Building a VCO

For my second DIY Eurorack module I picked the Befaco Even VCO. There are a number of DIY VCOs on the market but this one kept rising to the top of my list. Available in full kit, price is decent, good reviews.

It was considerably more complex and dense, of course, than the ADSR I did. Fortunately I didn’t have to build it with my crappy old soldering iron; I have a new one. I spent several hours last weekend getting most of it done before running into an apparent issue with the panel hardware. After querying with Befaco and with an online forum I got assurances my approach was correct so I went ahead and finished the build on Monday.

Then the moment of truth. I plugged it in, hooked up the oscilloscope to the pulse wave output, and… there was a pulse wave! I checked the other outputs. All good! Astonishing!

About those outputs: It has the full set of usual suspects — pulse, sawtooth, triangle, and sine. And then a fifth output, labeled “Even”, which gives the module its name. Befaco calls it a “waveshape with emphasis on even harmonics” which is something of an understatement. What it actually is is a sine wave at the fundamental frequency added to a square wave at double that frequency. Since a sine wave has no harmonics at all besides the fundamental, adding that to a waveform an octave higher gives you something with only the fundamental plus even harmonics. In fact, since a square wave has only odd harmonics, when you end up with is harmonics 1, 2, 6, 10, 14… 

So anyway, I proceeded to go through the calibration, and that seemed to go smoothly. The module has a 10-position rotary switch labeled 32′, 16′, 8′, 4′, …, 1/4′, 1/8′, 1/16′. You tune to C5 at the 1′ setting and then adjust trimmers to get it in tune at 8′ and 1/4′. When I was done it was in tune within a couple cents over the full ten octave range. Impressive.

Then I connected it to the Mother-32.

KB CV to the module V/OCT input, module output to EXT AUDIO and to a tuner. I found that the FM and H SYNC inputs work. And… I found the V/OCT inputs seemed not to track correctly. If I played C4 on the keyboard and tuned the VCO to C4+0 cents, then playing C5 would give me C5-17 cents and C3 would give me C3+17 cents. Further octaves were more out of tune. Yikes.

Could the voltages coming off the octave switch have been wrong? I checked and, no, they weren’t. That then suggested there was an incorrect resistance either on the octave switch or on both of the V/OCT inputs.

Or… that the KB CV I was sending it was wrong.

It looked pretty good (though not perfect) when I checked the voltages at the end of a cable with the other end plugged into KB CV. But tonight I tried plugging the other end into the VCO and checking the voltages at the jack… and they were wrong. -2.96 V at C1 to 3.95 V at C8. The deviation at each octave from exactly 1 V/oct was almost perfectly consistent with the mistuning.

So then I went looking in the Moog forums and found several threads on the subject of which this was one:

tl;dr: I’m not the only one. This seems to be a misfeature of the Mother-32. Suggested solution there is to run it through a buffered mult, of which I have none, so that’s probably my next module.

That’s the bad news. The good news is, it looks as though I built this VCO to function perfectly on the first try!

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