The Doepfer A119 module has a balanced mono external input. My MFB Tanzbar has a stereo output. Both are TRS connections (tip, ring, sleeve), but configured differently. So if you want to pull your dancing bear audio into your modular you either need two input modules or you need to find a way to sum the stereo channels into a mono channel. With a mixer in between this is no problem. Just take an aux output and plug that into the A119.

But what if you’d want to connect the drum machine, or any other stereo source for that matter, directly to the A119? This is where the different TRS connections come into play. The balanced mono input on the Doepfer expects the tip and the ring leads to carry a hot and a cold version of the same signal. The cold signal being the polarity-inverted version of the hot signal. The A119 then takes the difference of the hot and the cold signals to put it back together into one mono signal. This theoretically means that if you feed it a stereo signal with no inversion going on, everything that is panned exactly in the middle of the stereo image effectively gets cancelled.

Let me explain. Imagine the exact same sine wave on two channels. At any point in time, the difference between the two waves is zero. So all sound that is equally present in the left and right channels will not make it to the outputs of the A119. And it doesn’t. I tried. While it is in a way funny to hear nothing more than some faint toms and claps, and to see the theory actually holds up in practice, it’s not very functional to bring your massive sounding beat into your modular.
So I needed to find a way to sum the stereo output to a mono signal without the use of an external mixer. I quickly found out that:

1. You shouldn’t use an insert cable, aka stereo to mono splitter in reverse because of impedance issues.
2. Off-the-shelve summing cables do not exist.
3. Making a summing cable yourself is not that complicated.

I found this source and decided it was trustworthy: I took a TRS cable, got rid of the TRS connector on one end, soldered the resistors into place and wired up a TS jack. And lo and behold, it works. Off course this is not a balanced mono signal. But the Tanzbar outputs are not balanced anyway and the A119 handles an unbalanced mono signal just as well. So now I’m back to what it’s all about: filtering, overdriving, gating, delaying and bit crushing those beats in my modular.

Good luck building your own. Drop me a line if you need help.