aion ZVEX MEGALITH User manual

MEGALITH DUAL DRIVE 1
PROJECT NAME
MEGALITH
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
ZVEX®Box of Rock & Distortron
A two-channel drive composed of cascaded MOSFET boost stages. The drive channel and boost channel
can be combined or selected independently.
Amp-like overdrive / distortion 1.0.0 (2021-11-12)
BUILD DIFFICULTY
Intermediate
Actual size is 2.3” x 2.14” (main board) and 2.3” x 1.28” (bypass board).

MEGALITH DUAL DRIVE 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 Megalith Dual Drive is a hybrid of the ZVEX Box of Rock and Distortron, two similar pedals
composed of cascaded SHO boost stages.
The Box of Rock came first in 2006. In addition to the three cascaded stages, it also includes a fourth
SHO stage at the end for switchable volume boost. The Distortron followed in 2009 as part of the lower-
cost “Vextron” series (which also included the Mastotron). The Distortron omits the boost, but adds two
toggle switches for gain (low or high) and “subs” or bass (3 different settings).
By combining the Distortron’s switches with the Box of Rock’s extra boost stage, we can have the best of
both worlds. The Megalith is a touch-sensitive low- to medium-gain pedal that overdrives naturally and
responds very well to the guitar’s volume knob.
The Box of Rock was designed as a 2-channel effect, with the SHO as the “always on” clean foundation to
which the drive channel can be added (as seen in Zvex’s original demo video). The boost comes after the
drive when both are engaged. This is a reasonable way of using it, but it won’t work with every rig.
Because of this, the Megalith adds an internal slide switch allowing the order to be changed so that the
boost comes before the drive. In this mode, the boost stacks with the drive and adds to the gain rather
than just boosting the volume, as in the default ordering.
Note that since the gain stages are identical to individual SHO circuits, the two gain controls (the main
circuit gain and the boost level) exhibit the characteristic “crackle” of the SHO when they are adjusted,
since they are directly biasing the MOSFETs. This is normal.
USAGE
The Megalith has four knobs, two toggles, and one internal slide switch:
• Gain controls the amount of gain in the first stage of the drive section.
• Tone is a Big Muff-style “balance” control that pans between treble and bass emphasis.
• Drive Level sets the overall output of the drive signal.
• Boost Level controls the gain of the boost stage.
• Gain (toggle) selects between three different gain modes.
• Sub (toggle) selects between three different bass cutoff frequencies after the third drive stage.
• Order (internal slide switch) changes the order so that drive comes before boost (default operation)
or boost comes before drive.

MEGALITH DUAL DRIVE 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 1M Metal film resistor, 1/4W
R2 1M Metal film resistor, 1/4W
R3 5k1 Metal film resistor, 1/4W
R4 470k Metal film resistor, 1/4W
R5 1M Metal film resistor, 1/4W
R6 1M Metal film resistor, 1/4W
R7 180R Metal film resistor, 1/4W
R8 5k1 Metal film resistor, 1/4W
R9 1M Metal film resistor, 1/4W
R10 1M Metal film resistor, 1/4W
R11 5k1 Metal film resistor, 1/4W
R12 330R Metal film resistor, 1/4W
R13 390R Metal film resistor, 1/4W
R14 47k Metal film resistor, 1/4W
R15 82k Metal film resistor, 1/4W
R16 9k1 Metal film resistor, 1/4W 10k in original. See build notes.
R17 9k1 Metal film resistor, 1/4W 10k in original. See build notes.
R18 1M Metal film resistor, 1/4W
R19 1M Metal film resistor, 1/4W
R20 1M Metal film resistor, 1/4W
R21 5k1 Metal film resistor, 1/4W
R22 47k Metal film resistor, 1/4W
R23 82R Metal film resistor, 1/4W
RPD 1M Metal film resistor, 1/4W
LEDR 4k7 Metal film resistor, 1/4W LED current-limiting resistor. Adjust value to change LED brightness.
LEDB 4k7 Metal film resistor, 1/4W LED current-limiting resistor. Adjust value to change LED brightness.
C1 100n Film capacitor, 7.2 x 2.5mm
C2 22n Film capacitor, 7.2 x 2.5mm
C3 470pF MLCC capacitor, NP0/C0G
C4 22n Film capacitor, 7.2 x 2.5mm
C5 56n Film capacitor, 7.2 x 2.5mm
C6 47n Film capacitor, 7.2 x 2.5mm

MEGALITH DUAL DRIVE 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
C7 470n Film capacitor, 7.2 x 3.5mm Original uses 820n for C7. If you can find one, omit C8 and C9. See build
notes for more info.
C8 330n Film capacitor, 7.2 x 3mm
C9 22n Film capacitor, 7.2 x 2.5mm
C10 22n Film capacitor, 7.2 x 2.5mm
C11 100n Film capacitor, 7.2 x 2.5mm
C12 2n2 Film capacitor, 7.2 x 2.5mm 2n in original. See build notes.
C13 2n2 Film capacitor, 7.2 x 2.5mm 2n in original. See build notes.
C14 100n Film capacitor, 7.2 x 2.5mm
C15 1uF Film capacitor, 7.2 x 3.5mm
C16 10uF Electrolytic capacitor, 5mm
C17 100uF Electrolytic capacitor, 6.3mm
C18 100n MLCC capacitor, X7R
D1 1N5817 Schottky diode, DO-41
D2 1N5239B Zener diode, 9.1V, DO-35
D3 1N5239B Zener diode, 9.1V, DO-35
Q1 BS170 MOSFET, N-channel, TO-92 Can substitute 2N7000 (rotate 180 degrees)
Q2 BS170 MOSFET, N-channel, TO-92 Can substitute 2N7000 (rotate 180 degrees)
Q3 BS170 MOSFET, N-channel, TO-92 Can substitute 2N7000 (rotate 180 degrees)
Q4 BS170 MOSFET, N-channel, TO-92 Can substitute 2N7000 (rotate 180 degrees)
ORDER 4PDT slide Slide switch, 4PDT E-Switch EG4208
TONE 100kB 16mm right-angle PCB mount pot
GAIN 5kC 16mm right-angle PCB mount pot
D. LVL 100kB 16mm right-angle PCB mount pot
B. LVL 5kC 16mm right-angle PCB mount pot
HIGAIN SPDT cntr off Toggle switch, SPDT on-off-on
SUB SPDT cntr off Toggle switch, SPDT on-off-on
D.LED 5mm LED, 5mm, red diffused
B.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.
D.FSW 3PDT Stomp switch, 3PDT
B.FSW 3PDT Stomp switch, 3PDT
ENC 125B Enclosure, die-cast aluminum Can also use a Hammond 1590N1.

