Electric Druid FilterFX Construction Guide
www.electricdruid.net
Op-amps
Similarly, op-amp choice is not critical. Choose any 8-pin dual audio op-amp with the standard
pinout. TL072, LF353, or MC1458 will all work. Many more audiophile options are also possible!
Diodes
The power protection diode suggested is 1N5817. This diode is recommended because of its low
voltage drop at the sort of currents the pedal draws. Others will work but may reduce headroom
a little more.
The 1N4148 diode can be replaced with other small signal silicon diodes. 1N914 is a direct
replacement and can be considered identical.
Vactrols/optocouplers
The VTL5C3 vactrol optocouplers can be replaced with either other types of vactrol (which might
change response times and other behaviour - which could be good or bad, depending on your
objectives) or could be replaced with home-made opto’s made from and LDR and an LED stuck
together and wrapped with something to keep the light out (heatshrink or even electrical
insulation tape). R11 and R12 (both 120K) limit the dark resistance of the opto’s to something
reasonable, and you may find with other options that you need to change their values.
Ideas for potential upgrades or customizations
Adding CV inputs
Since the Electric Druid STOMPLFO chip operates using 0-5V control voltages like many other
Druid chips, it is possible to add CV control of the Offset(Frequency), Rate, Depth, or Waveform
controls.
Adding further expression pedal inputs
The circuit includes an expression pedal input to replace the
filter’s Frequency control, but a similar scheme can be used
for the other controls if required.
A normally-closed stereo/TRS jack socket should be wired
in, so the front panel control can be used when an
expression pedal is not inserted.
The typical expression pedal wiring is for the Sleeve to be
grounded, the Ring to carry the reference voltage, and the
CV return on the tip. The diagram shows how this wiring
relates to the PCB, the jack, and the pot.
However, this is not the only possible wiring,
and your expression pedal may not be this way!
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