EFM RXCV Economy Midi 2 CV converter
The RXCV was the 2nd module i built. It's now succeeded by the EFM 3503 midiport.

The circuit is designed to be powered by a central modular supply of either +/-12V or +/-15V. Actually it refused to give 1V/Oct until i used +/-15V. The datasheet says the LM324 only goes up to +V - 1.5V. That would mean it can just about reach 10V on a 12V supply, but it's very close to its maximum at that point, which usually suggests it's not linear anymore.
There is an on board +5V regulator for the PIC, DAC, inserter and opto coupler. Midi data is read by the PIC. The PIC interprets the data and switches it's outputs to comply with the midi messages. It will respond to pitch, pitch bend, modulation, key down and more that one key down messages all others are ignored. Pitch and pitch bend control voltage outputs are obtained by switching a dual 7528 DAC. Gate, Glide (Slide), and Mod controls are generated by the PIC. The delay generator is built from a complementary fet pair in a 4007. It's normal state is high. When triggered the output from the 4007 goes low and then rises. The time it take to do this is set by the Mod Delay pot and is variable from immediate response to almost a full second. First draining then charging the 10uF capacitor C10. When connected to a VCA used to control a LFOs output level, it has the effect of delaying the Mod and then gently applying it smoothly and slowly much like you would do naturally with a mod lever. You can elect to have automatic mod after every note played by plugging the Key Gate output into the Delay Input .
If you setup the module forget about the -10.56 at pins 4 & 18 of the DAC! This might be the value that tom needed for his setup but the calculated value would be -10.66V (for a 8bit DAC we have 128 steps with 1/12V each, that gives us 128*0.83=10.66). For setup just play the highest C on your keyboard and adjust the scale trimmer T1 until you get the exact voltage (i.e. 7V for C7).
To bring the gate to an usable value i lowered the value of R16:
last update 11 JAN 2012
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