Minimoog clone VCA
This is the VCA circuit from the
Minimoog, with a couple of additions.
The Minimoog VCA is using only discrete transistors as active
components. It has two cascaded stages that can be voltage
controlled individually. After that there is a discrete buffer
amplifier.
In the Minimoog, the first stage is controlled by the envelope
generator and the second stage is controlled by the footpedal. I
reversed this, as having the EG on the second input gives much
better signal to noise ratio. The footpedal input is used for
amplitude modulation in my module.
I added an opamp to get the output amplitude up to my standard of
10 volts peak to peak. Originally, the filter could output around
5 volts peak to peak, without excessive distortion. So a 2X
amplification was enough.
Another opamp was used as an input adder. After the opamp, I
connected a stereo pot and some resistors. The other variable
resistor of the stereo pot was connected so that it reduces the
amplitude of the EG signal as the input signal is increased. This
enables me to overdrive the input stage, without changing the
output amplitude. The knob for this pot is labelled
"drive".
I also added an opamp to make the amplitude modulation amount
knob bipolar. So by turning the knob to the left you get negative
modulation and to the right positive.
Finally I added my signal indicator circuit.
To get the best performance, I
matched all transistor pairs. I also used Polycarbonate caps in
the signal path. There are no electrolytic caps in the signal
path in my version of the VCA. The Minimoog has electrolytic
output DC blocking caps.
I changed some of the resistor values so that the VCA would run
as well as possible on +-15 volts supply.
This VCA is essential to get that classic Minimoog sound. It's not just the filter, as many people think. With the drive knob, I can get an even larger palette of sounds, than a genuine Minimoog. It can convert a triangle wave to an almost perfect sine. If you turn it even further, you get soft clipping. The sonic effect of this depends on the input signal. It seems to be the most dramatic when you set the filter to very high resonance.
This is not a very high fidelity
VCA, but the noise level is quite low. There is some distortion
even at low drive settings, but not more than most other good
VCAs, I suppose. There is also some CV feedthrough that causes a
slight click when very short attack times are used. This is
normally masked by the input signal. Maybe it even adds to the
punchiness of the sound.
I have trimmed the offset to get no DC shift before the DC
blocking capacitor. But there is some AC-coupling of the CV
signals anyway.
My circuit board for the Minimoog
VCA consists of four VCAs on a single 150x100 mm board. The
signal indicators are on a separate board, that fits underneath
the main board.
The four VCAs on the module are designed to work in pairs, for
stereo effects. If you want to create a stereo effect form a mono
signal, you can gang two VCAs with a flip of a toggle switch.
This connects all four inputs to both VCAs. So you can mix up to
four signals and then add for exemple autopanning on them.
Circuit board layout (PDF-file)
Component placement (PDF-file)
Schematics (GIF-file)
This is how the Minimoog VCA module looks from the back. Note the written numbers on the transistors. This is measurements for the matching.
The module seen from above...
...and from below.
