Multiple multiples

The multiples module is the only module I’ve planned for Kosmodrome that’s designed by someone else in Kosmo format. Everything else is more or less based upon something designed for Eurorack or other format and konverted to Kosmo by me.

But this is the Buffered and Passive Multiples by Cory Torpin, a.k.a. CTorp, of the LMNC Discourse group. The circuit’s based on a design by Dave Jones, and Cory did the PCB layout and a front panel. (Incidentally, the horstronic. Buffered Multiple I have in my Eurorack row is also based on Jones’s design.) There are two of the Jones buffered multiples, each with three outputs, as well as two sets of four jacks wired as passive multiples. Cory sent me a board and panel set some time ago in a trade, and I finally got around to building it.

I’ve made a decision, though, that I want the Kosmodrome modules to be visually more or less uniform. Not a crazy quilt of different colors, styles, and fonts. So partly for that reason, while I did use Cory’s panel, I made an overlay for it in my own style.

The other reason for the overlay is… Cory’s panel isn’t actually consistent with the PCB. Originally the intent was for the inputs to the buffered multiples to be at the bottom, but some things got switched around on the PCB so the right one’s input ended up on the top, but then the panel shows the left one’s input on the top instead.

The module doesn’t work at all with an input plugged in there! Took me quite a while to diagnose the problem. Anyway, I corrected it in the overlay.

There are trim pots to adjust the gain to unity. They’re supposed to be 2kΩ but my lying spreadsheet lied to me and said I had nine 2k trimmers when I only had five, so I ended up using a 10k for one of them.

Here’s how I did the calibration: I used my expression pedal interface output as a voltage source, so I could vary it with my foot while using my hands to adjust the trim pots. Instead of measuring the input voltage and then trying to match the output voltage, I measured the voltage difference between the input and the output. With 0V input you get a small (under about 10 mV) difference which is fairly insensitive to the trim pot setting, due to op amp offsets. So I moved to a large voltage input and adjusted the trim pot until I saw about the same difference value. At that point the voltage difference doesn’t vary when you vary the input. That’s the point where you have unity gain, with a little offset.

If you want to make one of these you can grab the Gerbers for the PCBs and panel from Cory’s Github:

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