Minimoog VCA cloning report

[jorgen.bergfors at idg.se]

>nice work,

Thanks!

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

>I think it'd change vca overdrive charachteristic?

Yes, maybe. But the voltage between each resistor would remain the same.
I suspect that changing the supply from 10 to 15 volt does more to the overdrive characteristics!

>> 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.

More likely the changed supply voltage, I think.

>> 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'll check if it starts to let signal though at exactly 0 volts or maybe some small negative voltage.

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

Do the service manual say anything about voltage levels at different points in the circuit?

/Jorgen