AN-7: Virtual Analog (VA) Korg35 Highpass Filter
by Will Pirkle
This App Note derives the equations for the Korg35 resonant HPF found on the Korg MS10 and early MS20. The block diagram is synthesized from the equations and the delay-less feedback loop is resolved. The interesting 6dB/octave behavior in this 2nd Order filter is also revealed and emulated. 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 Korg35 HPF with naive Nonlinear Processing added in the feedback loop.
UPDATE: I have updated the Korg35 HPF model with a newer and greatly simplified version that eliminates three filtering blocks and has more simplified equations and implementation. The frequency response and transfer functions are identical to the original. The new version also simplifies the nonlinear processing.