aion AURUM User manual

AURUM AMP OVERDRIVE 1
PROJECT NAME
AURUM
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
BOSS®OD-2 Turbo Overdrive
The first in a series of legendary pedals using BOSS’s “discrete op-amp” topology that overdrives more
like an amplifier than traditional diode-clipping circuits.
JFET overdrive 1.0.0 (2021-11-26)
BUILD DIFFICULTY
Intermediate
Actual size is 2.3” x 2.42” (main board) and 1.78” x 0.86” (bypass board).

AURUM AMP OVERDRIVE 2
TABLE OF CONTENTS
1Project Overview 9Drill Template
2Introduction & Usage 10 Enclosure Layout
3-5 Parts List 11 Wiring Diagram
6-7 Build Notes 12 Licensing
8Schematic 12 Document Revisions
INTRODUCTION
The Aurum Amp Overdrive is based BOSS®OD-2 Turbo Overdrive, notable as the first appearance of
Boss’s “discrete op-amp” circuit topology which uses 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. As a bit of trivia,
the OD-2 is also one of only three fully discrete circuits ever made by BOSS (no op-amps), alongside the
FZ-3 Fuzz and the DS-2 Turbo Distortion.
The OD-2 was first released in 1985 as the successor to the OD-1. In 1994, it was replaced by the OD-
2r. The “r” in this case stands for “remote”, since it allowed an external footswitch to change between the
two channels on the fly. However, the OD-2r also corrected one one of the most persistent flaws of the
original unit, which is that it could barely get to unity gain at the highest volume setting.
The Aurum is more specifically based on the “Turbo” mode of the circuit, adapting the OD-2r version
of the circuit with the gain recovery stage and a couple of other minor tweaks. The non-”Turbo” mode
is a completely separate drive circuit within the pedal, and generally speaking it’s unremarkable when
compared to the Turbo mode.
The OD-2 was officially succeeded by the OD-3 in 1997, although the BD-2 Blues Driver can be seen as
a “rebranded” in-between member of the same series since it shares much of the same circuitry as both
the OD-2 and OD-3. However, all three circuits sound pretty different, and they each bring their own
character to the table.
The original OD-2 uses the 2SK117-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 Aurum has three controls:
• Drive increases the gain of the first discrete op-amp stage, which overloads the second stage and
pushes it into clipping.
• Tone is a passive treble cut after the second gain stage.
• Volume sets the overall output of the effect signal.

AURUM 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 4k7 Metal film resistor, 1/4W
R6 2k2 Metal film resistor, 1/4W
R7 4k7 Metal film resistor, 1/4W
R8 4k7 Metal film resistor, 1/4W Not present in original OD-2. Use a jumper if building that version.
R9 2k2 Metal film resistor, 1/4W
R10 100k Metal film resistor, 1/4W
R11 10k Metal film resistor, 1/4W
R12 100k Metal film resistor, 1/4W
R13 4k7 Metal film resistor, 1/4W
R14 2k2 Metal film resistor, 1/4W
R15 4k7 Metal film resistor, 1/4W
R16 270k Metal film resistor, 1/4W
R17 1k8 Metal film resistor, 1/4W
R18 820R Metal film resistor, 1/4W
R19 10k Metal film resistor, 1/4W
R20 4k7 Metal film resistor, 1/4W
R21 100k Metal film resistor, 1/4W
R22 4k7 Metal film resistor, 1/4W
R23 4k7 Metal film resistor, 1/4W
R24 4k7 Metal film resistor, 1/4W
R25 4k7 Metal film resistor, 1/4W
R26 680R Metal film resistor, 1/4W
R27 47k Metal film resistor, 1/4W
R28 470k Metal film resistor, 1/4W
R29 10k Metal film resistor, 1/4W
R30 100k Metal film resistor, 1/4W
R31 1k Metal film resistor, 1/4W
R32 4k7 Metal film resistor, 1/4W

