NLC1025 Double Penetration 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. 

more documentation: nlc dp filter documents

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