Monday, 24 August 2015

V/Oct Calibration Tool

I'm currently upgrading a few of my first DIY synthesizers by adding MIDI control and perhaps also renovating the front panel. I also want to re-calibrate the tracking of all the oscillators of the first synths I built in order to have a proper volt/octave scale for the MIDI/CV converters to work with. To that end, I built a small V/Oct calibration tool based on an old circuit by Ray Wilson of Music From Outer Space that he mentions in one of his earlier VCO circuits. In contrast to Ray's tool, mine spans a 7 octave range by supplying voltages from 0 to 7 volts.

V/Oct calibration tool. Soon with enclosure.

I started by selecting a couple of resistors from a batch of 50 or so (10k 1% metal film) for equal resistance using a Wheatstone bridge. I managed to find two groups of 3 and 4 resistors that among them produced a midpoint voltage of 0.0mV (when measured using my cheapo DMM), with the two groups being 0.1mV apart. I don't know how that difference translates to resistance, but in any case they all now seem pretty equal to me. :-)

I used these resistors to build a multi-level voltage divider to produce the voltage steps starting with the 7 volts on top of the chain. Incidentally, I get the top 7 volts by simply swapping the 47k and 68k resistors in Ray's circuit (trimmed to precision using a 10k multi-turn trimpot). The calibration tool's power is taken from the synth's power supply. Two LEDs on the front panel show proper connection to both negative and positive supply voltages.

It took a bit more time to build than I anticipated (I did it on perfbord with the component leads providing the connections between parts), but now that it's finished I actually look forward to tediously calibrating all the oscillators.

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