Intro:
    Basically this was just a side project for something bigger (I'm not telling what... you'll see, just wait ;) ). It was greatly influenced by PAiA's MIDI2CV8, I just wanted to improve it and scale it down (I was successful, of course ;) ).
    I had quite a lot of troubles getting some good components for this thing (since I live in such small country without any Maxim or AD distributer): 12 bit DAC with serial interface was best I could find for reasonable price (I mean, damn, I don't want to spend 60$ on some cute Burr-Brown 16 bit DAC), and I couldn't find any better multiplexer than 4051 (this is the "weak link") - it has too large leakage current to keep the output ripple reasonably low - I'm stuck with about 3mV; however my Korg MS-10 (Hz/V input) seems to be quite happy with it even on lower notes (where relative error should reach over 5%).

Important: Before version 2.0 circuit had a major error which I didn't notice, because I was running my prototype from battery pack, which had too low voltage to make a circuit malfunction. Anyway, I was driving the multiplexer directly from the uPC pin, while the MUX's supply voltage is much higher
than 5V. MUX is of course CMOS which should be driven rail-to-rail, otherwise it just acts as a resistor. That was the case here. I sincerely apologize for such basic error!
 

Features in software version v2.0

Control outputs:
    - Pitch (64 semitones V/octave or Hz/V , changeable via SysEx message);
          note offset changeable from 0 to 63 via SysEx
    - Gate (separate normal [0V -> +5V] and S-trigger [floating -> GND] outputs)
    - Velocity [-5V -> +5V]
    - four consequential Continuous Controllers (separate [-5V -> +5V] and S-trigger);
          numbers from 0..3 to 31..34, changeable via SysEx
    - Pitch bend (separate [-5V -> +5V] and S-trigger)

Hz/V scaling is done in software via look-up table and features cca. five cent non-linearity error over 5 octaves (tested with MS-10 and tuner in Boss SE-70 effect)

Following features were added for controlling Korg Mono/Poly:

    - Sync tick output (S-trigger, tempo selectable via SysEx)
    - Portamento switch (S-trigger, controlled with CC #65)

All channel refresh period (worst,fictional case): 380 us
Output ripple (specified, but not tested): cca. 3mV

Zip file with schematics, PCB layout, documentation, and firmware (v2.0)
 

Slave board    New!

Feature: mixed pitch/pitchbend CV output with 4 different pitchbend ratios (selectable via SysEx)

Zip file with schematics, PCB layout, and partlist for slave board