AURUM AMP OVERDRIVE 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
R33 4k7 Metal film resistor, 1/4W
R34 4k7 Metal film resistor, 1/4W
R35 4k7 Metal film resistor, 1/4W
R36 10k Metal film resistor, 1/4W Use 16k for original OD-2.
R37 10k Metal film resistor, 1/4W Use 14k for original OD-2.
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 22n Film capacitor, 7.2 x 2.5mm
C2 18n Film capacitor, 7.2 x 2.5mm
C3 100n Film capacitor, 7.2 x 2.5mm
C4 100pF MLCC capacitor, NP0/C0G
C5 22n Film capacitor, 7.2 x 2.5mm
C6 6n8 Film capacitor, 7.2 x 2.5mm
C7 10n Film capacitor, 7.2 x 2.5mm
C8 150n Film capacitor, 7.2 x 2.5mm
C9 100pF MLCC capacitor, NP0/C0G
C10 10n Film capacitor, 7.2 x 2.5mm
C11 18n Film capacitor, 7.2 x 2.5mm
C12 27n Film capacitor, 7.2 x 2.5mm Original OD-2 uses 22n. See build notes.
C13 1uF Film capacitor, 7.2 x 3.5mm
C14 10uF Electrolytic capacitor, 5mm
C15 1uF Film capacitor, 7.2 x 3.5mm
C16 22n Film capacitor, 7.2 x 2.5mm
C17 1uF Film capacitor, 7.2 x 3.5mm
C18 10uF Electrolytic capacitor, 5mm Power supply filter capacitor.
C19 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C20 47uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.
C21 10uF Electrolytic capacitor, 5mm Power supply filter capacitor.
C22 10uF Electrolytic capacitor, 5mm Power supply filter capacitor.
C23 47uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.
C24 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C25 100n MLCC capacitor, X7R Power supply filter capacitor.
Z1 1N4738A Zener diode, 8.2V, DO-41 Original OD-2 uses 6.2V zener, e.g. 1N4735A. See build notes.
D1 1N5817 Schottky diode, DO-41
Q1 2SK209-GR JFET, N-channel, SOT-23 Both originals use 2SK117-GR. 2SK209-GR is the SMD equivalent.
Q2 2SK209-GR JFET, N-channel, SOT-23 Both originals use 2SK117-GR. 2SK209-GR is the SMD equivalent.
Q3 2SK209-GR JFET, N-channel, SOT-23 Both originals use 2SK117-GR. 2SK209-GR is the SMD equivalent.

AURUM AMP OVERDRIVE 5
PARTS LIST, CONT.
PART VALUE TYPE NOTES
Q4 2N3906 BJT transistor, PNP, TO-92 Substitute. Original uses 2SA970-GR.
Q5 2SK209-GR JFET, N-channel, SOT-23 Both originals use 2SK117-GR. 2SK209-GR is the SMD equivalent.
Q6 2SK209-GR JFET, N-channel, SOT-23 Both originals use 2SK117-GR. 2SK209-GR is the SMD equivalent.
Q7 2N3906 BJT transistor, PNP, TO-92 Substitute. Original uses 2SA970-GR.
Q8 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC3378-GR.
Q9 2N3906 BJT transistor, PNP, TO-92 Substitute. Original uses 2SA1335-GR.
Q10 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC2458-GR.
Q11 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC2458-GR.
Q12 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC2458-GR.
DRIVE 250kA 16mm right-angle PCB mount pot
TONE 10kB 16mm right-angle PCB mount pot
VOLUME 50kA 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.

AURUM 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 2SK117-GR and 2SK184-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 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 by one pad, as shown:
BJT JFET

