AN-6: Virtual Analog (VA) Diode Ladder Filter

by Will Pirkle

This App Note derives the equations for the VA Diode Ladder Filter. The Diode Ladder Filter was originally designed for the EMS VCS3 synth but is most famous as the resonant filter from the Roland TB-303 BassLine monophonic synth. The complete filter equations are derived and a block diagram is synthesized. Actual filter measurements have excellent agreement with the theoretical frequency response. Both linear and “budget” nonlinear versions are presented and implemented. The filter performs well across the spectrum and self-oscillates as expected without becoming unstable. The plot below is the measured output of the Plug-In for various values of feedback k.


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