How to “upgrade” your Behringer Model D to a duophonic synth

The Behringer Model D is a great analog synth – and by design it’s monophonic. However, if you’ve seen my channel before, you know I’m not going to let that stop me from trying to play more than one note at a time with it, something that has a unique characteristic in synthesizers that are intended to be monophonic.

You can’t control any of its 3 oscillators separately, but there’s a fourth oscillator that can be controlled independently, and that’s the filter – via a dedicated CV input – and when you crank up emphasis all the way, it resonates and creates its own tone. The nice thing about the D, is that there’s resonance both in high pass and in low pass modes, which opens up a lot of sonic possibilities.

This does require a little configuration trick. When playing duo-phonically, the keyboard sends two notes – so we need a way to tell the D’s oscillators to play one, and the filter to play the other.

In the particular case of the Arturia Keystep, which is what I use in this clip, that’s achieved by setting note priority on the D to LOW, and CV out priority on the Keystep to HIGH. CV then controls the high note, and MIDI the low one. This trick lets you play two notes at once on the D.

The final piece in the puzzle is that the Behringer Model D’s CV tracking on the filter doesn’t quite track at 1 volt per octave, but it’s close enough. If you’re playing the filter notes in a one octave range, depending on your sensitivity, that might not be noticeable, but it does become apparent across a few octaves. The way to solve this is to interject a module like Befaco’s A*B+C, which, with a bit of fiddling back and forth, can produce a result which tunes quite nicely. Since I don’t have the module, I used the techniques shown in my “hybrid modular” video to use a virtual version of that module in VCV Rack.

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