aion HELIODOR AMP OVERDRIVE User manual

HELIODOR AMP OVERDRIVE 1
PROJECT NAME
HELIODOR
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
BOSS®OD-3 Overdrive
The final entry in a series of legendary pedals using BOSS’s “discrete op-amp” topology that overdrives
more like an amplifier than traditional drive circuits.
JFET overdrive 1.0.1 (2022-02-01)
BUILD DIFFICULTY
Intermediate
Actual size is 2.3” x 2.42” (main board) and 1.78” x 0.86” (bypass board).

HELIODOR AMP OVERDRIVE 2
TABLE OF CONTENTS
1Project Overview 8Drill Template
2Introduction & Usage 9Enclosure Layout
3-5 Parts List 10 Wiring Diagram
6Build Notes 11 Licensing
7Schematic 11 Document Revisions
INTRODUCTION
The Heliodor Amp Overdrive is based on the BOSS®OD-3 Overdrive, released in 1997 as the successor
to the OD-2r Turbo Overdrive.
The OD-3 continues in the tradition of the OD-2/r and BD-2 Blues Driver with BOSS’s signature
“discrete op-amp” topology, using two JFETs and a PNP transistor to create a variable gain stage that can
be controlled like an op-amp but clips gracefully when overdriven. Unlike the two earlier units, though,
the OD-2 uses clipping diodes in the this stage to achieve a more traditional soft clipping.
Due to the circuit similarities with the BD-2, it’s been theorized that the OD-3 was actually designed as
the successor to the Blues Driver, but that BOSS didn’t want to make changes to such a commercially
successful pedal—and as a result, the OD-3 was released under the Overdrive umbrella.
Regardless of what went on in the product development meetings, the OD-3 does borrow ideas from
both the OD-2 and BD-2 and manages to improve on both of them from a design standpoint. The Blues
Driver may get more attention, but the OD-3 remains the crowning achievement of BOSS’s discrete op-
amp topology.
The Heliodor is a direct adaptation of the OD-3 with no changes other than converting to true bypass.
The original OD-3 uses the 2SK184-GR JFET, a low-cutoff type similar to J201, but no longer made in
through-hole format. 2SK209-GR is the SMD version that is still in production and will perform exactly
the same as the originals. Each of the JFETs have extra pads for soldering SMD parts, but if you don’t
feel confident in your SMD skills, Aion FX sells 2SK209-GR pre-soldered to adapter boards to be used in
through-hole applications.
USAGE
The Heliodor has three controls:
• Drive increases the gain of the discrete op-amp stage, which clips on its own as well as pushing the
second JFET stage into clipping.
• Tone is a passive treble cut before the output volume control.
• Volume sets the overall output of the effect signal.

HELIODOR AMP OVERDRIVE 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 10k Metal film resistor, 1/4W
R2 1M Metal film resistor, 1/4W
R3 10k Metal film resistor, 1/4W
R4 100k Metal film resistor, 1/4W
R5 10k Metal film resistor, 1/4W
R6 10k Metal film resistor, 1/4W
R7 3k3 Metal film resistor, 1/4W
R8 10k Metal film resistor, 1/4W
R9 220k Metal film resistor, 1/4W
R10 220k Metal film resistor, 1/4W
R11 2k7 Metal film resistor, 1/4W
R12 6k8 Metal film resistor, 1/4W
R13 470R Metal film resistor, 1/4W
R14 3k3 Metal film resistor, 1/4W
R15 2k7 Metal film resistor, 1/4W
R16 100k Metal film resistor, 1/4W
R17 33k Metal film resistor, 1/4W
R18 10k Metal film resistor, 1/4W
R19 1k Metal film resistor, 1/4W
R20 2k2 Metal film resistor, 1/4W
R21 4k7 Metal film resistor, 1/4W
R22 47k Metal film resistor, 1/4W
R23 150k Metal film resistor, 1/4W
R24 1k Metal film resistor, 1/4W
R25 6k8 Metal film resistor, 1/4W
R26 12k Metal film resistor, 1/4W
R27 4k7 Metal film resistor, 1/4W
R28 12k Metal film resistor, 1/4W
R29 4k7 Metal film resistor, 1/4W
R30 100k Metal film resistor, 1/4W
R31 10k Metal film resistor, 1/4W
R32 100k Metal film resistor, 1/4W

