Minimoog clone VCF

The core of this filter is almost exactly the same circuit as in the Minimoog. Only a couple of minor changes had to be made, to be able to run it on +-15 volts, instead of +-10 volts. These changes were developed by René Schmitz, and you can find the schematics on his web site.
To get the best performance, I matched all transistors and capacitors in the transistor ladder. Of course, the transistors for the differential amplifier were also matched.

I added an opamp to get the output amplitude up to my standard of 10 volts peak to peak. Another opamp vas used as an input adder. The input impedance of the original circuit (10k) was a little low for my taste. I also added opamps to make the modulation amount knobs 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.

This filter really sounds sweet. It has the classic Minomoog sound, and noise is quite low. I don't have genuine Minimoog to compare, but maybe my filter is a little quieter thanks to the BC560 low noise transistors.

Circuit board layout (PDF-file)

Component placement (PDF-file)

This is how the Minimoog VCF module looks from the back. Note the written numbers on the transistors. This is measurements for the matching.

The resonance control (labeled "emphasis") uses a reverse taper potentiometer on the Minimoog. These are quite difficult to obtain. I tried with a linear pot, but the function was not at all satisfactory. The solution was to use a normal logarithmic taper pot, with gears to change the direction of rotation.

The assembly containing the reversed emphasis pot and the TM pot. The gears are 40 teeth, module 0,3.

This picture shows the pot assebmly installed. Note the compact construction.