aion SAPPHIRE User manual

SAPPHIRE AMP OVERDRIVE 1
PROJECT NAME
SAPPHIRE
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
BOSS®BD-2 Blues Driver
One of the most popular pedals in the BOSS lineup, it uses their distinctive “discrete op-amp” topology
to deliver an amp-like drive resembling vintage Fender tweed or blackface amps.
Amp-like 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).

SAPPHIRE AMP OVERDRIVE 2
TABLE OF CONTENTS
1Project Overview 10 Drill Template
2Introduction & Usage 11 Enclosure Layout
3-5 Parts List 12 Wiring Diagram
6-8 Build Notes 13 Licensing
9Schematic 13 Document Revisions
INTRODUCTION
The Sapphire Amp Overdrive is based on the BOSS®BD-2 Blues Driver, one of their most popular
pedals in continuous production since it was first released in 1995. It’s been used by a wide variety of
artists including Mike McCready (Pearl Jam), Robert Smith (The Cure), Tom Morello, and even Prince.
The Blues Driver bears many similarities to the OD-2 Turbo Overdrive from 1988. This was the first
appearance of Boss’s “discrete op-amp” circuit topology that 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.
The Blues Driver starts with a circuit very similar to the OD-2, and then adds a slight bass boost to
the end to give it the signature fullness, along with some clipping diodes and a fixed-position Fender
tonestack for shaping the tone.
Despite its overwhelming popularity, the BD-2 is not without its flaws. On higher gain settings, the note
decay has been described as “fizzy” or “splatty”. And while the bass-heavy EQ is distinctive, it’s not as
flexible as other more transparent drives. Because of this, it’s one of the most frequently modded pedals
out there, with Keeley’s and Analogman’s mods being the most popular.
Another notable one is the Galaxie Mod from Machine Head Pedals, which is documented in the build
notes on page 7 and is strongly recommended. It’s an extensive mod that replaces half of the passive
components, completely transforming the pedal into something new.
The Sapphire is based on the stock BD-2 circuit converted to true bypass. The bass boost has been made
variable, so it can be adjusted between the stock value and the Galaxie value.
The original BD-2 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 Sapphire has four controls:
• Drive is a dual-gang control that simultaneously increases the gain of both discrete op-amp stages.
• Tone is a passive treble cut after the second gain stage.
• Bass is an active bass boost at the end of the circuit.
• Volume sets the overall output of the effect signal.

SAPPHIRE 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 220k Metal film resistor, 1/4W
R5 4k7 Metal film resistor, 1/4W
R6 2k2 Metal film resistor, 1/4W
R7 1k5 Metal film resistor, 1/4W Galaxie mod: 10k. See build notes for full mod.
R8 22k Metal film resistor, 1/4W
R9 510k Metal film resistor, 1/4W Omit if using 250kA drive pot.
R10 2k2 Metal film resistor, 1/4W
R11 330k Metal film resistor, 1/4W Galaxie mod: omit (leave empty). See build notes for full mod.
R12 100k Metal film resistor, 1/4W Galaxie mod: 22n CAPACITOR. See build notes for full mod.
R13 1M Metal film resistor, 1/4W Galaxie mod: 6n8 CAPACITOR. See build notes for full mod.
R14 15k Metal film resistor, 1/4W Galaxie mod: JUMPER. See build notes for full mod.
R15 1M Metal film resistor, 1/4W Galaxie mod: 100k. See build notes for full mod.
R16 4k7 Metal film resistor, 1/4W
R17 2k2 Metal film resistor, 1/4W
R18 2k2 Metal film resistor, 1/4W Galaxie mod: 10k. See build notes for full mod.
R19 33k Metal film resistor, 1/4W
R20 510k Metal film resistor, 1/4W Omit if using 250kA drive pot.
R21 2k2 Metal film resistor, 1/4W
R22 5k6 Metal film resistor, 1/4W Galaxie mod: 2k2. See build notes for full mod.
R23 470k Metal film resistor, 1/4W
R24 6k8 Metal film resistor, 1/4W
R25 1k2 Metal film resistor, 1/4W
R26 10k Metal film resistor, 1/4W
R27 470k Metal film resistor, 1/4W
R28 100k 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 2k2 Metal film resistor, 1/4W

