aion Obscura User manual

OBSCURA OCTAVE BLEND 1
PROJECT NAME
OBSCURA
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
Prescription Electronics COB
Based on the octave section of the fOXX Tone Machine, this circuit adds a clean blend and a knob to
change the character of the octave effect.
Octave fuzz 1.0.0 (2023-09-08)
BUILD DIFFICULTY
Easy
Actual size is 2.3” x 1.86” (main board) and 1.78” x 0.86” (bypass board).

OBSCURA OCTAVE BLEND 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 Obscura Octave Blend is an adaptation of the Prescription Electronics COB (Clean Octave Blend),
an octave fuzz with blendable clean signal.
Prescription Electronics (PEI) was one of the original boutique pedal companies, founded in 1993 by
Jack Brossart in Portland, OR. The COB was introduced within a couple of years and was produced up
until the company closed its doors in 2014. PEI made a brief comeback in 2019 and reissued most of
their lineup, but after Jack’s passing that same summer, they seem to have disappeared for good.
The COB is a very unique effect. The octave section is clearly based on the fOXX Tone Machine with a
few value tweaks, but it’s missing both a tone and a gain control. Instead, the Octave knob loads down
the octave stage in a way that does not actually add or remove the octave, but changes its character. At
some settings it’s much more suitable for blending, and at others it sounds good on its own with no clean
signal. There are a few great YouTube demos of the COB if you want to get an idea of how it works and
what it’s capable of, but suffice it to say, it’s far from a typical octave fuzz.
The Obscura is a direct clone of the COB except for one addition: a switch that disables the octave,
adapted from our Vulcan (fOXX Tone Machine) project. Its behavior is very different in this circuit.
Typically you’ll want to leave it in the down (stock) position because the octave is the whole point of
the effect. But there are some interesting and unique sounds in the other two modes, especially in
combination with the blend control, so we felt like it was a worthwhile addition.
USAGE
The Obscura has the following controls:
• Octave changes the character of the octave signal, from a glitchy tone reminiscent of a ring
modulator to a more traditional octave fuzz.
• Blend pans between the clean and effect signal.
• Volume controls the overall output of the effect, which comes after the blend.
• Octave (toggle switch) selects between octave, no octave (original) and no octave (modified).

OBSCURA OCTAVE BLEND 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 (most 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 22k Metal film resistor, 1/4W
R2 150k Metal film resistor, 1/4W
R3 1k5 Metal film resistor, 1/4W
R4 47k Metal film resistor, 1/4W
R5 560k Metal film resistor, 1/4W
R6 120k Metal film resistor, 1/4W
R7 10k Metal film resistor, 1/4W
R8 10k Metal film resistor, 1/4W
R9 680k Metal film resistor, 1/4W
R10 36k Metal film resistor, 1/4W
R11 47k Metal film resistor, 1/4W
R12 1k Metal film resistor, 1/4W
R13 47k Metal film resistor, 1/4W
R14 100k Metal film resistor, 1/4W
R15 100k Metal film resistor, 1/4W
R16 4k7 Metal film resistor, 1/4W
R17 4k7 Metal film resistor, 1/4W
R18 100k Metal film resistor, 1/4W
R19 100k Metal film resistor, 1/4W
R20 100k Metal film resistor, 1/4W
R21 100R Metal film resistor, 1/4W Power supply filter resistor.
RPD 2M2 Metal film resistor, 1/4W Input pulldown resistor. Can be as low as 1M.
LEDR 10k Metal film resistor, 1/4W LED current-limiting resistor. Adjust value to change LED brightness.
C1 1uF Film capacitor, 7.2 x 3.5mm
C2 100n Film capacitor, 7.2 x 2.5mm
C3 10uF Electrolytic capacitor, 5mm
C4 100n Film capacitor, 7.2 x 2.5mm
C5 100n Film capacitor, 7.2 x 2.5mm
C6 1n Film capacitor, 7.2 x 2.5mm
C7 10uF Electrolytic capacitor, 5mm
C8 10uF Electrolytic capacitor, 5mm
C9 10uF Electrolytic capacitor, 5mm

