MIDImplant is described as the smallest MIDI to Control Voltage (MIDI2CV) converter ever made. As long as your vintage synthesizer has volt/oct response (in contrary to Hz/volt) you can use this little board to add MIDI to it.

It’s calibrated at 1V/oct, but any ratio is possible, from 0.3V/oct to 2V/oct. Hz/V response will be available too, as one of the features of new firmware being under development now. You only need to drill a hole for MIDI DIN socket, and connect a few wires inside your synthesizer. Basic soldering skills are needed as well as a bit of knowledge about your synth. Its service manual, or at least schematic is most helpful.

Features:

• Up to 61 keyboard range for standard 1V/oct ratio
• Fast response – valid CV and Gate voltages appear 0.3ms after MIDI transmission ends.
• Voltages are generated with precision better than 2 cents (2/100 of a semitone) at 1V/oct ratio
• User selectable Volt/octave ratio with resolution of less than 1mV/oct, easily adjusted with use of MIDI keyboard – no trimming screwdriver needed
• Standard version works with V-trigger type of gating signal. Gate ON level is equal to the power supply voltage delivered to MIDImplant. For S-trigger compatible synths, order MIDImplant-S.
• Each output of MIDImplant can be easily configured by the user to generate voltage representation of MIDI signals like:
  • MIDI notes with velocity. One input represents pitch, the other one velocity, while GATE1 controls the note duration.
  • MIDI notes on 2 different channels. Each Gate output works together with paired CV output.
  • MIDI pitchbend with resolution of more than 4000 steps
  • Any of 128 available MIDI Continuous Controller, e.g. volume, modulation, etc.
  • Channel Aftertouch, and Polyphonic Aftertouch
  • MIDI Program Change – voltage level represents Program number received via MIDI
• Both inputs can work independently, that means one can be notes, the other – velocity. Or both can represent pitch of notes on different channels. Or one can be Pitchbender, and the other one – modulation. Combinations are endless.
• Each input can work on user defined MIDI channel, but if MIDI OMNI ON mode is selected, both outputs respond to MIDI signals on any channel
• Both CV outputs can generate voltages in range between 0V and 5V
• dimmensions: 15x36mm (0.6”x1.4”) without MIDI socket 
Details are available at the MIDImplant site.