Electric Druid FilterFX Construction Guide
www.electricdruid.net
Offboard components
Note that the BOM above doesn’t include offboard components. These are a matter of taste, but
the basics are listed below.
• Enclosure, PCB fits Hammond 1590BB or Eddystone 29830PSLA
• Mono 1/4”/6.35mm Input jack
• Mono 1/4”/6.35mm Output jack
• Optional stereo (TRS) 1/4”/6.35mm Expression pedal jack - must have switched contacts to
allow normal operation.
• Optional Sync input jack - The PCB allows space for a 1/4”/6.35mm mono jack, but you may
prefer a 3.5mm jack to allow easy synchronisation with a Eurorack synth, for example.
• Stomp switch, 3PDT for Bypass switching
• Stomp switch, SPST momentary, for Tap Tempo
• Power Input socket, 2.1mm. Sockets with an external nut are much easier, since you can wire
them and test the board without it in the enclosure.
• 4 x Knobs
Because of the current drain of this pedal, we don’t recommend using batteries instead of a power
adaptor. Ok,
maybe
, if you’ve got 9V rechargeables.
Component choices and substitutions
Very few of the components in the circuit are especially critical and a unit built with non-ideal
components will likely still work fine.
Resistors
In the interests of lowest noise, we recommend you use 1% metal film resistors.
Changing the LED resistors
Depending what LEDs you use, you may find the LEDs are too bright or not bright enough for
your taste. The Bypass LED is powered from the 9V supply with a 1K series resistor (R15, bottom
left below the 2M2). The LFO LED is powered from 5V with a 470R resistor (R16, furthest right of
the resistors between the switches).
Capacitors
Use good quality polypropylene or polyester film capacitors. The board allows either 0.2”/5mm or
0.3”/7.5mm lead spacing for the film capacitors. The two integrator capacitors C2 and C3 are
especially important, since they’re crucial to the filter’s operation. The circuit is unlikely to be able
to achieve self-oscillation at high resonance settings because of the differences in the optos, but
close matching between the filter caps will help increase the possibility.
Transistors
Transistor choice is not critical. Any medium gain, low noise NPN device will work. The board
expects a transistor with the EBC pinout, like the 2N3904. If you only have transistors with the
alternative CBE pinout like the BC547, you can fit the transistor back-to-front.
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