AURUM AMP OVERDRIVE 7
BUILD NOTES, CONT.
OD-2 / OD-2r differences
The OD-2r is an updated version of the OD-2 that was released in 1994, nine years after the original
OD-2. The major feature was the addition of the “remote” jack allowing the channel to be changed with
an external footswitch. While BOSS did not mention any other changes when marketing the new unit,
they did sneak in a few updates internally and the two units do not sound exactly the same.
First, there’s an added two-transistor volume recovery stage so that the unit has more volume available.
The original OD-2 barely got to unity gain at max volume—a persistent complaint from users—but the
added volume recovery solves this issue entirely.
Second, the tone control capacitor has been increased from 22n to 27n. This allows for slightly more
treble cut at the minimum setting, but with the treble control set high, it makes essentially no difference.
Third, a minimum drive resistor has been added in series with the drive control. This is largely
responsible for the OD-2r’s perception of having slightly more gain than the original, although it really
just impacts the knob positions and the gain itself is not increased.
Fourth, the supply voltage for the Turbo stage has been increased from 5.6V to 7.6V by changing the Z1
zener from 6.2V to 8.2V.
Lastly, the bias voltage is different. While the original OD-2 had a trimmer for the Turbo mode’s bias
voltage, the OD-2r omits this and just uses fixed resistors. It’s not known what parameters they used
in the OD-2 to determine where to set the trimmer in the factory, but the nominal 2.6V bias voltage in
the schematic is less than half of the 5.6V supply, so it’s may have been as simple as setting the trimmer
to 2.6V. However, if it was performance-based with 2.6V being just a rough target, then we don’t have
enough information. We only know this trimmed voltage wasn’t important enough to keep in the OD-2r.
The improvements in the OD-2r make it the definitive version of the circuit, and it’s recommended to
build the Aurum to OD-2r specs, which is the default configuration in the parts list.
Building the original OD-2
If you want to build a straight OD-2, make the following changes.
1. Use 22n for C12 instead of 27n.
2. Jumper R8 instead of using a 4k7 resistor.
3. Use 1N4735A for Z1 (6.2V zener) instead of 1N4738A.
4. Use 16k for R36 and 14k for R37, which will give 2.6V bias from a 5.6V source.
5. Run a wire from the middle pin of the Tone control to pin 3 of the Volume control, and omit all of
the components in between: C13-C15, R21-26, and Q8-9. No traces need to be cut, just leave the
spaces empty.
This list is provided for completion’s sake, but since the volume recovery stage fixes a severe
shortcoming and does not color the tone, it’s recommended to skip #5 and keep this added stage in an
OD-2 build.

SCHEMATIC
AURUM AMP OVERDRIVE 8
IN
OUT
1N5817
100uF
10kB
50kA
+9V
100n
GND
2M2
10k
22n
1M
VB
2SK209-GR
10k
VA
GND
100k
4k7
2k2
4k7
2N3906
4k7
2k2
100pF
GND
GND
18n
100n
100k
10k
22n
6n8
GND
100k
4k7
2k2
4k7
2N3906
270k
1k8
100pF
GND
GND
150n
GND
10k
10n
GND
18n
27n
GND
GND
2N3904
470k
2N5088
VB
VA
10k
GND
1k
100k
GND
1uF
100uF
GND
VA
4k74k7
VB
GND
47uF
GND
4k7
2N5088
10uF
GND
250kA
10n
GND GND
820R
4k7
22n
47k
1uF
100k
VB
4k7
2N3906
4k7
4k7
680R
VA
VA VA
GND
4k7
GND
10uF
1uF
4k7
10uF
GND
1N4738A
GND
10uF
GND
10k10k
GND
47uF
GND
VC
VD
VD
VC
VC
VC
VD
VC
VC
VC
GND
D1
C24
TONE
1
2
3
VOLUME
1
2
3
C25
RPD
R1
C1
R2
Q1
R3
Q2
R4
Q3
R5
R6
R7
Q4
R8
R9
C4
C2
C3
R10
R11
C5
C6
Q5
R12
Q6
R13
R14
R15
Q7
R16
R17
C9
C8
R19
C10
C11
C12
Q8
R28
Q10
R29
R31
R30
C17
C19
R33R34
C20
R32
Q11
2N5088
Q12
C18
DRIVE
1
2
3
C7
R18
R20
C16
R27
C13
R21
R22
Q9
R23
R25
R26
R24
C14
C15
R35
C21
Z1
C22
R36R37
C23
GND
2SK209-GR
2SK209-GR

AURUM AMP OVERDRIVE 9
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

AURUM AMP OVERDRIVE 10
ENCLOSURE LAYOUT
Enclosure is shown without jacks. See next page for jack layout and wiring.
125B