aion CORVUS User manual

CORVUS DISTORTION / SUSTAINER 1
PROJECT NAME
CORVUS
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
EHX Big Muff Pi (1978 Op-Amp)
The 1978 redesign of the classic Big Muff Pi, using op-amps instead of transistors. Famously used by
Billy Corgan on the Smashing Pumpkins album Siamese Dream.
Fuzz / Distortion 1.0.1 (2021-04-20)
Actual size is 2.3” x 1.86” (main board) and 1.78” x 0.87” (bypass board).
BUILD DIFFICULTY
Easy

CORVUS DISTORTION / SUSTAINER 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 Corvus Fuzz is a clone of the 1977-1980 “op-amp” EHX Big Muff Pi, a redesign of the classic Big
Muff circuit using op-amps instead of transistor gain stages. It was designed by Howard Davis, a brilliant
EHX engineer who also invented the Memory Man delay and Electric Mistress flanger as well as the
Deluxe Big Muff Pi.
He was tasked in the redesign with getting a similar sound to the Big Muff, but the circuit itself bears no
resemblance to the earlier 4-transistor circuit apart from the tone control. Therefore, while it was the
only commercially-available version of the Big Muff for several years, today it’s seen as a fully separate
circuit. (In 2017, EHX reissued this version of the Big Muff, formally adopting its formerly unofficial
name, the “Op-Amp Big Muff Pi”.)
The Corvus is based on the 1978 revision of the Big Muff, which added a “Tone Bypass” switch. As
inferred by the name, this switch fully bypasses the tone stack for a more raw and grinding sound.
The Corvus is a direct adaptation of the 1978 circuit except for the inclusion of an optional modification
to the gain control, originally conceived by Analogguru. This converts the op-amp configuration into a
variable gain stage, whereas the stock unit uses a fixed gain configuration followed by a volume control.
With this modification, the op-amp isn’t amplifying the signal any more than it needs to. However, the
original version can also be built if you prefer (see build notes for details).
USAGE
The Corvus has the following controls:
• Sustain controls the amount of gain that is fed into the clipping stage.
• Tone pans between a low emphasis and high emphasis, with an overall scooped tone in the middle.
• Tone Bypass (toggle switch) bypasses the tone stack entirely, increasing the output level and giving
it a more aggressive bite.
• Volume is the overall output level.

CORVUS DISTORTION / SUSTAINER 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 56k Metal film resistor, 1/4W
R2 330k Metal film resistor, 1/4W
R3 10k Metal film resistor, 1/4W
R4 47k Metal film resistor, 1/4W
R5 560k Metal film resistor, 1/4W
R6 62k Metal film resistor, 1/4W
R7 47R Metal film resistor, 1/4W Modification; omit for stock version. See build notes.
R8 OMIT 47R in stock version. See build notes.
R9 8k2 Metal film resistor, 1/4W
R10 470k Metal film resistor, 1/4W
R11 5k6 Metal film resistor, 1/4W
R12 1k2 Metal film resistor, 1/4W
R13 47k Metal film resistor, 1/4W
R14 47k Metal film resistor, 1/4W
R15 100k Metal film resistor, 1/4W
R16 220k Metal film resistor, 1/4W
R17 220k Metal film resistor, 1/4W
R18 820k Metal film resistor, 1/4W
R19 1M Metal film resistor, 1/4W
R20 47R Metal film resistor, 1/4W
RPD 2M2 Metal film resistor, 1/4W
LEDR 4k7 Metal film resistor, 1/4W
C1 150n Film capacitor, 7.2 x 2.5mm
C2 10n Film capacitor, 7.2 x 2.5mm
C3 4n7 Film capacitor, 7.2 x 2.5mm
C4 10uF Electrolytic capacitor, 5mm
C5 10uF Electrolytic capacitor, 5mm Modification; jumper for stock version. See build notes.
C6 1uF Film capacitor, 7.2 x 3.5mm
C7 330pF MLCC capacitor, NP0/C0G
C8 1uF Film capacitor, 7.2 x 3.5mm

