Most of the mods I found here:I was wondering about the Jürgen Haible mod for VCO-to-VCF. Do you have a link to the description or some instructions or schematics, please?
The instructions regarding the VCO-to-VCF mod is rather thin:
The original info regarding Jürgen Haible´s Oberheim mod is here:
I couldn´t use the mod directly transfered to the PolySix. Only on my third attempt did it work, and in the end I used the one of the VCA´s of Bergfotron:
I built 6 x VCAs based on the LM13600, but didn´t use the output transistors in the chip, as explained here, but used TL074 as buffers. Also, I only build one exponential converter, and it could easily drive all 6 VCAs.
In addition I put in switches so I could turn each VCO-to-VCF effect on and off, and build a pot as a voltage divider to give the circiut the required control.
The last (or primary!) part is to know where to hook up this extra circuit to the Polysix analog circuit board: the signal output of each VCO and the control input summer of each VCF.