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VCO OUT > VCF IN mod strange behavior
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VCO OUT > VCF IN mod strange behavior

by sebiiksbcs on Tue Nov 10, 2015 2:03 am

Hi all, hope this forum is still active?

I did the VCO out / VCF in mod exactly as described here: http://www.ladyada.net/wiki/x0x/vcfmods (Updated Mod for VCO out/VCF in) and now the following happens:

If I connect an external synth, the external signal just doesn't seem to fully "get in there" and I hear a weird distorted version of the signal, and the x0x filter cutoff just kinda warbles and barbles (and only if I turn the resonance fully CW). But if I simply send the VCO out signal back to the VCF in via the mod, everything works fine.

Now this: If I run any external signal (including the x0xb0x' own VCO) through a guitar pedal (EHX Holy Stain) and then into the x0x filter input, everything works fine. I have the feeling it might have to do with input and output impedances? (The Holy Stain's output impedance is 570 ohms, not sure about the other synths' impedances)

I experimented with different values for the cap (anything from 100nf to 100uF) and resistor (4.7k to 47k) that run from the input jack tip to the VCF input but get very similar results. I am currently going with a 10uF cap and 22k resistor.

Does anyone have an idea what could be happening here? Thanks all

sebiiksbcs
 
Posts: 5
Joined: Mon Oct 26, 2015 5:34 am

Re: VCO OUT > VCF IN mod strange behavior

by sebiiksbcs on Sun Apr 10, 2016 3:58 am

In case anyone else is having this problem: Soldering a 100k resistor in series after the capacitor that takes the signal back into the filter (to R70/Q12) seems to have fixed it.

sebiiksbcs
 
Posts: 5
Joined: Mon Oct 26, 2015 5:34 am

Re: VCO OUT > VCF IN mod strange behavior

by sebiiksbcs on Thu Aug 04, 2016 1:51 am

Turns out the mod does work with some devices and doesn't with others. Impedance problem? Total n00b in this area.

For example if I connect my Alesis Micron to the x0x VCF input it just doesn't seem to push any sound through except some crackles. And the same crackles at both the line outputs and headphones. The Micron's line outputs are balanced by the way, is that a factor? (I only feed the hot signal and ground into the x0x though)

sebiiksbcs
 
Posts: 5
Joined: Mon Oct 26, 2015 5:34 am

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