Modeling the Korg35 Lowpass and Highpass Filters

Pirkle W. Modeling the Korg35 Lowpass and Highpass Filters. In: AES Convention 135. Audio Engineering Society; 2013. Paper 103. Available from: https://aes.org/publications/elibrary-page/?id=16938

@inproceedings{Pirkle2013_16938,
  author = {Pirkle, Will},
  title = {{Modeling the Korg35 Lowpass and Highpass Filters}},
  booktitle = {AES Convention 135},
  note = {Paper 103},
  year = {2013},
  month = oct,
  publisher = {Audio Engineering Society},
  url = {https://aes.org/publications/elibrary-page/?id=16938}
}

TY  - CPAPER
TI  - Modeling the Korg35 Lowpass and Highpass Filters
AU  - Pirkle, Will
T2  - AES Convention 135
M1  - Paper 103
PY  - 2013
DA  - 2013/10/06
UR  - https://aes.org/publications/elibrary-page/?id=16938
PB  - Audio Engineering Society
LA  - en
AB  - The Korg35 Filter is a voltage controlled Sallen-Key topology capable of producing both lowpass (LPF) and highpass (HPF) filter responses. It is known for its ability to self-oscillate as well as its saturated or distorted output as the damping factor of the filter approaches zero where self-oscillation occurs. Both LPF and HPF are second order but the highpass version features a 6 dB/octave roll off instead of the usual 12 dB/octave giving it a distinctive sound with more bass response. The analog Sallen-Key topology uses a delay-less positive feedback loop to implement the Q control of the filter. The saturation circuit is also inside this loop. Rather than use a typical biquad or state variable filter, we choose to use Virtual Analog (VA) filters [1] as building blocks to replicate the analog filter topology directly from its signal flow graph, including the delay-less loop as well as the saturation circuit. Both loaded and unloaded (lossy) versions of the Sallen-Key topology were designed and implemented in C++ with the point of self-oscillation exactly matching both analog transfer functions. Sample code and extra documentation are available at www.willpirkle.com.
ER  -