Strange twins

Last month I built a dual VCA module.

100 mm wide dual VCA module

If this looks familiar it’s because it’s not the first Kosmo format MFOS dual VCA I’ve made. I did the first about two years ago. But it wasn’t quite the same:

75 mm wide dual VCA module

That one was built around the commercially available MFOS circuit board, and was 75 mm wide, the first 75 mm module I ever made.

I wanted another pair of VCAs and I liked the MFOS circuit. But the MFOS PCB was designed for Ray Wilson’s own synth format, with no power header — just solder pads for power and ground — and no board mounted panel components. That meant a lot of wiring, and a little auxiliary circuit board for the Eurorack style power header. This time around I decided I’d do my own PCB layout to avoid all that.

I also decided to make it 100 mm wide. You can fit four 1/4″ jacks across 75 mm but only just barely. If board mounted, the PCB has to be just about 75 mm wide with no margin for keeping away from adjacent modules. That spacing also means the patch cable plugs are crowded together in a way I don’t like, making it hard to plug and unplug one without dislodging others. I never did a four jacks across 75 mm board again. I’ve standardized on 25 mm spacing, and a four jacks across module is (at least) 100 mm wide.

So I designed the new PCB, and I made a mistake. I got the (board mounted) pots switched around on the PCB, with the CV attenuators on the top and the offsets on the bottom. That wasn’t what I intended and I could have redone the PCBs, but since I didn’t discover the problem until I’d soldered it up, I decided just rearranging the front panel to match would be easier.

I could have left the original module alone. The tight jack spacing wasn’t really causing that much trouble. But I felt the standardized spacing was better, and the thought of having the CV attenuators on top in one module and the offsets on top in the other bothered me.

So I pulled out the old module, unmounted all the panel components, lengthened some wires as needed to get them to reach further, and put everything behind another copy of the new panel.

(I also added a normal connection from the output of the first VCA to the input of the second. That’s designed into the new PCB; it’s a wire on the old module.)

Now the two modules are twins. From the front. Look behind, and they’re not quite identical.

Two 100 mm wide dual VCA modules, front and back