HELIODOR AMP OVERDRIVE 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
R33 1k Metal film resistor, 1/4W
R34 10k Metal film resistor, 1/4W
R35 10k Metal film resistor, 1/4W
RPD 2M2 Metal film resistor, 1/4W Input pulldown resistor. Can be as low as 1M.
LEDR 4k7 Metal film resistor, 1/4W LED current-limiting resistor. Adjust value to change LED brightness.
C1 47n Film capacitor, 7.2 x 2.5mm
C2 18n Film capacitor, 7.2 x 2.5mm
C3 1uF Film capacitor, 7.2 x 3.5mm
C4 10n Film capacitor, 7.2 x 2.5mm
C5 56n Film capacitor, 7.2 x 2.5mm
C6 1uF Film capacitor, 7.2 x 3.5mm
C7 470n Film capacitor, 7.2 x 3mm
C8 47pF MLCC capacitor, NP0/C0G Not in original unit. See build notes.
C9 56n Film capacitor, 7.2 x 2.5mm
C10 1n2 Film capacitor, 7.2 x 2.5mm
C11 470n Film capacitor, 7.2 x 3mm
C12 10uF Electrolytic capacitor, 5mm
C13 12n Film capacitor, 7.2 x 2.5mm
C14 1uF Film capacitor, 7.2 x 3.5mm
C15 10uF Electrolytic capacitor, 5mm
C16 6n8 Film capacitor, 7.2 x 2.5mm
C17 18n Film capacitor, 7.2 x 2.5mm
C18 33n Film capacitor, 7.2 x 2.5mm
C19 22n Film capacitor, 7.2 x 2.5mm
C20 1uF Film capacitor, 7.2 x 3.5mm
C21 1uF Film capacitor, 7.2 x 3.5mm
C22 47uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.
C23 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C24 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
D5 1N914 Fast-switching diode, DO-35
D6 1N914 Fast-switching diode, DO-35
D7 1N914 Fast-switching diode, DO-35

HELIODOR AMP OVERDRIVE 5
PARTS LIST, CONT.
PART VALUE TYPE NOTES
Q1 2SK209-GR BJT transistor, PNP, TO-92 Original uses 2SK184-GR. 2SK209-GR is the SMD equivalent.
Q2 2SK209-GR JFET, N-channel, SOT-23 Original uses 2SK184-GR. 2SK209-GR is the SMD equivalent.
Q3 2SK209-GR JFET, N-channel, SOT-23 Original uses 2SK184-GR. 2SK209-GR is the SMD equivalent.
Q4 2SK209-GR BJT transistor, PNP, TO-92 Original uses 2SK184-GR. 2SK209-GR is the SMD equivalent.
Q5 2N3906 BJT transistor, NPN, TO-92 Substitute. Original uses 2SA1048.
Q6 2SK209-GR BJT transistor, PNP, TO-92 Original uses 2SK184-GR. 2SK209-GR is the SMD equivalent.
Q7 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC2458-GR.
IC1 JRC4558D Operational amplifier, DIP8 Substitute. Original uses M5218AL.
IC1-S DIP-8 socket IC socket, DIP-8
DRIVE 100kA 16mm right-angle PCB mount pot
TONE 10kB 16mm right-angle PCB mount pot
VOLUME 100kB 16mm right-angle PCB mount pot
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.

