Paia 4700 vcfs affect vco pitch why?

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bloodyhell
Learning to Wiggle
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Paia 4700 vcfs affect vco pitch why?

Post by bloodyhell » Fri Jul 19, 2019 9:46 pm

So the symptom is that if I patch any of the vco's into any of the filters, the pitch drops slightly (but irritatingly noticeably) near the high end of the cutoff. Granted that my whole system came with a single 2720 type power supply to power some 12 modules, could that be the problem? Or is that an issue within the filter's design, since this happens with any of the two filters involved? Or is it an entirely different bug? Your 2.cents are highly appreciated.

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diophantine
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Post by diophantine » Fri Jul 19, 2019 10:07 pm

Can you test by unplugging all modules from the power supply except for one VCF and one VCO and see if the problem still persists between those two modules?

bloodyhell
Learning to Wiggle
Posts: 36
Joined: Sun Dec 01, 2013 3:05 am

Post by bloodyhell » Sat Jul 20, 2019 6:33 pm

Took out all but one vco and one vcf. The modules were attached to the power bus with alligator clips, yikes..

I connected both modules as close to the beginning of the supply bus as possible, and started testing as follows:

Patch:
VCO (saw) > VCF input, VCF output (lp) > headphones
Bias 0v-+5v > VCO cv, Bias -5v-+5v > VCF cv

Started testing with vcf, cutoff threshold, level and Q set to low and then slowly increased the cutoff. the pitch started to be slightly affected with the cutoff close to the high end.

Then I set Q to high - pitch is affected.
Switched cutoff threshold to high - pitch is affected.
Switched out the vco - pitch is affected.
Switched out vcf - pitch is affected.
Disconnected the 4720 vco and connected the vco from my 2720 to the vcf - pitch is NOT affected.

Conclusion: The pitch seems to be always affected when the 4720 vco is connected to the 4730 filter and the cutoff reaches the high end, independent from the Q setting. The problem must be caused somewhere within the 4700 system between the 4720 vco and the 4730 vcf, or in the bus rail where both are connected. I don't know where to go from there beyond that.

PS: Could there be some kind of crosstalk going on between the cutoff signal and the vco cv? Did anybody else deal with this kind of problem and what did resolve it?

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