Extending VCF frequency range

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Extending VCF frequency range

Post by crossword »

The range of the frequency knob on the 9700 VCF front panel is very limited. I notice that the low end is limited by a resistor between the pot and ground, and the high end is presumably limited by being connected to ground instead of V+.

Is there a reason for this?

Secondly, can I safely short that resistor and connect the top of the Freq pot to V+ to make the pot cover the full range of the filter? Any reason this would not work or be a bad idea?
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Re: Extending VCF frequency range

Post by PAiA-Scott »

I believe the range of the panel control for Frequency on the 9730 VCF is reduced the way it is so it is more like a fine adjust, with gross control via the normally-connected modulator, or external control inputs.

Try connecting the output of a modulator, set to cycle, back to the gate input to hold it 'on' and the control for the amount of modulator to the VCF Frequency can then be used as a knob to manually sweep the filter over a wider range -- I don't have a P9700S set-up to try this right now and it's been a while since I last tried it, but it might have to be the VCA ADSR modulator for the latching action. One of the 9744 CV Source modules can be used for this panel controlled cv.

Or yes, the circuit could be changed too with the potentiometer for the Frequency voltage having more than 0v connected at the clockwise end of the rotation. A direct connection to V+ would provide the most possible range and resistors from this cw terminal to this V+ positive dc supply would reduce the range.

Remember for best filtering action, attenuate the VCO waves using VCA L or R level control/mix before running through the VCF where increased resonance or Q settings can result in more gain than is attainable and uncontrolled oscillation.
Scott Lee
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