DP Filter

Moog / EMS filter

NLC Double Penetration (DP) Filter

Everybody loves the Moog ladder filter, right? Minimoog, 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, not ‘what are the offspring like’.
Let’s just see them in coitus!

This circuit replaces the diodes and transistors of the ladders with opto-isolators, which are components that transfer signals from diodes to transistors as infra-red light.
The interesting effect is that the transistor filter does not need an audio input and is dependent on the diode ladder to operate. This is an analogue filter containing no micro-processors.

There are outputs for each filter, plus a summed output and a difference output.

There is a switch to swap the resonance feedback paths between the filters which can make things go a bit nuts. Depending on settings and CV inputs, you can get evolving chaotic patterns, glitch or semi-normal behaviour.

Very interesting timbres and harmonics can be obtained by patching the transistor ladder, summed or difference outputs back into one of the diode ladder inputs.

Controls

Each filter has pots for Q (resonance), frequency cut-off and to set the level of the CV1 input.

As mentioned, the switch swaps the resonance feedback paths or leaves them as normal.

Sockets

The diode ladder has two signal inputs, two CV inputs and an output.

The transistor ladder obtains its audio signal from the diode ladder so does not have its own signal input. It does have an FM input, two CV inputs and an output.

There is also a summed output and a difference output where the signals from each section are added or together and the difference between them is available.

https://www.nonlinearcircuits.com/modules/p/double-penetration-dp-filter?rq=DP%20filter