Minimoog VCA cloning report

[jorgen.bergfors at idg.se]

Hi all.
During the weekend, I fired up my Minimoog VCA clone. It worked on the first try, so the circuit board layout was obviously OK. I run it on 15 instead of 10 volts, but that seems to be no problem. Currennt consumption from the positive supply is 32 mA. That might be a little high for a VCA, but acceptable. There is a chain of resistors between positive supply and ground that is only 1020 ohms in total. This accounts for much of the power consumption.
Maybe these resistors all could be increased by a factor of 1.5?
I did change one resistor though. The 62 ohm resistor at the output stage was increased to 82 ohms. Otherwise it would not clip symmetrically.
It took some experimenting to find out how this VCA actually works:
If both CVs are "maxed out", there is very little distorsion or rounding of the waveform. Instead it clips rather abruptly when the output amplitude reaches 8V p-p.
But if you turn down the VCs, especially the second one, there is a lot of rounding. It then can turn a triangle into a very good sine.
So I connected a stereo pot and a couple of resistors in a way that I can reduce the second CV and increase the input signal at the same time. This way, the output amplitude stays virtually the same, but the distorsion changes. I'm gonna add this pot to my front panel and label it "drive".
The senitivity of the CV inputs is way too high, by the way. It is around 1 volt for full effect. Beyond that, you just don't get more gain.
I wonder what CV level standard the Minimoog uses. For 10V CVs I had to change the CV input resistors to 1M  (still not quite enough) and 560k respectively. They were originally 68k and 33k.
I definitely would connect the envelope to the second CV input instead of the first. That gives a lot less noise. I wonder if Moog really connected it to the first CV, like in Marjan's schematics?
All in all an interesting circuit, that can act both as a VCA and a distorsion unit!

/Jorgen

P.S. can anybody explain what the diodes at the output are good for? The conduct if the signal goes above the positive supply or below ground. At this point the signal is biased to 6V. But how could it ever go above the positive supply?