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. 

Using this module

Firstly, the transistor ladder will not work by itself. The diode ladder can be used as a stand-alone filter if desired. The audio signal to the diode ladder is fed to the transistor ladder as infra-red light by the photo-couplers. So in general use, just use the diode ladder inputs, for most pot settings you will be able to get filtered signals from all four outputs. The freq cut-off and Q of each filter can be modulated independently. So why have an input for the transistor ladder? If you insert an audio signal here it will have an FM effect on the audio signal coming in from the diode ladder, so consider it another modulation input. If the signal on the transistor input is of a similar frequency to the one on the diode filter input, you can get some pretty good filtered harmonics happening. The Q swap switch swaps the Q feedback paths of the 2 filters. The effect is quite brutal though can be tamed with careful tweaking of the Q pots. At some settings you will get chaos (unverified, the maths is beyond me) but the module will take on a life of it’s on. Sometimes you will hear the original audio signal, other times it will glitch away or no sound at all can be heard. This behaviour can of course be semi-controlled by CV.