Music and Synth DIY

Eurorack, Schematics, Synth DIY, Through hole

MIDIMUSO CV12 Eurorack conversion panel/PCB (finished)

MIDIMUSO CV12 Eurorack conversion panel/PCB, designed in KiCad.

The MIDIMUSO CV-12 and ORAC PCB is an excellent and inexpensive MIDI to CV converter. I’ve used the chip in several projects, especially as a MIDI to trigger converter for switching circuits.

I’ve had a breadboarded Eurorack conversion of the ORAC PCB in my rack for a while, and it was due a proper replacement.

All I’ve done here is make a PCB with pin sockets lined up with the three 2×4 2.54mm pin header ports on the ORAC board, and connected them to three rows of six Thonkiconn sockets, and added a Eurorack power header and filtering, Type A & B TRS Midi in, and a 14 HP panel.

There are three versions of the CV12 PCB – I’ve designed this around the latest board, version 1.2. I don’t think the port positions have moved since version 1.1, so it should be OK for the previous board, but I haven’t tested/measured it.

I’ve read reports of the V/oct tracking not scaling properly if the power drops below the required 12v – adding a protection diode in series would cause a voltage drop, so I’m using a keyed header for reverse polarity protection.

Boards ordered today from JLCPCB and waiting for a test build. I’ll post the Gerbers and KiCad files here once I’ve confirmed it works.

Update #1

Got the boards back from JLCPCB, and built them along with a fresh MIDIMUSO CV12 PCB.

Panel, power input board, and main board

The 5 pin DIN midi socket and the 12v DC power socket need to be left unplaced. The 7805 and the LM317 voltage regulators, and the 5k multi-turn trimmer, need to be placed on the rear of the board.

The male double headers for the ports on the ORAC PCB connect with female headers on my conversion board, and the power filter board sits on top with m3 spacers.

Completed module

There are four flying wires: +12v & GND, and MIDI current source/sink. Bit of a compromise but hey ho.

As the MIDIMUSO chip has several different modes, from 11 MIDI to trigger outputs to 4 note polyphonic MIDI to CV, it didn’t make sense to put anything on the panel other than the port groupings and outputs: the modes and their outputs are detailed in the manual.

I made this purely for my own use, but I’ll post the gerbers, IBOM, KiCad files and a small build guide in case anyone wants one, or wants to build on it/modify it – once I’ve had a chance to fully test it. So far, it seems to be working perfectly.

NB: I’ve put TRS MIDI INs for both type A and type B – there’s no protection against plugging both in at the same time, so don’t do that.

I tested this briefly with an Arturia Beatstep – this is neither MIDI TRS type A or type B, but will work if you plug a normal mono 3.5mm cable into the TRS B socket.

Update #2

OK, this looks like it’s all working properly. If you want to build one, just upload the gerber files for the two boards and the panel to JLCPCB or the like. Use at your discretion, while I’ve tried my best to make sure there are no errors, I’m not making any guarantees. If you find any problems, drop me a an email –

Feel free to remix, rehash, adapt, build, resell, whatever…

Update #3

Although this worked OK with digital modules with quantised V/oct inputs, analog VCOs produced a fluctuating modulated pitch, which looks like op amp oscillation. Updated the board to include resistors after the op amps’ feedback loop which seems to have corrected it. I can also see about 40mv of a 62k frequency AC wave riding on the DC, which I presume is the first order filter not quite getting all of the PIC’s PWM smoothed, but doesn’t seem to affect it.

Here are the Gerbers, KiCad files, IBOM and build guide. – please note I haven’t had the updated boards back to test yet – I’ll update this when I have.

CV12 eurorack conversion by Keith Gentry is licensed under CC BY 4.0