
IC1
R19
R20
C2
C3
R2
C4
R3
R4
C5
1
2
3
8
6
74
IC2
C6
C7
R5
R6
R7
C8C9
OPTO
1
2 4
6
R18R17
R8
C10
D2
D3
R9
C11
C12
2
3
1
IC3A
IC3B
6
5
7
84
R10
LEVEL
1
2
3
C13
R11
D4
C15
R12
R13
R14
R15
R16
Q1
C16
MODE
2
3
1
BASS
2
3
1
C14
GND

DYNAMO REACTIVE DISTORTION 7
DRILL TEMPLATE
Cut out this drill template, fold the edges and tape it to the enclosure. Before drilling, it’s recommended
to first use a center punch for each of the holes to help guide the drill bit.
Ensure that this template is printed at 100% or “Actual Size”. You can double-check this by measuring
the scale on the printed page.
Top jack layout assumes the use of closed-frame jacks like the Switchcraft 111X. If you’d rather use
open-frame jacks, please refer to the Open-Frame Jack Drill Template for the top side.
LED hole drill size assumes the use of a 5mm LED bezel, available from several parts suppliers. Adjust
size accordingly if using something different, such as a 3mm bezel, a plastic bezel, or just a plain LED.
ø3/8” ø1/2”
0.385”
0.625” 0.625”
x: 0, y: -1.775
ø15/32”
x: -0.775, y: -1.775
ø5/16”
ø3/8”
125B
OUT
TONE
BASS
MODE
DRIVE
LEVEL GAIN
FOOTSWITCHLED
DC IN
x: -0.65, y: +1.71 x: 0.65, y: +1.71
ø9/32” ø9/32”
x: -0.65, y: +0.41
ø9/32”
x: 0.65, y: +0.41
ø9/32”
CENTER (0,0)
x: 0, y: +0.83
ø1/4”
x: 0, y: +1.29
ø1/4”
0 1 2
CM
0 1
INCH

DYNAMO REACTIVE DISTORTION 8
ENCLOSURE LAYOUT
Enclosure is shown without jacks. See next page for jack layout and wiring.
125B

DYNAMO REACTIVE DISTORTION 9
WIRING DIAGRAM
125B
IN +VGND NC NC OUT
PCB
IN
GND +V +V JACK GND JACK
OUTIN
GND GND PCB
OUT

DYNAMO REACTIVE DISTORTION 10
LICENSE & USAGE
No direct support is offered for these projects beyond the provided documentation. It’s assumed
that you have at least some experience building pedals before starting one of these. Replacements and
refunds cannotbe offered unless it can be shown that the circuit or documentation are in error.
All of these circuits have been tested in good faith in their base configurations. However, not all the
modifications or variations have necessarily been tested. These are offered only as suggestions based
on the experience and opinions of others.
Projects may be used for commercial endeavors in any quantity unless specifically noted. No
attribution is necessary, though a link back is always greatly appreciated. The only usage restrictions
are that (1) you cannot resell the PCB as part of a kit without prior arrangement, and (2) you cannot
“goop” the circuit, scratch off the screenprint, or otherwise obfuscate the circuit to disguise its source.
(In other words: you don’t have to go out of your way to advertise the fact that you use these PCBs, but
please don’t go out of your way to hide it. The guitar effects industry needs more transparency, not less!)
DOCUMENT REVISIONS
1.0.0 (2023-07-04)
Initial release.
Table of contents
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