aion FRACTAL PARAMETRIC OVERDRIVE User manual

FRACTAL PARAMETRIC OVERDRIVE 1
PROJECT NAME
FRACTAL
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
Pearl®OD-05 Over Drive
A versatile overdrive with a single-band parametric EQ, allowing adjustable boost or cut from 100 Hz to
4kHz.
Parametric overdrive 1.0.0 (2022-04-08)
BUILD DIFFICULTY
Intermediate
Actual size is 2.3” x 1.86” (main board) and 1.78” x 0.87” (bypass board).

FRACTAL PARAMETRIC OVERDRIVE 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 Fractal Parametric Overdrive is based on the Pearl OD-05 Over Drive, a curiously obscure effect
from the early 80’s when Pearl (yes, the drum company!) briefly dipped into the guitar effects pedal
business. The result was a line of very well-engineered pedals that unfortunately never got the attention
they deserved.
Even 40 years later, this pedal still is relatively unknown. There are no commercial clones and it’s
relatively unexplored in the DIY scene. But those who have played through one universally praise it for
its tonal flexibility.
The OD-05 has a standard feedback-diode clipping stage like a Tube Screamer, but the magic is what
comes in front of it: a parametric equalizer that allows either a cut or boost of up to 15dB in a range from
100 Hz to 4 KHz.
With this flexible preclipping EQ, you can get some incredible sounds out of it. You can crank the mids
for a cocked-wah sound, or tune the circuit to enhance the resonant peak of your specific guitar and
amp. You can even get a scooped-mids sound if that’s what you’re after, something that’s pretty rare in
an overdrive.
The updated version of the Fractal includes one new feature: the fixed post-clipping treble cut can be
dialed out for a slight treble boost. Since the parametric EQ only affects a single frequency band, this can
allow for control over a second frequency, albeit with reduced functionality. Turned to zero, it’s identical
to the stock circuit.
USAGE
The Fractal has five controls:
• Drive controls the amount of gain from the op amp that is fed through the feedback clipping diodes.
• Frequency sweeps through the midrange from 100Hz to 4kHz, setting the frequency that is either
boosted or cut by the EQ Gain control.
• EQ Gain is the boost or cut of the frequency set with the Frequency knob. It provides up to +/-15dB
gain. At the center position, the response is flat and the Frequency knob has no effect.
• Tone adds back some of the treble via a passive filter. At zero, it’s equivalent to a stock OD-05.
• Level is the output level of the effect.

FRACTAL PARAMETRIC 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 1k Metal film resistor, 1/4W
R2 1M Metal film resistor, 1/4W
R3 10k Metal film resistor, 1/4W
R4 680k Metal film resistor, 1/4W
R5 270k Metal film resistor, 1/4W
R6 22k Metal film resistor, 1/4W
R7 22k Metal film resistor, 1/4W
R8 22k Metal film resistor, 1/4W
R9 3k3 Metal film resistor, 1/4W
R10 2k2 Metal film resistor, 1/4W
R11 2k2 Metal film resistor, 1/4W
R12 22k Metal film resistor, 1/4W
R13 22k Metal film resistor, 1/4W
R14 22k Metal film resistor, 1/4W
R15 22k Metal film resistor, 1/4W
R16 10k Metal film resistor, 1/4W
R17 10k Metal film resistor, 1/4W
R18 10k Metal film resistor, 1/4W
R19 1M Metal film resistor, 1/4W
R20 1k Metal film resistor, 1/4W
R21 1k Metal film resistor, 1/4W
R22 10k Metal film resistor, 1/4W
R23 1k Metal film resistor, 1/4W
R24 470R Metal film resistor, 1/4W
R25 10k Metal film resistor, 1/4W
R26 10k Metal film resistor, 1/4W
R27 10k Metal film resistor, 1/4W
R28 470R 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 33n Film capacitor, 7.2 x 2.5mm
C2 1uF Film capacitor, 7.2 x 3.5mm

FRACTAL PARAMETRIC OVERDRIVE 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
C3 10uF Electrolytic capacitor, 5mm
C4 1uF Film capacitor, 7.2 x 3.5mm
C5 4n7 Film capacitor, 7.2 x 2.5mm
C6 68n Film capacitor, 7.2 x 2.5mm
C7 100n Film capacitor, 7.2 x 2.5mm
C8 470pF MLCC capacitor, NP0/C0G
C9 100n Film capacitor, 7.2 x 2.5mm
C10 47pF MLCC capacitor, NP0/C0G
C11 220n Film capacitor, 7.2 x 2.5mm
C12 15n Film capacitor, 7.2 x 2.5mm
C13 1uF Film capacitor, 7.2 x 3.5mm
C14 47uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.
C15 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C16 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C17 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
Q1-Q4 2N5088 BJT transistor, NPN, TO-92 Substitute. Original uses 2SC2240-GR.
IC1 JRC4558D Operational amplifier, DIP8
IC1-S DIP-8 socket IC socket, DIP-8
IC2 JRC4558D Operational amplifier, DIP8
IC2-S DIP-8 socket IC socket, DIP-8
DRIVE 100kB 16mm right-angle PCB mount pot
FREQ. 100kC dual 16mm dual-gang PCB mount pot
EQ 10kB 16mm right-angle PCB mount pot Original OD-05 uses center detent, which is useful if you can find it.
TONE 10kB 16mm right-angle PCB mount pot
VOL. 10kA 16mm right-angle PCB mount pot Original OD-05 uses 10kB, but audio taper provides better control.
LED 5mm LED, 5mm, red diffused
IN 1/4" stereo 1/4" phone jack, closed frame Switchcraft 112BX 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.