MEGALITH DUAL DRIVE 5
BUILD NOTES
C7 capacitor
C7 is an 820n capacitor in the original. While these are still produced, they are only available from
a small number of manufacturers and may be hard to find. Presumably ZVEX chose this value after
experimentation since 1uF is a lot more common.
The Megalith PCB adds footprints for three capacitors in parallel (C7, C8 and C9) so you can mix and
match values to add up to 820n. The easiest combination is 470n/330n/22n (822n total), and this is
what is used in the parts list as the default. However, you could also use other combinations such as
three 270n capacitors or 470n/220n/120n, both of which add up to 810n.
2-pole filter values
Between the tone control and drive level, there is 2-pole filter formed by R16/C12 and R17/C13. In the
original Box of Rock, the two resistors are 10k and the two capacitors are 2n.
2n is no longer a standard value of capacitor, typically only available in green mylar “chiclet” types. If 2n2
capacitors are used for C12 and C13, then the resistors need to be 9k1 to preserve the same frequency
response. This is the default arrangement in the parts list.
If 2n capacitors are used for C12/C13, then R16/17 should instead be the original 10k.
MOSFET substitution
The BS170 and 2N7000 are nearly identical in specifications, but the pinout is reversed between the
two. If you have 2N7000 handy, they’ll sound the same in this circuit, but they need to be rotated 180
degrees from the silkscreen which assumes BS170.

MEGALITH DUAL DRIVE 6
SCHEMATIC
GND
1M
100kB
5kC
100kB
1M
1M
BS170
1N5239
GND
5k1
100n
VA
22n
GND
470k
470pF
1M
1M
BS170
GND
5k1
VA
22n
180R
GND
1M
1M
BS170
GND
5k1
VA
390R
56n
47k
22n
100n
82k
GND
GND
9k1
2n2
9k1
2n2
GND GND
GND
GND
470n47n
330R
GND
330n 22n
IN
DRIVE
RPD
DRIVE LEVEL
1
2
3
GAIN
1
2
3
TONE
1
2
3
R1
R2
Q1
D2
R3
C1
C2
R4
C3
R5
R6
Q2
R8
C4
R7
R9
R10
Q3
R11
R13
C5
R14
C10
C11
R15
R16
C12
R17
C13
HI GAIN
2
3 1C7C6
SUB
2
3
1
R12
C8 C9
GND
1M
5kC
1M
1M
BS170
1N5239
GND
5k1
100n
VA
GND
1uF
47k
GND
10uF
B
BOOST IN
OOST OUT
R18
BOOST LEVEL
1
2
3
R19
R20
Q4
D3
R21
C14
C15
R22
C16
TO
DRIVE
IN
TO
OUTPUT
JACK
FROM
INPUT
JACK
TO
BOOST
IN
FROM
BOOST
OUT
FROM
MAIN
OUT
4k7
5MM
1N5817
100uF
GND
+9V
100n
VA
82R
4k7
5MM
+9V
LEDR
DRIVE LED
D1
C17
A2
A3
A1
DRIVE A
B2
B3 B1
DRIVE B
C2
C3
C1
DRIVE C
C18
R23
LEDB
BOOST LED
A2
A3
A1
BOOST A
B2
B3 B1
BOOST B
C2
C3
C1
BOOST C
SW1A
2
3
1
SW1B
5
6
4
SW1C
8
9
7
GND
GND
GND
DRIVE
OUT

MEGALITH DUAL DRIVE 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.
ø3/8” ø1/2”
0.385”
0.625” 0.625”
ø3/8”
OUT DC IN
125B
LEVEL
DRIVE
LEVEL
BOOST
GAIN
TONE
SUB
HI GAIN
BOOST DRIVE
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: -0.765, y: -1.675
ø15/32”
x: -0.765, y: -1.675
ø15/32”
x: -0.765, y: -1.10
ø5mm
x: +0.765, y: -1.10
ø5mm
CENTER (0,0)
x: 0, y: +0.41
ø1/4”
x: 0, y: +0.41, ø1/4”
0 1 2
CM
0 1
INCH

MEGALITH DUAL DRIVE 8
ENCLOSURE LAYOUT
Enclosure is shown without jacks. See next page for jack layout and wiring.
125B

MEGALITH DUAL DRIVE 9
WIRING DIAGRAM
125B
D.IN +VGND D.OUT B.IN B.OUT
DRV
IN
GND +V +V JACK GND JACK
OUTIN
DRV BOOST BOOST
OUTOUT IN

MEGALITH DUAL DRIVE 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 (2021-11-12)
Initial release.