OBG VCF TLA OMG (part 1)

Having learned something about KiCad, I started thinking about putting it to use. I’ve got thoughts of a couple of modules I’d kind of like, both of which I’d have to breadboard and debug before finalizing the PCBs and building them.

So I thought, meanwhile, maybe I should work on a module using an existing circuit, no breadboarding required.

I headed over to the late Ray Wilson’s Music From Outer Space and found a likely candidate: This state variable VCF from the “MFOS Oldies But Goodies” list. (Not to be confused with another, later, more complex state variable VCF design in the main modules list for which you can get a Eurorack version from SynthCube.) That page shows the schematic and also a PCB layout. Of course it’s not a Eurorack design. It runs on ±12V and uses Eurorack compatible signals, but there’s no Eurorack power header — just pads to solder wires connecting to +12V, -12V, and ground. As for the front panel, you’re on your own. Wilson used his own preferred format, and mounted hardware to the panel and connected it to the PCB via wires. I’m not sure how he connected the PCBs and panels mechanically. What’s common in Eurorack is to have some or all the panel components mounted on the PCB, eliminating the need to wire them and providing a mechanical connection.

I’d address those concerns later. First I just replicated Wilson’s schematic in KiCad. Then I more or less replicated his PCB layout. So far so good.

Adding the Eurorack power header was pretty easy. I also put in diodes to protect against power reversal.

Next, dealing with panel hardware. There were five jacks (signal and CV in; LP, BP, and HP out) and two pots (frequency and resonance). I modified the PCB layout to include all of those along one edge, facing horizontally. That would mean the PCB would be mounted perpendicular to the panel, which could be quite narrow.

I stared at the design until I decided I wasn’t going to find anything else wrong with it, then uploaded it to JLCPCB and ordered five boards (for $6.40 plus postage). And then of course I found something wrong with it. I’d placed the pots incorrectly, too far from the edge of the board. Well, that could be worked around, just mount the pots to the panel and wire them to the PCB. (Meanwhile I fixed the positioning in KiCad.)

But the more I thought about that front panel the more I hated it. It would have five jacks and two knobs all in one vertical column. They’d be so close together, even the Davies clone knobs would be too large for comfort.

A lot of Eurorack users like narrow, 2HP or 3HP or 4HP panels. You can cram a lot into a case with narrow modules. Me, not so much. I like to get a clear, uncluttered view of things, and I like to not have to squeeze my hand into confined spaces to change something. Yes. I like big knobs and I cannot lie.

(My favorite knob so far: the ten-position octave switch on the Befaco Even VCO. It’s big and it clicks and it’s beautiful.)

So I went back to the drawing board KiCad for a fresh start. I threw out Wilson’s PCB layout and worked up my own. I decided to go with a sandwich design: Vertical jacks and pots on one board, parallel to the panel, with a header to plug the second board, also parallel and containing most of the circuitry, into. Had I made a greater effort to squeeze things I might have been able to get everything into a 50 mm wide board (with THT parts; SMD could be narrower) but I gave myself more breathing room with something closer to 60 mm design. Wide, yeah, but that’s okay. I even took the opportunity to add a second signal input as well as three more pots, as signal and CV attenuators. The jacks/controls board is about 56 mm by 91 mm and the main board is about 51 mm by 72 mm, both under the 100 mm by 100 mm limit for the lowest prices.

I stared at those boards, decided I wasn’t going to find any more problems, and uploaded them — this time to PCBWay. I ordered ten of each. Five would’ve been plenty, ten was wasteful I suppose, but it was the same price: $10 plus postage, minus a $5 coupon.

Then the front panel. I’d designed something, 12HP wide, before designing the PCBs, but how to produce it? I experimented a bit with toner transfer onto aluminum with some success, but not a lot. I decided to go ahead and try making a PCB panel. Back into KiCad. I was up all night (for the first time in, what, decades?) and didn’t at the time succeed in creating slotted mounting holes that JLCPCB’s software was happy with. Finally I just went with round mounting holes and sent it to JLCPCB. Eurorack panels are longer than 100 mm, so cost more than $5, and in the case of PCBWay, a good deal more. But at JLCPCB I got five for $6.70.

(I did figure out slotted mounting holes later, I think.)

So that’s that. I’ll be getting PCBs from JLCPCB (they’ve shipped), which I’ll probably admire and then toss out, and more PCBs from PCBWay, at least one of each of which I’ll use, and panels from JLCPCB, one of which I’ll use. Assuming all goes well. And assuming I get the parts, but that’s another tale…

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