I built the next utility module of the three I bought last month, the AI Synthesis AI002 Mixer. This was nowhere near as simple as the MiniAtt but much simpler than the Looping ADSR (let alone the Even VCO) and didn’t take much time at all.

When I plugged in to test I noticed a couple of things that seemed wrong. First, the gain with the Mix Out knob at the 12:00 position was much less than the unity the documentation claimed. With Mix Out close to fully clockwise it got up to about 1.4x gain, but then the sawtooth I was putting in started to distort and, as I turned the knob those last few degrees, it turned into a square wave, the amplitude nose dived, and at full clockwise the output signal disappeared entirely.

I got in touch with Abe of AI synthesis and dug up the schematic, and discovered I’d been working off an out of date build guide. (There’s an HTML version and a PDF version, and the PDF was obsolete.) In the up to date version is a recommended modification, clipping one leg of the Mix Out pot. That completely fixed the strange behavior near the pot limit. It didn’t fix the gain, in fact the 1.4x gain I’d had before was now lower and it was about unity at full clockwise. But Abe told me the documentation was wrong, and maximum gain of unity was correct.

He apologized if that wasn’t what I was expecting, but I told him it was no problem. I just was concerned that there might be a problem causing the gain to be too low. There’s no problem, the gain is what it should be, and I’m okay with that. I put the front panel on and it’s in my rack now.

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