Mikrophonics

My latest orders from PCBWay arrived yesterday! Printed circuit boards and front panels.

This may have been an odd choice for my first Kosmo module, but it’s not like I didn’t have a decent set of bread and butter modules already. Which is what’s available so far from LMNC, so if I wanted something more offbeat I knew I’d have to design or adapt it myself. And since I’m still learning KiCad and getting used to Kosmo a simple module seemed a good idea. I’ve been interested in Music Thing Modular‘s Mikrophonie for a while now, and it filled the bill.

Mikrophonie, which is named after a pair of compositions by Karlheinz Stockhausen, is simply a piezo pickup and preamplifier mounted behind a front panel. You can scratch on the front panel, tap it, thump the synth case, and so on, to produce audio signals. There’s also an input jack so you can feed it sounds from an external pickup.

I could have bought a Mikrophonie PCB and used that. You can’t buy the board alone, only in combination with the front panel, but that would have been okay. However, it’s out of stock at Modular Addict, SynthCube apparently doesn’t carry it, and Thonk is not shipping modules at this time due to the pandemic. But Music Thing Modular’s designs are open source hardware. (Several companies have marketed their own modules based upon Mikrophonie.) So I decided to download the schematic and make my own PCB.

I copied the schematic in KiCad with almost no changes — just used Schottky diodes in series on the power buses instead of switching diodes to ground, because I like that better. Even though it’s a proven circuit I did throw it together on a solderless breadboard. It was messy but it worked.

Then I laid out a PCB. Time to pick a name for the project, and I couldn’t resist calling it Mikrokosmos — after the collection of piano pieces by Bela Bartok, not the song by BTS. I could have just reproduced the Eurorack PCB, moving some of the panel hardware off-board, but I chose to create a version tailored to Kosmo.

I also designed a front panel, initially in Inkscape and imported into KiCad. Inspired by Music Thing’s panel, I left an area on the front unmasked with a complicated design in the copper which I hoped would be good for scratching on. The hole for the standoff screw became the epicenter of the copper design.

I had both fabricated by PCBWay. I was very pleased with the results, really pleased with how the front panel came out.

The circuit’s very simple: One dual op amp, six resistors, four diodes, four capacitors, a pot, two jacks, a power header, a piezo pickup. The board went together quickly, maybe too quickly, because I initially mounted the pot on the wrong side. I had to cut it off and put another on the back. Fortunately I did have a spare.

I used a double stick adhesive strip from Scotch to attach the piezo to the back of the panel. If it falls off tomorrow I’ll try something else. It’s holding for now.

Plugged it in, it worked! The scratch area produces nice results. With the gain up I started getting mild feedback — maybe could have gotten less mild if I’d pushed it, but I didn’t. I haven’t tested the external input.

So, success! Unfortunately I don’t have a case to mount it in yet. Home Depot‘s open but I’m not interested in going there until after the pandemic’s subsided.

My PCB and front panel designs are available on my Github.

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