Minimoog VCA cloning report

[>>>marjan<<<]

Hi Jorgen,

nice work,

> Maybe these resistors all could be increased by a factor of 1.5?

I think it'd change vca overdrive charachteristic?

> 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.

maybe it's up to a specific transistor used


> 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".

this is very interesting thing...

> 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 guess mini's envelopes have negative bias, look at the divider for the
first stage current
source transistor it's around -0.25V so you need less than 1V Vbe to
drive it in saturation
Maybe there should be some negative bias adder at the input to interface
it
with normal signals, or to change mentioned divider voltage value but I
suspect
it'd change the whole behaviour.
I'm maybe completely wrong here...

> 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?

I have vca scan from the original service manual and it's the first one.

> 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?

I put such protective diodes at the input of things, but here I think
it's used to protect
your output gear (amp) while powering up mini when some transients
happen.
Also there's an ext input, nobody can stop you putting 220V AC in there
:)


marjan