OBSCURA OCTAVE BLEND 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
C10 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C11 100n MLCC capacitor, X7R Power supply filter capacitor.
D1 1N5817 Schottky diode, DO-41
D2 1N4001 Rectifier diode, DO-41
D3 1N4001 Rectifier diode, DO-41
Q1 2N3904 BJT transistor, NPN, TO-92
Q2 2N3904 BJT transistor, NPN, TO-92
Q3 2N3904 BJT transistor, NPN, TO-92
Q4 2N3904 BJT transistor, NPN, TO-92
BLEND 250kB 16mm right-angle PCB mount pot
OCTAVE 1MB 16mm right-angle PCB mount pot
VOLUME 100kB 16mm right-angle PCB mount pot
OCT. SW. SPDT cntr off Toggle switch, SPDT on-off-on
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.
BATT Battery snap 9V battery snap Optional. Use the soft plastic type—the hard-shell type will not fit.
FSW 3PDT Stomp switch, 3PDT
ENC 125B Enclosure, die-cast aluminum Can also use a Hammond 1590N1.

OBSCURA OCTAVE BLEND 5
BUILD NOTES
Octave knob functionality
The Octave knob does not necessarily reduce or increase the octave effect, but changes the character
of it. All the way down, the Q3 transistor is severely loaded in a way that weakens the signal and cuts
highs. This can increase the octave effect or turn it into a glitchy ring-mod type of sound. At these lower
settings, the octave seems to be almost isolated from the main signal, which makes it great for adding
into an otherwise clean sound with the blend knob.
As you turn the Octave knob up, it gets closer to a standard octave fuzz tone, more like the fOXX Tone
Machine it’s based on. But as with all of these transistor-based octave effects, its behavior will vary
wildly depending on the guitar and the surrounding signal chain.
Octave switch
The octave in this effect is generated by splitting the signal into two, one in-phase and one out of phase,
and then rectifying the signals to cancel out half of the waveform of each. The signals are then combined
back together which emphasizes the octave overtone.
The octave switch lets you disable half of the phase splitter to cancel out the octave effect. However, the
non-octave signal still passes through a series diode (D2) which introduces something called “crossover
distortion”. While this crossover distortion is part of the sound of the COB, it sounds very good without
it as well and justifies having its own setting.
As a result, the octave switch has been modified to have Octave, No Octave (original) and No Octave
(modified) settings. Since it’s primarily an octave effect, it obviously is going to work best in the Octave
mode, but there are some interesting sounds to be discovered in the other two positions.

SCHEMATIC
OBSCURA OCTAVE BLEND 6
GND
2M2
1N5817
100uF
1MB
100kB
GND
250kB
+9V
100n
VA
22k
1uF
150k
2N3904
1k5 47k
VA
100R
560k
GND GND
100n
120k
2N3904
GND
10k
VA
GND
10uF
10k
680k
GND
36k
100n
2N3904
47k
VA
1k
47k
GND
100k
100n
GND GND
100k
4k7
GND
1n
2N3904
4k7
VA
10uF10uF
100k 100k
GND GND
1N4001
1N4001
100k
VA
10uF
GND
IN
OUT
RPD
D1
C10
OCTAVE
1
2
3
VOLUME
1
2
3
BLEND
1
2
3
C11
R1
C1
R2
Q1
R3 R4
R21
R5
C2
R6
Q2
R7
C3
R8
R9
R10
C4
Q3
R11R12
R13
R14
C5
R15
R16
C6
Q4
R17
C7C8
R18 R19
D2
D3
R20
C9
2
3 1
OCT. SW
GND

OBSCURA OCTAVE BLEND 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.
0 1 2
CM
0 1
INCH
x: -0.65, y: +1.71 x: 0.65, y: +1.71
ø9/32” ø9/32”
x: 0, y: +0.66
ø9/32”
x: 0, y: -1.20
ø15/32”
x: -0.775, y: -1.20
ø5/16”
CENTER (0,0)
ø3/8” ø1/2”
0.385”
0.625” 0.625”
ø3/8”
OUT DC IN
125B
VOLUME
ON/OFF
OCTAVE
BLEND
OCTAVE
FOOTSWITCHLED
x: +0.85, y: +0.66
ø1/4”

OBSCURA OCTAVE BLEND 8
ENCLOSURE LAYOUT
Enclosure is shown without jacks. See next page for jack layout and wiring.
125B

OBSCURA OCTAVE BLEND 9
WIRING DIAGRAM
125B
IN +VGND N.C. N.C. OUT
PCB
IN
GND +V +V JACK GND JACK
OUTIN
GND GND PCB
OUT
Shown with optional 9V battery. If battery is omitted, both jacks can be mono rather than one being stereo.
Leave the far-right lug of the DC jack unconnected.

OBSCURA OCTAVE BLEND 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 cannot be 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 (2023-09-08)
Initial release.