Minimoog clone VCF + VCA mk II (AMORE)

The prototype version of the board. There are a few differences compared to the final board.

This AMORE module combines clones of the Minimoog VCF and VCA on one board. The core circuits are the same as on the Minimoog clone VCF and VCA mk I. The main difference is that I managed to fit the circuits on a half size board in this version. Apart from that, there are two electrical differences: This version uses a simple VCA to achieve voltage controlled resonance. On the mark 1, on the other hand, this is done by means of a Vactrol type optocoupler. Secondly, the bypass feature on the mk I has been omitted in this version.
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.

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. This filter really sounds sweet. It has the classic Minimoog 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 BC550C low noise transistors.

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 foot pedal. I reversed this, as having the EG on the second input gives much better signal to noise ratio. The foot pedal input is intended for use with type of amplitude modulation except envelopes in my module.
To get the best performance, I matched all transistor pairs. I also used polyester 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 is not a very high fidelity VCA, but the noise level is quite low. There is some distortion, but not more than most other good VCAs, I suppose. There is also some CV feed through 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.

Example waveforms

Here are some examples with the filter fed a 1 kHz sawtooth from the Dual VCO and the filter cutoff set to roughly 4 kHz. Other ratios between oscillator frequency and filter cutoff frequency will of course produce different waveforms.

This is the effect of varying the cutoff frequency. As you can see, the amplitude is reduced when the cutoff frequency is lowered.

The effect of increasing resonance. As you can see, the amplitude gets lower when the resonance is increased.

The resonance turned up to self oscillation. The first picture is with input signal (same as above examples) and the other are with no signal – just the filter oscillation.

Overdriving the filter's input results in soft clipping.

Building the module

Bill of materials

You should have access to the parts in the general bill of materials.
In addition, you need the following less common parts:

5.62 kohm 0.1% resistors (2)
20 ohm trimmer (1)
10 µF non-polar electrolytic capacitor (1)

Matching

This module needs matching of transistors, using the Bergfotron transistor matcher. See circuit diagram for information on which transistor pairs need matching.
There might be some advantage to also matching the four 22n capacitors.

Trimming

There are two trimmers, marked balance 1 and 2, for nulling the DC offset of the VCA. Measure before the DC blocking capacitor at the output and adjust the trimmers so there is minimal change in the DC voltage when you sweep both CVs.
The VCF has three trimmers. ”Regen cal” adjusts the maximum resonance. Adjust it so that the filter just starts to oscillate at some fairly low cutoff frequency. This adjustment is done according to personal taste. "Scale" sets the volts per octave scaling of the filter. Set the filter to self oscillation and feed connector pin 1 with exactly 1, 2, 3, 4 volts and measure the frequency. Trim until you get a doubling of the frequency for each volt's increase. "Range" adjusts the cutoff frequency and can be set to taste, so the cutoff knob on the front panel covers the range you want. 

Skill level required: MEDIUM

The board is rather densely populated. Otherwise there are no particular difficulties and the circuit uses no hard to find parts.

Circuit board layout

Component placement

VCF schematics

VCA schematics