Calibrate VCO Tracking

How to Calibrate the Tracking of the Bermuda or Mino

Cody Iott

 

When exact pitch is expected, you may find the pitch of 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

 

Step 1:

With your voltage source at 0 volts, tune the VCO to 55Hz. This is an A note. Youtube reference note.

 

Step 2:

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. If you are sharp, adjust the trimmer with a small counter-clockwise adjustment. 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 adjusting the trimmer as needed. Each time you adjust the trimmer, you will need to go back and retune the VCO to 55Hz  at 0V before increasing the voltage to 1V again. Eventually you should get exactly 110Hz without adjusting the trimmer. When that happens, move on to step 3.

 

Step 3:

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.

 

Step 4:

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.

 

Step 5:

Increase to 4V and get 880Hz.

 

Step 6:

At the 5V mark, you would expect to so 1760Hz.  With the Bermuda 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 the simple Bermuda circuit. 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 and email and let me know.