Audio mixer

A new module today, an audio mixer. Yippee. Yeah, boring, but essential.

It’s intended for in-synth mixing of audio signals — not as an output module (that’s another story). This one, like the DC mixer, has three inputs, each with a slide attenuator, a general level control, and two outputs. Unlike the DC mixer it has AC coupled inputs to remove any DC level. Nor does it try to add a DC level, so there’s no offset control. For audio there’s not a lot of use for an inverted output, so both outputs are identical. That makes for a very simple circuit — one dual op amp, a few resistors and capacitors.

This one has an aluminum front panel. The PCB fabricator I use has just recently started offering aluminum as a substrate. Single sided only, which is no problem for front panels, really, but the holes for obvious reasons are not plated and specifying NPTH in KiCad introduces some complications. I came up with something that would work and had it made, and then I found out you actually can submit a design with plated through holes specified; they just will ignore the plating part. So you can use the same design you’d use for an FR4 panel. “Single sided” means there’s no copper or solder mask on the back, but they do silkscreen both sides. The back side looks like bare aluminum but has a non conducting layer on it. If you want to ground the aluminum you need to sand a bit of that off. Cost for single layer aluminum is the same as for double layer FR4.

I was surprised to find there didn’t seem to be much difference in stiffness. I did a crude experiment with putting a weight load on panels to see how much they sag and there wasn’t a significant difference. The aluminum surface is a little smoother than the FR4 so the appearance is a bit nicer, though the silkscreen is as low resolution as ever. Good enough for me.

Schematic, KiCad design files, Gerbers, documentation in GitHub repo.

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