Vulcan Drive (version 1.
0)
Sandbox Distortion
The Vulcan Drive originally started out as a Ross Distortion project. However, this circuit is so similar to the DOD250
and the MXR Distortion Plus I have made the changes to the PCB and bill of materials below to allow for these circuits
to be built around this PCB. Adding a few SIL sockets you could create a distortion that is a combination of all 3 and
assist you in learning what effect these component changes have in this simple and common op-amp circuit.
1590a
The board spacing will fit a 1590a enclosure. You can mount 9mm pots directly to the board.
Bill of Materials
Ross Distortion Variant
Resistors Capacitors
R1 4.7k C1 1nf
R2 10k C2 10nf
R3 1 Meg C3 47nf
R4 1 Meg C4 10nf
R5 1 Meg C5 1uf
R6 1 Meg C6 1uf
R7 10k C7 1nf
R8 1 Meg C8 Empty
Diode IC
D1 1n4001 IC1 RC4558
D2 1n270 (any ge diode)
D3 1n270 (any ge diode) Potentiometer
D4 Jumper Volume 50ka
Drive 500kc
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Distortion Plus Variant
Resistors Capacitors
R1 4.7k C1 1nf
R2 10k C2 10nf
R3 1 Meg C3 47nf
R4 1 Meg C4 10nf
R5 1 Meg C5 1uf
R6 1 Meg C6 1uf
R7 10k C7 1nf
R8 2.2 Meg C8 10pf
Diode IC
D1 1n4001 IC1 RC4558
D2 1n270 (any ge diode)
D3 1n270 (any ge diode) Potentiometer
D4 1n270 (any ge diode) Volume 10ka
Drive 500kc
DOD 250 Overdrive Variant
Resistors Capacitors
R1 4.7k C1 jumper
R2 10k C2 10nf
R3 1 Meg C3 4.7nf
R4 470k C4 10nf
R5 22k C5 10uf
R6 22k C6 4.7uf
R7 10k C7 1nf
R8 2.2 Meg C8 22pf
Diode IC
D1 1n4001 IC1 RC4558
D2 1n4148(any si diode)
D3 1n4148(any si diode) Potentiometer
D4 1n4148(any si diode) Volume 100k Log
Gain 500k Log
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Modification Ideas & Circuit Notes
Ross Distortion / Distortion Plus “2 in 1”
Compare the components in the bill of materials for these 2 circuits and you will find that they are almost
identical. In fact the pull down resistor and volume control will have very little effect on the output of this
circuit, these 2 component differences can be ignored. This leaves only 2 components, the ceramic
capacitor, and extra ge diode on the distortion plus. These could be added to switches for an ‘on the fly’ 2
in 1 circuit selector. You will experience a volume drop with the distortion plus as you add the extra ge
diode to the circuit (essentially, more output signal to ground).
Diodes
D3 and D4 are connected in series. If you are only going to use one diode in this position a jumper is
required for the empty slot. Experiment with the diodes in these slots and the quantity. The capacitor C7
drops highs to ground. Experiment with different values for this capacitor or swap it out for another diode
slot for additional clipping.
Potentiometers
The “original” potentiometer values and taper used in these circuits are inconsistent. If any of the above values are
hard to find, you should be fine using a 500k or 1M linear for gain and 100k log for volume in all 3 circuits.
Gain Stage Output Cap
Capacitor C6 was originally a tantalum which is a polarized capacitor. As this capacitor is in the signal path
it can be replaced with a film cap for the Distortion Plus and the Ross Distortion.
Dual Op-Amp
This pcb layout is essentially true to the original (including the gain pot). However the Ross Distortion uses an
RC4558 (dual op-amp) whereas the Distortion Plus and DOD250 use the single op-amp LM741. To ‘standardize’ I
have opted to use a dual op-amp pinout for this layout and terminating the 2nd, unused op-amp. There are a slew of
dual op-amps with the same pinout as the RC4558 that you can use in its place so don’t forget to socket the IC!
Changing the IC will have a subtle effect on the resulting tone of the circuit.
The following are a handful of possible substitutes for the RC4558:
RC4559 NE5532 LM833 LM1458
TL072 LF353 NTE928M OP275
OPA2604 OPA2111 OPA2132 OPA2134
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Boxing up your effect
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Watch my offboard wiring tutorial for information on wiring this effect up inside an enclosure (with LED,
stompswitch, etc)
http://www.youtube.com/watch?v=z6fpwU8RY_0
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