SAPPHIRE AMP OVERDRIVE 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
R33 10k Metal film resistor, 1/4W
R34 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 100n Film capacitor, 7.2 x 2.5mm Galaxie mod: 10n. See build notes for full mod.
C3 150n Film capacitor, 7.2 x 2.5mm Galaxie mod: 39n. See build notes for full mod.
C4 47pF MLCC capacitor, NP0/C0G Galaxie mod: 100pF. See build notes for full mod.
C5 47pF MLCC capacitor, NP0/C0G
C6 220pF MLCC capacitor, NP0/C0G Galaxie mod: omit (leave empty). See build notes for full mod.
C7 100n Film capacitor, 7.2 x 2.5mm Galaxie mod: 10k RESISTOR. See build notes for full mod.
C8 47n Film capacitor, 7.2 x 2.5mm Galaxie mod: 100k RESISTOR. See build notes for full mod.
C9 2n2 Film capacitor, 7.2 x 2.5mm Galaxie mod: 47n. See build notes for full mod.
C10 100pF MLCC capacitor, NP0/C0G Galaxie mod: 220pF. See build notes for full mod.
C11 1uF Film capacitor, 7.2 x 3.5mm Galaxie mod: 68n, 82n or 100n. See build notes for full mod.
C12 100pF MLCC capacitor, NP0/C0G
C13 6n8 Film capacitor, 7.2 x 2.5mm
C14 5n6 Film capacitor, 7.2 x 2.5mm
C15 18n Film capacitor, 7.2 x 2.5mm Galaxie mod: 33n. See build notes for full mod.
C16 18n Film capacitor, 7.2 x 2.5mm Galaxie mod: 22n. See build notes for full mod.
C17 47n Film capacitor, 7.2 x 2.5mm
C18 2n2 Film capacitor, 7.2 x 2.5mm Galaxie mod: omit (leave empty). See build notes for full mod.
C19 56n Film capacitor, 7.2 x 2.5mm
C20 56n Film capacitor, 7.2 x 2.5mm
C21 1uF Film capacitor, 7.2 x 3.5mm
C22 1uF Film capacitor, 7.2 x 3.5mm
C23 47uF Electrolytic capacitor, 5mm Power supply filter capacitor.
C24 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C25 47uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.
C26 47uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.
C27 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C28 100n MLCC capacitor, X7R Power supply filter capacitor.
D1 1N5817 511-1N5817
D2 1N914 512-1N914 Galaxie mod: omit (leave empty). See build notes for full mod.
D3 1N914 512-1N914 Galaxie mod: omit (leave empty). See build notes for full mod.
D4 1N914 512-1N914 Galaxie mod: omit (leave empty). See build notes for full mod.
D5 1N914 512-1N914 Galaxie mod: omit (leave empty). See build notes for full mod.

SAPPHIRE AMP OVERDRIVE 5
PARTS LIST, CONT.
PART VALUE TYPE NOTES
D6 1N914 Fast-switching diode, DO-35 Galaxie mod: omit (leave empty). See build notes for full mod.
D7 1N914 Fast-switching diode, DO-35 Galaxie mod: omit (leave empty). See build notes for full mod.
Q1 2SK209-GR JFET, N-channel, SOT-23 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 2N3906 BJT transistor, PNP, TO-92 Substitute. Original uses 2SA1335-GR.
Q5 2SK209-GR JFET, N-channel, SOT-23 Original uses 2SK184-GR. 2SK209-GR is the SMD equivalent.
Q6 2SK209-GR JFET, N-channel, SOT-23 Original uses 2SK184-GR. 2SK209-GR is the SMD equivalent.
Q7 2N3906 BJT transistor, PNP, TO-92 Substitute. Original uses 2SA1335-GR.
Q8 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC2459-GR.
Q9 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC2459-GR.
Q10 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC2459-GR.
IC1 JRC4558D Operational amplifier, DIP8 Substitute. Original uses M5218AL (SIL-8).
IC1-S DIP-8 socket IC socket, DIP-8 Substitute. Original uses 2SA970-GR.
GAIN 500kA dual 16mm right-angle PCB mount pot Substitute for 250kA in original. Use 510k for R9 and R20 to drop the
value to 250k. Omit these resistors if using an actual 250kA pot.
BASS 25kB 16mm right-angle PCB mount pot
TONE 10kB 16mm right-angle PCB mount pot
VOLUME 100kA 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.

SAPPHIRE AMP OVERDRIVE 6
BUILD NOTES
Using SMD JFETs
The 2SK184-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 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

