MIDI-CV Converter

I recently finished a MIDI to Control Voltage converter I’ve been working on for awhile. Here’s a photo of it before I installed it in my synthesizer’s cabinet:


(That’s a flickr-hosted photo so you can click on it to find a larger version.) I thought I’d put this photo up not because it’s a stellar example of my DIY skillz–it isn’t–but because I was amused at how it inadvertently ended up being a little showcase of circuit-construction techniques. On the right we have a PC board I laid out and had fabricated by BatchPCB. You may recall having seen this PC board on my blog previously. If you look closely at the photo above you’ll see that the chip labeled “HDAC” has been removed from the board. That was the quad 16-bit digital-to-analog converter I was going to use for pitch CVs. I don’t know what happened but I burned up two of those chips (at about $25 a pop) before giving up. I don’t know whether I made a layout error, whether my power supply was bad, whether I had overlooked some key piece of information in the spec sheet, or whether I just plain had bad luck. In any case, after the Magic Smoke came out of the second chip I decided to try a different DAC.

That decision led to the small board in the center of the row. That’s a PC board I etched myself. On its underside is a quad 12-bit DAC which actually has better INL specs than the 16-bit unit has. It also has the bonus feature of not burning up when I apply power to it.

On the left of the row is the brains of the system. It’s an Atmel ATMega128 proto board from Futurlec, mounted on a piece of stripboard with inline sockets. Just to its left you can see the optoisolator for the physical MIDI interface and a JTAG header for writing the code. (No ISP for me–life’s too short to prototype without source-level debuggers.) An ATMega128 is overkill for this application but I had the proto board lying around already. It talks to the DACs over SPI and TWI via the jumper cables between the boards.

So, there you have it: stripboard, hand-etching, and commercial fabrication all in one project. In true lo-tek fashion they’re all hot-glued to a piece of stiff cardboard. This is the sort of thing that I can get away with because my modular synthesizer sits on a table and doesn’t get hauled around the country on tour, unlike some people’s synthesizers.

By adam

Go ahead, try to summarize yourself in a sentence or two.


  1. Very quiet. The S/N is exceptional. 🙂

    It works very well, as far as I’ve pushed it. My biggest concern was the timing, i.e. the ability to deliver gate signals without an objectionable amount of jitter. In the light testing I’ve done so far it has no audible jitter. Hopefully that will be true when I’m pushing data through all four channels. The firmware is currently set up so that it responds in a monophonic manner to MIDI channels 1-4 with velocity and modulation data mapped to the low-res DACs. If I find that it can’t deal with four channels of full data without getting sloppy I’ll have to tune the code. So far, though, I’m entirely happy with its performance.
    I did discover that my VCOs are tracking a bit sharp so I’ll have to recalibrate them. Apparently the commercial MIDI-CV interface I replaced with this one wasn’t as accurate.

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