Double Penetration (DP) Filter

Everybody loves the Moog ladder filter, right? TB-303 etc.....
And everyone loves the EMS diode ladder, the Pink Floyd characteristic synth sound, yeh?

So what happens when we mate these two filters?
No, I don't mean what are the offspring like
Lets just see them in coitus!

This circuit replaces the diodes and transistors of the ladders with opto-isolators.
The interesting effect is that the transistor filter does not need an audio input,
The audio from the diode ladder is sent across as infra-red light and turns on the transistors.

There are outputs for each filter, plus summed output and a difference output.
The difference output is quite unique, never heard anything like it.

There is a switch to swap the resonance feedback paths between the filters
which tends to make thing go a bit nuts.

See pdf for close-up view of the schematic pictured below.
Component values are not given as this is still in proto-type stage and some values WILL change.
Once fully fettled, PCBs and full schematic will be available.

schematic (large view - 1st draft)


The PCB is 1.8 x 3.0", pots spacing 1". Will suit Eurorack or 4U systems