Help modifying SSI2164 based Filter

DIY STUFF
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Hello,

Been snooping the LMNC forums for a long time, and I have learned lots of great DIY stuff here. Using the knowledge gained I’ve built a few cool synth projects too.

Currently, I’m building a “white noise” machine that can sweep between LP and BP filters and want to use the SSI2164 chip as the filter core. The device will run on a dual ±5V supply. I’m borrowing the filter core schematic from MI Ripples/Shruthi & SSI “Designing Voltage Controlled Filters for
Synthesizers with the SSI2164” document.

I haven’t been able to find a version of a 2164 core-based 4-pole LP filter without resonance. For my application resonance isn’t required so I’d like to omit it.

Can I simply remove the OTA or OPamp where the resonance taps into the last filter stage and feeds back to the input? That’s what I’ve done in the linked schematic image and replaced it with a 33kresistor. Also shown is the resonance part of the circuit I removed. I breadboarded this up but so far no dice, just a high-pitched oscillation.

I know the MI Ripples euro rack module will run on ±5v since I can run the one I built using that power source, and MI Shruthi runs on ±5v so I’m ruling that out as a potential issue.

Before I start pulling my hair out over what’s wrong I figured I should probably ask if this filter core schematic would even function as-is. Much of the design has been stripped away.

Thanks!


link to the schematic image if it’s too small to see

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yes, Q is just a feedback path from the output back to the input through an inverting opamp I think, so omitting it should be fine.

the older version of the datasheet has some more basic VCF building blocks (single poles etc) which aren’t in the newer ones, I’ve still got a copy on my site here:

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To either clarify or confuse, I’m not sure which, I think the answer is to get rid of the feedback path entirely.

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Got it, and thanks for the link to that datasheet. So removing the entire Q feedback trace is probably the way to go.

Appreciate the super fast feedback!

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