VCF, grounded

Last May Day I wrote:

I don’t think the VCF is working as intended, though. It’s definitely not working the way I expect. I thought it was better when I built it but right now, no. It does do some sort of low pass filtering, the filter cutoff changes with the knob, and the resonance knob does something. But the resonance knob does nothing evident until it starts distorting, and the cutoff knob doesn’t seem to do much except at the lower end…

I finally talked myself into investigating. I’ve read that some filters do better if you attenuate the input signal somewhat, so I tried that and it did seem to result in slightly better behavior, but still not good. On closer examination it seemed the main problem was that a lot of unfiltered signal was bleeding through into the output.

I couldn’t find any assembly problems so I ended up emailing Michael Barton, who suggested some things to check, and one of them was it: I’d completely forgotten to ground the tip switch on the HP input jack. Once I fixed that most of the bleed through went away. There’s still a tiny bit getting through but I can live with it.

The other thing that went away when I grounded the tip switch was self oscillation at high resonance. But previously I’d thought self oscillation kicked in too early and so, following advice in the build guide, I’d replaced one of the resistors with a lower value. I restored that resistor to its specified value and self oscillation came back at a reasonably high resonance setting.

I’m happy. The problematic behavior earlier prompted me to decide to put a second VCF into Kosmodrome. Now that’s not so urgent, but I’ll do it anyway.

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