FRACTAL PARAMETRIC OVERDRIVE 5
BUILD NOTES
Treble tone control
The original OD-05 has a steep 1kHz treble cut after the clipping stage to compensate for boosted
treble earlier in the circuit and round out the tone.
By making this treble cut adjustable, we can allow some of the treble to be restored, which can be useful
with some EQ settings—for example, using the parametric EQ to cut the midrange, and then boosting
the treble for a more pronounced scooped-mids sound.
This is a new modification that has been added to the updated version of the Fractal. It replaces the
clipping diode switch from the legacy version, which is not very useful in this particular circuit.
At zero, it’s identical to the stock circuit, so there’s no good reason to leave it off—but if you do want to
omit it, use 10k for R24 and then jumper all three pads of the Tone pot together.

SCHEMATIC
FRACTAL PARAMETRIC OVERDRIVE 6
GND
2M2
1N5817
100uF
10kB
10kB
GND
100kB
10kA
+9V
100n
VA
33n
1k
1M
10k10k
47uF
VB
GND GND
VB
2N5088
2N5088
2N5088
2N5088
VA
470R
100uF
GND
VC
10k
GND
1uF
270k 680k
VC
GND
22k
VC
22k
22k
10uF
GND
GND
VC
3k3
GND
1uF
JRC4558D
JRC4558D
JRC4558D
JRC4558D
VA VA
GND GND
2k2
100kC
100kC
2k2
68n
GND
4n7
22k
22k
VB
22k 22k
100n
10k
10k
VB
10k
470pF
100n
1M
VB
47pF
1k
1k
10k
1k
220n
GND
470R
15n
GND
VA
10k
1uF
GND
IN
OUT
RPD
D1
C16
EQ GAIN
1
2
3
TONE
1
2
3
DRIVE
1
2
3
VOLUME
1
2
3
C17
C1
R1
R2
R26R27
C14
Q1
Q2
Q3
Q4
R28
C15
R3
C2
R5 R4
R6R7
R8
C3
R9
C4
2
3
1
IC1A
84
6
5
7
IC1B
2
3
1
IC2A
6
5
7
IC2B
84
R10
FREQUENCYA
1
2
3
1
2
3
FREQUENCYB
R11
C6
C5
R12
R13
R14 R15
C7
R16
R17
R18
C8
C9
R19
C10
D2
D3
R20
R21
R22
R23
C11
R24
C12
R25
C13
GND

FRACTAL PARAMETRIC OVERDRIVE 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.
x: -0.775, y: +1.71 x: +0.775, y: +1.71
ø9/32” ø9/32”
x: -0.775, y: +0.41
ø9/32”
x: 0, y: +1.71
ø9/32”
x: +0.775, y: +0.41
ø9/32”
x: 0, y: -1.20
ø15/32”
x: -0.775, y: -1.20
ø5/16”
CENTER (0,0)
125B
FREQUENCYTONEDRIVE
EQ GAINVOLUME
FOOTSWITCHLED
ø3/8” ø1/2”
0.385”
0.625” 0.625”
ø3/8”
OUT DC IN
0 1 2
CM
0 1
INCH

FRACTAL PARAMETRIC OVERDRIVE 8
ENCLOSURE LAYOUT
Enclosure is shown without jacks. See next page for jack layout and wiring.
Note: The upper pads for the dual-gang frequency potentiometer appear to be cut in half. This is
intentional! These are called a plated half-holes or castellated holes, and they are used so that the PCB
can lay flat across the pots instead of angling upward for the dual pot.
Solder it like you would if they were normal pads, but bend the top pins forward slightly so they make
contact with the edge of the pads.
125B

FRACTAL PARAMETRIC OVERDRIVE 9
WIRING DIAGRAM
125B
IN +VGND N.C. N.C. OUT
PCB
IN
GND +V +V JACK GND JACK
OUTIN
GND GND PCB
OUT

FRACTAL PARAMETRIC OVERDRIVE 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 (2022-04-08)
Initial release.