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Electronics => Beginners => Topic started by: workbench on June 09, 2019, 01:01:06 am

Title: Need help understanding a malfunction. VC Resonance Circuit of a VCF
Post by: workbench on June 09, 2019, 01:01:06 am
I made a PCB for a diode ladder filter.
The template was the YuSynth EMS clone.
My problem now is that the filter is permanently oscillated as if the resonance pot were fully turned on,
even if resonance is fully turned to the left.

The voltage at the gate terminal of the N channel j-fets is in the correct range between -0.1V and -2.4V.

The oscillation stops when I supply a negative voltage at RES-CV input.
While the voltage is applied, the Resonace potentiometer behaves normally.
I have already tried to reduce R31 to 220k without success.

Can someone tell me where the problem lies?

I attach the complete schematic in the appendix

(https://i.ibb.co/xhBLkPg/res-pot-error.png)
Title: Re: Need help understanding a malfunction. VC Resonance Circuit of a VCF
Post by: Audioguru on June 09, 2019, 01:34:08 am
Your opamp has no power supply so it will do nothing.
The datasheet for the 2N3819 Jfet shows that when the gate voltage is only -2.4V then some of them will not turn off. All of them turn off when the gate voltage is -8V.
Title: Re: Need help understanding a malfunction. VC Resonance Circuit of a VCF
Post by: workbench on June 09, 2019, 02:08:28 am
The supply for the opamp can be seen in the PDF. it is connected to + 12V and -12V.