The LFO alternative

I’ve been procrastinating on synth building lately. Finally yesterday I figured out that it wasn’t that I didn’t want to work on anything. It was that I didn’t want to work on the thing I was in the middle of working on. So I put it aside and worked on something else.

Cory Torpin recently released seven modules he calls the “k25 series” — an attenuator, an attenuverter, a dual gate to trigger, a glide, a sub oscillator, an AR envelope generator, and an LFO. They’re all Kosmo format and 25 mm wide, hence the name. Coincidentally I had about 25 mm of Kosmo space left in Kosmogenesis:

and no LFO other than the one in the Mother-32. Cory had boards and panels for sale but he’d also released the Gerber files, and I had some other boards I wanted fabbed, so I added his LFO boards and panel to the order. They arrived a couple days ago. It looked to be a nice easy build — one IC, a few capacitors, several resistors, not much else — perfect as an antidote to the more complicated stuff I was avoiding.

I decided to throw it together on a solderless breadboard first, as I’ve been tending to do lately before soldering. I wasn’t planning any mods or anything, but I just wanted to verify the circuit, check that its outputs were what I wanted and not needing any tweaks to get the output amplitudes or the frequency range where I prefer them, and make sure I knew which way around I wanted the bicolor LED. For the latter, a lot of people like red to indicate positive voltage and green for negative, in analogy with red wire insulation for the positive rail. In my brain, though, LEDs remind me more of traffic lights than wire insulation, and I think of green as positive (accelerate) and red as negative (decelerate).

The circuit mostly worked right away, except that at the highest frequencies the triangle wave got very distorted. Not something I was very concerned about. There’s a switch to select which integrating capacitor to use (on the breadboard I just moved a jumper wire) and with the large cap I was seeing about 0.014 to 2.5 Hz; with the small cap, about 1.4 to 250 Hz. Seemed good. Max amplitudes were about ±7 V with the triangle wave, a little higher with the square wave.

So I went ahead and built the module. I was right, it was an easy build. Once done it was near bedtime, so I put it aside without testing it. Sam Battle advises never testing a new module at the end of the day. You’re too likely to spend hours trying to debug when you should be getting your sleep, or to go to bed frustrated, or to make mistakes leading to a larger failure rate for modules tested at night versus modules tested in the morning. (A viewer on today’s builder’s livestream posited that there are pixies that come around and fix your mistakes overnight. I need better pixies for this other thing I’ve been working on.) I did take a picture, though. Then noticed I hadn’t put the IC in yet. Installing ICs is probably another thing you shouldn’t do late at night.

This morning I checked for shorts and correct power to the IC, then plugged it in and turned it on, and it worked right away. The triangle wave looked fine at all frequencies; guess that was just a breadboard thing. The LFO had earned its knobs, so I put them on and took a glamour shot.

Unlike Cory’s buffered and passive multiples, this one gets to keep Cory’s front panel design, because it’s not going into Kosmodrome, and it’s way too late to impose front panel uniformity on Kosmogenesis.

I do note a couple things I’d do differently if it were my design. I’d put the controls further apart, even if it meant losing the crystal graphic. But maybe it wouldn’t, because I’d leave out the level control — I prefer controlling levels on input, not on output. I might also adjust the capacitor values to make a larger overlap between the low and high frequency ranges. But these are very minor things. It does just fine as a small, simple, low cost LFO, and for sure it’s better than a 25 mm gap in the case.

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