AN-5: Virtual Analog (VA) Korg35 Lowpass Filter
by Will Pirkle
This App Note derives the equations for the Korg35 resonant LPF found on the Korg MS10, early MS20 and Monotron series. The block diagram is synthesized from the equations and the delay-less feedback loop is resolved. A nonlinear processing block is added to simulate the clipping circuit in the original; both the naive and budget versions of nonlinear processing are implemented so that you can experiment with your own variations. The filter performs as expected and self-oscillates nicely, especially with the nonlinear processing block engaged. The block diagram below is for the Korg35LPF with naive Nonlinear Processing added in the feedback loop.
UPDATE v3.0: I have updated the Korg35 LPF to streamline the project as well as explaining why self oscillation occurs at K = 2 (rather than K = 3 in the analog version). In v3.0, I derive the equations for the filter directly from the signal flow graph (rather than the conceptual block diagram) which results in one less filter block to implement. There is no difference in the transfer function, frequency response or sound of the updated filter.
AN-5 PDF File (v3.0)