SAPPHIRE AMP OVERDRIVE 7
BUILD NOTES, CONT.
Galaxie Mod
The Galaxie Mod created by Keith Aviles of Machine Head Pedals (known as 11 Gauge on the TDPRI
forums) is without question the most extensive BD-2 mod that has been developed. This modification—
or more accurately, major surgery—changes out about half of the components to completely restructure
the pedal. It lowers the gain and fully eliminates the fizz while improving the tone control’s usefulness.
For a few of the parts, capacitors are replaced with resistors and vice versa. To accommodate this, some
of the component footprints on the Sapphire PCB are composites that allow either 0.2” film capacitors
or 0.3” resistors to be used, so the pads look like a sideways number 8. The PCB silkscreen shows the
components from the stock BD-2.
The parts list includes alternate values for the mod, but for completion’s sake, here is the full list.
C11: This value can be tailored to your preference. The original mod uses 100n, but Keith suggests that
82n or 68n may be better alternates if you have them.
R24: In the Galaxie Mod, this resistor is changed to 33k. However, the Sapphire’s bass control is a
variable resistor in series with R24, and at max rotation it’s nearly 33k in value (6.8k + 25k = 31.8k). It’s
recommended to leave R24 as the stock 6.8k, but you could increase it to 8.2k if you want full clockwise
rotation to be closer to 33k.
PART STOCK VALUE MOD VALUE
C2 100n 10n
C3 150n 39n
C4 47pF 100pF
C6 220pF Omit (leave empty)
C7 100n 10k resistor
C8 47n 100k resistor
C9 2n2 47n
C10 100pF 220pF
C11 1uF 68n, 82n or 100n (see note)
C15 18n 33n
C16 18n 22n
PART STOCK VALUE MOD VALUE
C18 2n2 Omit (leave empty)
R7 1k5 10k
R11 330k Omit (leave empty)
R12 100k 22n capacitor
R13 1M 6n8 capacitor
R14 15k Jumper (bare wire)
R15 1M 100k
R18 2k2 10k
R22 5k6 2k2
R24 6k8 see note
D2-D7 1N914 Omit (leave empty)

SAPPHIRE AMP OVERDRIVE 8
BUILD NOTES, CONT.
Gain pot value
The original BD-2 uses a 250kA dual-gang potentiometer. These are not available from any of the usual
suppliers except for Tayda Electronics, who carries a straight-pin knurled-shaft type. These can be made
to fit the Sapphire PCB with some effort, but it’s much easier to just use a 500kA dual potentiometer
which is readily available in the right-angle format used in this project.
Parallel resistors (R9 and R20) have been added to each half of the gain control to drop the 500k value
down to an effective value of 250k. The taper is affected slightly, but it’s not noticeable unless you’re
doing an A/B test against an original and trying to match knob positions. If using the stock 250kA value,
omit R9 and R20.
Q2/3 and Q5/6
It has been suggested that Q2/3 and Q5/6 (the JFET differential pairs that make up the two discrete op-
amp gain stages) should be matched in their VGS(off) value for for best performance.
This may be true in theory, but we took measurements of the JFETs in an original Blues Driver and the
two pairs were not matched any more closely than any of the other non-paired JFETs, which suggests
they were chosen at random. It doesn’t appear that BOSS used any sort of matching in the manufacture
of the Blues Driver and so it’s not something we recommend giving any consideration.

SAPPHIRE AMP OVERDRIVE 9
SCHEMATIC
IN
OUT
1N914
1N5817
100uF
10kB
100kA
+9V
100n
GND
47n
1M
VB
2SK209-GR
2SK209-GR
2SK209-GR
10k
VA
GND
220k
4k7
2k2
1k5
2N3906
22k
2k2
47pF
GND
GND
100n
VA
VA
VA
150n 250kA
OMIT (510k)
47pF
220pF
100k
330k
100n
47n
1M
15k
GND
GND
VB
1N9141N914
1N914 1N914
GND
2n2
1M
4k7
2k2
2k2
2N3906
33k
2k2
100pF
GND
GND
VA
VA
VA
1uF
OMIT (510k)
100pF
GND
VB
250kA
6n8
5k6
5n6
GND
18n
18n
GND GND
25kB
JRC4558D
47n
470k
VB
2n2
6k8
56n
1k2
2N5088
VA
10k
470k
VB
56n
GND
100k
2N5088
VB
VA
10k
GND
100k
GND
1uF
1uF
100uF
GND
VA
10k
10k
VB
GND
47uF
GND
2k2
2N5088
47uF
GND
JRC4558D
VA
GND
47uF
GND
D1
C27
TONE
1
2
3
VOLUME
1
2
3
C28
C1
R2
Q1
R3
Q2
R4
Q3
R5
R6
R7
Q4
R8
R10
C4
C2 C3 GAINB
1
2
3
R9
C5
C6
R12
R11
C7
C8
R13
R14
D2D3
D4 D5
C9
Q5
R15
Q6
R16
R17
R18
Q7
R19
R21
C10
C11
R20
C12
GAINA
1
2
3
C13
R22
C14
C15
C16
1
2
3
BASS
6
5
7
IC1B
C17
D6
D7 R23
C18
R24
C19
R25
Q8
R26 R27
C20
R28
Q9
R29 R30
C21
C22
C24
R33
R34
C25
R32
Q10
C23
2
3
1
IC1A
84
C26
GND

SAPPHIRE AMP OVERDRIVE 10
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: -0.65, 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”
x: 0, y: -1.775
ø15/32”
x: -0.775, y: -1.775
ø5/16”
CENTER (0,0)
ø3/8”
125B
OUT
VOLUME GAIN
BASS TREBLE
FOOTSWITCHLED
DC IN
0 1 2
CM
0 1
INCH