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The DuoFonik is a dual SSM2044 VCF from Neutron Sound.  The SSM2044 is an integrated circuit designed for electronic music, for making 4-pole low-pass filters.  This module contains two of them, A and B, with features to enhance their use in tandem.

The PC board is small enough to fit many different modular synthesizer format panels.  This one is built with the Front Panel Express panel created by the designer, which features switches that allow the A pots to control the B filter in tandem with the A filter.  Unfortunately, there wasn’t room on the panel for signal input attenuators.

The panel design squishes the MOTM format, so that four pots can fit vertically above three jacks.  The board is affixed to the panel by 2-inch standoffs that fasten to the front panel using flush-mounted black screws (visible from the front).  Here’s a photo comparing it to a standard MOTM module (the 485).

Construction

The PC board is chock-a-block full.  Most of the resistors stand on end.  The bypass capacitors are those tiny surface mounts, soldered to the back of the board, that require the occasional expression of colorful language during soldering.  Here’s a photo of my assembled board.  Note the 10n polystyrene capacitors – Mouser #23PS310.  They are a tight fit.  Note that I chose normal resistors for R17 and R117 instead of tempco ones.

There is a circuit flaw on the REV 1 board that requires replacing C9 with a jumper.  I found out about it after I finished the board, so my C9 is installed.  Here’s a photo of my fix, including the addition of a C9 on the bottom of the board.  The red circle shows the shorting where C9 is originally installed, and the yellow cap above is the replacement.  Note that it goes between U2 pin 13 and the opposite end of R12, rather than to pin 13.  I did that to match the circuit at R112 and C109.  And it was convenient.

All connections to the panel are made via headers along one side of the board, making it a fairly simple affair to remove the board completely for repairs, if needed.  I made one change to the panel wiring scheme:  B IN is normally connected to the A IN by the jack switch lug.  That way whatever is patched to A IN also goes to B IN, if nothing’s patched there.

Calibration

Each filter has three trimpots:  scale (1V/octave), initial frequency, and self oscillation.  A seventh trimpot adjusts the balance of the two outputs into the on-board mixer.   Calibration was straight-forward.

1. I adjusted the self-oscillation trimpot, at a frequency of about 1 KHz, until the oscillation started around 75% rotation of the RESONANCE pot.  (The filter oscillates well at all frequencies.)
2. With the filter oscillating (high resonance setting), I adjusted the 1v/octave trimpot.  I found that the tracking was not wide range, so I chose a middle frequency and calibrated the 1V/octave input to jump from 400 Hz to 800 Hz when 1 volt was applied to the 1V/octave CV input.
3. I adjusted the initial frequency trimpot so that with the FREQUENCY pot fully anticlockwise the lowest frequency was 32 Hz.  The top end then fell between 15 and 16 KHz.  There was a little difference in the frequency range for the A and B filters.  Since the scale trim also impacts the FREQUENCY panel pot, it should be possible to get them both fairly close, if you take the time, but with the caveat that this interacts with 1V/octave.
4. The null trimpot adjustment is made by applying a 10V p-p triangle wave from an external VCO to both the A and B inputs.  Turn the FREQUENCY pot to the max and the RESONANCE pot to the min.  Switch the A input to INVERT.  The two signals should cancel each other.  Adjust the trimpot to get the smallest signal from the MIX OUT.

Sample Sound

I made this during the calibration procedure.  It’s a sawtooth wave, attenuated to about 5V p-p through the A filter.  I used the Binary Zone to make a stepped CV sequence, sending the output to the FREQ CV input, and the inverted CV output to the VCO FM input.  So when the VCO steps to a lower frequency, the VCF steps to a higher.  I twiddled the FREQ and RES controls during the recording.  This is not definitive!  See the Neutron site for the good demos.

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