How to Calibrate the Tracking of Analog VCOs
When exact pitch is expected, you may find the pitch of your analog VCO either falls flat or goes sharp after a couple octaves (or even after a couple notes!). No worries, sounds like the tracking just needs a little adjusting. Set aside 15-20 minutes to calibrate your own VCO. Once it's finished, it should not need to be re-calibrated for many years (if ever).
What you need:
- A voltage source capable of exact 1 volt increments. An analog keyboard controller such as the Arturia Keystep will serve this purpose.
- And either, a frequency counter (best method), a guitar tuner, or your ears and a reference piano/synth
- A very small flathead screwdriver
With the module already installed, turn on your system and let it "warm up" for 10 minutes.
With your voltage source at 0 volts, tune the VCO to 55Hz. This is an A note. Youtube reference note.
Now increase your voltage source to 1V (go up an octave on your keyboard). The frequency should be exactly 110Hz. If you are flat, adjust the front panel trimmer with a small clockwise adjustment (try 1-3 full rotations to start). The pitch should actually get lower when you adjust the trimmer. That's to be expected.
If you are sharp, adjust the trimmer with a small counter-clockwise adjustment. The pitch will get sharper. This may seem wrong or counter intuitive, but keep going!
Now, bring your voltage source back to 0V, and adjust the Frequency and Tune knobs back to 55Hz again. Repeat going up an octave and you should be closer to 110Hz than you were before. Keep repeating until you are spot on. Eventually you should get exactly 110Hz without adjusting the trimmer. When that happens, move on to step 3. But remember(!), each time you adjust the trimmer, you will need to go back and retune the VCO to 55Hz at 0V before increasing the voltage again.
Increase your voltage source to 2V. The frequency should be 220Hz. Keep making minor adjustments to the tracking trimmer until you can successfully track 2 octaves. Remember that each time you make an adjustment to the tracking trimmer, you must reset the VCO to 55Hz at 0V source.
Increase to 3V and you should get 440Hz. By this point, only very fine adjustments to the trimmer are needed. You should be 90% calibrated.
Increase to 4V and get 880Hz.
At the 5V mark, you would expect to see 1760Hz. With the Bermuda and Paradox VCO, don't be surprised if you start to fall a little flat here. Right around the 1.5 - 2kHz range, the bermuda starts to fall flat no matter what. Don't undo all the good calibrating work you've done so far. This is a limitation of their simple circuitry. With other VCOs, this might be a good point to start adjusting any "high frequency compensation" trimmers that are included. The Mino VCO has high frequency compensation built in and no adjustment is necessary.
How'd that go? Is there a mistake in this guide? Still something you're unsure about? Send me an email and let me know.