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.
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