HELIODOR AMP OVERDRIVE 6
BUILD NOTES
Using SMD JFETs
The 2SK117-GR JFET is no longer available in through-hole format. This PCB uses a hybrid through-
hole/SMD outline for each JFET. An extra “G” (gate) pad is included to accommodate surface-mount
devices without the need for adapters.
SMD JFETs should be oriented as follows:
All surface-mount JFETs use the same pinout, so this configuration will fit any type that we’re aware of.
However, always check the datasheet if you’re uncertain—they’re difficult to desolder.
Using through-hole adapters
If you’re not confident in your ability to work with surface-mount parts, Aion FX offers 2SK209-GR
JFETs (the SMD version of 2SK184-GR and 2SK117-GR) that come pre-soldered to adapters for use
in through-hole designs. These are from the same manufacturer as the ones used in the original BOSS
pedals and will perform identically.
Using old-stock transistors
Toshiba has not manufacturered through-hole transistors and JFETs in many years, but it’s still possible
to find the 2SK184-GR and 2SK117-GR as well as the four types of BJT transistors used in the original.
However, be aware that these follow the Japanese pinout conventions, whereas the PCB layout is set up
for USA conventions since there are a lot more widely-available substitutes in this format.
For those using original Toshiba through-hole JFETs or BJTs, an extra pad has been added to the left of
the transistor outline (drain for JFETs, emitter for BJTs) so that the Japanese pinout can be easily used
without needing to twist the legs around. In both cases, the transistor should be rotated 180 degrees
from the silkscreen and shifted over one pad, as shown:
C8 capacitor
C8 (47pF) is not present in the original unit, but it’s good practice to have a small-value feedback
capacitor when dealing with these types of gain stages to prevent high-frequency oscillation. The OD-2
and BD-2 do use this capacitor in similar circuitry.
BJT JFET

SCHEMATIC
HELIODOR AMP OVERDRIVE 7
IN
OUT
1N914
1N5817
100uF
10kB
100kB
+9V
100n
GND
2M2
10k
47n
1M
VB
2SK209-GR
10k
VA
GND
220k
6k8
2k7
470R
2N3906
3k3
2k7
47pF
GND
GND
VA
VA
VA
470n
10n
10k 10k
56n
3k3
GND
VB
1N914
1N914
1N914
1N914
GND
56n
33k
2SK209-GR
1k
VA
33n
22n
GND GND
JRC4558D
VB
100k
2N5088
VB
VA
10k
GND
1k
100k
GND
1uF
1uF
VA
10k
10k
VB
GND
47uF
GND
JRC4558D
VA
GND
18n
100k
VB
2SK209-GR
2SK209-GR
10k
VA
GND
1uF
100kA
1uF
220k
GND
GND
100k
1n2
GND
GND
2k2
GND
10uF
GND
10k
470n
4k7
12n
1uF
GND
47k
150k
VB
6k8
1k
10uF
GND
6n8
4k7
12k
18n
4k7
12k
GND
D1
C23
TONE
1
2
3
VOLUME
1
2
3
C24
RPD
R1
C1
R2
Q1
R3
Q3
R9
Q4
R12
R11
R13
Q5
R14
R15
C8
C7
C4
R8 R6
C5
R7
D2 D3
D4
D5
C9
R17
Q6
R19
C18
C19
6
5
7
IC1B
R30
Q7
R31
R33
R32
C20
C21
R34
R35
C22
2
3
1
IC1A
84
C2
R4
Q2
R5
C3
DRIVE
1
2
3
C6
R10
R16
C10
R20
C12
R18
C11
D6 D7
R21
C13
C14
R22
R23
R25
R24
C15
C16
R27
R26 C17 R29
R28
GND

HELIODOR AMP OVERDRIVE 8
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
VOLUME DRIVE
TONE
FOOTSWITCHLED
DC IN
x: -0.65, y: +1.71 x: 0.65, y: +1.71
ø9/32” ø9/32”
x: 0, y: +0.66
ø9/32”
CENTER (0,0)
0 1 2
CM
0 1
INCH

HELIODOR AMP OVERDRIVE 9
ENCLOSURE LAYOUT
Enclosure is shown without jacks. See next page for jack layout and wiring.
125B

HELIODOR AMP OVERDRIVE 10
WIRING DIAGRAM
125B
IN +VGND NC NC OUT
PCB
IN
GND +V +V JACK GND JACK
OUTIN
GND GND PCB
OUT