Mobius PiLL

This is a chaotic noise module, it is generally not very nice unless you are into noise. In which case, hello!!

There are several papers describing ways to couple PLL (phase locked loop) ICs to create chaos. I have been sitting on a stack of them for years but found the actual circuits quite limited in a synth.

Eventually it clicked that the low-pass filter needed to be a bit fancier than a pot and a capacitor, so this version gets two Buchla style low pass gate/filters.

The circuit contains two PLL chips and two low pass filters. The PLLs are voltage controlled together, the filters are controlled individually.

Normally in a PLL circuit, the output of the phase detector goes thru a low pass stage and then the output of this is used to control the VCO. In this version, the low pass filter outputs are fed to the non-inverting input of an op amp for it's own VCO and the inverting input of an op amp for the other VCO. Maybe the pic describes it best, this is from

Intermittent Chaos

in a Mutually Coupled PLL’s

System (Shirahama et al), tho I have seen similar circuits in other papers.

It doesn't need an input signal to lock to, it can run on its own, but it can be fun to give it a signal anyway. The other inputs are for Freq CV, LPG1 CV and LPG2 CV. Two of the outputs are from the LPGs and two are from the PLL VCO, these two are pulse/squarewaves, very nice for feeding into filters.

The PLLs and filters are all vactrol controlled but the PCB is a unpatented NLC black-box design, so no need to buy any expensive vactrols.