Benjolin filter resonance

[elmegil]

I have a PCB built from my own eagle files of the Benjolin schematic.

(for some details of what/why, see Muff's: https://www.muffwiggler.com/forum/topic-70928-0-125.html)

This is a prototype that, as I mention on Muff's, is not anything like what the final result of that project will be, but I figured:

1) I have the prototype boards, I should build one for myself
2) and be sure I understand the circuit better by doing so.

And yes, I've discussed the Muff's project with Rob, and that's why this particular prototype will never get fabbed again. Well, that and the mistakes... .


Anyway, I made a big stupid mistake when I was routing the PCB, and at one point I shuffled the op amps around to better fit the layout and forgot to flip them to preserve - input on top. So I have 7 op amps on my prototype that were backwards (+ to -, - to +).

Good enough, that's identified and fixed, lots of cuts & bodging but:

1) both oscillators work
2) xor & pwm work
3) filter cutoff works
4) rungler seems to work
5) T2 -> cutoff and rungler -> cutoff work

BUT

The resonance pot has zero effect.

I've gone back over component values, connections, etc, and I *believe* I don't have any more incorrectly routed traces.

Problem is, I'm not entirely following how U1.C (using Rob's designators) is supposed to behave, and how it regulates the resonance.

It actually seems to behave as though the resonance is, if not at max, very high. I get the characteristic LPF sound that you get when the resonance is high and you sweep the cutoff, where you distinctly hear the harmonics cutting in or out depending on the direction of the sweep. But looking at the output of U1.C shows very little if any activity there, and there is definitely no change in the response of the filter with adjustments to the resonance pot.

Anyone have:

1) description of that specific section of the circuit?
2) suggestions as to what might be wrong? I have double checked resistor values and connections particularly for the 510R -> 560p -> ground sections.


Thanks....

[elmegil]

Follow up:

I was making it too complicated (who knew!)

In making cuts to correct other PCB mistakes, I inadvertently cut the trace from U1.C output to the header for the resonance pot....

[soundworker]

I build serveral Benjolin last years for me and friends. It is a nice chaos generator and sounds good. Normally both VCO going to a comperator and it is not possible to get a "clean" VCO out.
I found an easy way to make it to a little synth with both VCOs on extra outs. Now ist is only chaotic if you like ...
Also have a frontdesigner file for frontpanel.

See www.klangbaukoeln.de