CORVUS DISTORTION / SUSTAINER 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
C9 100n Film capacitor, 7.2 x 2.5mm See build notes.
C10 120n Film capacitor, 7.2 x 2.5mm See build notes.
C11 150n Film capacitor, 7.2 x 2.5mm
C12 10uF Electrolytic capacitor, 5mm
C13 10uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.
C14 100uF Electrolytic capacitor, 6.3mm Reference voltage filter capacitor.
C15 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
IC1 RC4558P Operational amplifier, DIP8
IC1-S DIP-8 socket IC socket, DIP-8
IC2 LM741 Operational amplifier, DIP8
IC2-S DIP-8 socket IC socket, DIP-8
SUST. 10kB 16mm right-angle PCB mount pot
TONE 10kB 16mm right-angle PCB mount pot
VOL. 100kA 16mm right-angle PCB mount pot
T. BYP. SPDT toggle Toggle switch, SPDT on-on
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.

CORVUS DISTORTION / SUSTAINER 5
BUILD NOTES
Gain stage modication
The original 1978 Big Muff used a gain setup similar to the original Big Muff: a fixed gain stage with a
volume control after it that attenuate the level before the clipping stage.
Since it uses an op-amp rather than a transistor, it’s better to make the gain itself variable as opposed
to running it full-up and then cutting back the level afterward. So, the Corvus includes a modification
suggested by Analogguru that accomplishes this. C4 is added as a DC blocking capacitor, and the Sustain
control is wired back to the inverting input of the op-amp to control the gain directly.
The stock version of the Corvus uses this modified gain stage, so if you build it according to the parts
list, it will have this improvement. However, if you’d like to build the stock version, you’ll want to do the
following:
• C5: jumper
• R7: omit (leave empty)
• R8: 47R

SCHEMATIC
CORVUS DISTORTION / SUSTAINER 6
GND
2M2
1N5817
100uF
GND
+9V
100n
VA
10kB
100kA
220k
150n
56k
RC4558P
RC4558P
LM741
220k
GND
10uF
GND
1M 820k
GND GND
330k
10k 47k
10n
560k
62k
47R
GND
4n7
10uF
GND
10uF
GND
1uF
8k2
GND
470k
330pF
10kB
1uF
100n
5k6
1k2
GND
120n
GND
150n
47k
47k
GND
100k
GND
VB VC
10uF
VA
GND
VB
VC
OMIT
47R
VA
GND
IN
OUT
RPD
D1
C14
C15
SUSTAIN
1
2
3
VOLUME
1
2
3
R17
C1
R1 2
3
1
IC1A
6
5
7
IC1B
IC2
5
1
2
3
6
8
74
R16
C12
R19 R18
R2
R3 R4
C2
R5
R6
R7
C3
C4
C5
C6
R9
R10
D2 D3 D4
D5D6D7
C7
TONE
1
2
3
C8
C9
R11
R12
C10
C11
R13
R14
R15
C13
2
3
1
TONE_BYP
84
R8
R20
GND

CORVUS DISTORTION / SUSTAINER 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: -0.85, y: +0.66
ø1/4”
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
BYPASS
TONE
TONE
SUSTAIN
FOOTSWITCHLED

CORVUS DISTORTION / SUSTAINER 8
ENCLOSURE LAYOUT
Enclosure is shown without jacks. See next page for jack layout and wiring.
125B

CORVUS DISTORTION / SUSTAINER 9
WIRING DIAGRAM
125B
IN +VGND N.C. N.C. OUT
PCB
IN
GND +V +V JACK GND JACK
OUTIN
GND GND PCB
OUT

CORVUS DISTORTION / SUSTAINER 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
1.0.1 (2021-04-20)
Corrected control labels on drill template.
1.0.0 (2020-11-16)
Initial release.