Oakley Sound Quad SV-VCF Panel
Four Oakley Sound VCFs on another 19" blank panel. The stickers at the right give an approximate calibration for the knob positions, for the CV1 track, CV2 track, frequency, and Q. Unconventionally, I fix the knob pointers so that they point to the 12 O'Clock position when they are fully anti-clockwise. That way the knobs are easier to line-up, and I can use the approximate clock positions 0(12) to 10 with 300 degree rotation pots. This saves attempting to print and fit a scale for each control.
Audio Input 1, Audio Input 2, CV1 and CV1 inputs to VCF1 are applied to all other inputs via the switching sockets unless overridden by inserting a jack in the subsequent modules. The 1V/octave pitch CV input does not gang in this way, as the pitch CV connects to a manual control pot giving a fixed frequency capability. This is calibrated for A440 at setting 5, with roughly one octave per division. The On-board frequency pot then gives about +/-1 octave for fine fixed control when required. The 1V/octave pitch CV input overrides the manual frequency control. For flat Butterworth 12dB/octave HP or LP filtering the Q must be set to about 0.71 - the square root of 2 which is somewhere between setting 1 and 2 on the knobs. 24dB/octave filtering can by achieved by spliiting the 1V/octave pitch control voltage and feeding into two VCFs, cascading either HP and LP sections as desired. But for a flat response the Q of each filter then has to be set to Q1=0.54 and Q2=1.3, settings 0 and 5 on these filters. In fact the Q only seems to go down to 0.66 at setting 0, so that is as close as we can get on the low Q section. It is best to have the low Q section first in the chain, as this reduces the energy fed to the higher Q section which is more likely to clip if driven hard. These are version 1 VCFs and use CA3280s transconductance amps. The VCFs will only just about self oscillate. Newer versions use the more easily obtained LM13700s and I think have a mod to make them a bit more excitable at maximum Q setting.