Oh no, he’s back and he’s got a timbre!, or, Know when to fold (Part 2)

Hello world! Yes, that was a long break. Life’s been interesting, for instance, I’m retired now, but making a little bit of money teaching ukulele.

Which translates into: Hey, how about those cheap homebuilt synth modules, huh?

Until a few days ago I still had the unfinished NLC timbre! waiting for me on my workbench. Back in the month of mumble mumble I’d gotten as far as stuffing most of the resistors and discovering that somehow (probably having to do with placing orders way past bedtime) I was a couple short on 150ks. And that the shrouded power headers I had probably were not going to fit on the board. I ordered the resistors and unshrouded headers from Tayda, and they’ve been sitting on my desk, waiting patiently.

Actually a large part of the reason it took me so long to get back to it was that the workbench and the floor near it were an utter mess and I knew I should clean up, at least somewhat, before proceeding. Knowing it and doing it were two different things, but eventually I cleaned up. Even went so far as to vacuum the floor.

Yesterday I got back to work on the timbre!. I soldered everything before bed last night and this morning I tested. It worked… kind of.

That is, it folded an input sine wave, but it wouldn’t not fold it, even with both the panel pot and the trim pot turned all the way down. And the output level was too small.

That sounded like a problem described in the writeup, which I thought I could ignore, because it only applied if your signal levels were greater than ±5V. Or so I thought until I read it again and realized, no, it applied if your signal levels were greater than 5V peak to peak. Oops.

The recommended fix is to replace two resistors, a 100k and a 120k, with a 51k and a 220k, to reduce the input buffer gain and increase the output buffer gain respectively. So I unsoldered those two resistors and soldered in the new ones and tested again. The problem was worse!

“I didn’t just do what I think I did, did I?” I asked myself. I checked. I had. I’d gotten the two resistors switched around. Good thing I had one, count ’em, one 51k resistor left in my stash. (I probably could’ve unsoldered the resistors intact, but it was easier to cut them off.) Another round of resistor switching — this time I wrote myself a note to keep myself on the right track — and I had a working module.

More or less. The second input is intended for a triangle wave and has a simple tri-to-sine converter on it. Except, not sure, I think it might also need modification for higher amplitudes. It seems to want to convert a triangle wave into a square wave instead of a sine.

The tri-to-sine part of the circuit is pretty simple with just a handful of components. Unfortunately the jacks get mounted on the back of the circuit board right on top of the tri-to-sine solder pads, so debugging and fixing would be a massive pain. But I don’t really care. The only triangle source I have, the Befaco Even VCO, also has a sine output, so I don’t have much need for a tri-to-sine. And I find that the timbre! produces interesting results with a triangle wave plugged into either input. So I’m leaving it as it is.

It’s a cool module. If I were breadboarding it I’d probably use a different pot for the fold control — 50k I think might work better than 100k, which seems to more or less saturate the folding well before the clockwise limit is reached. But that, too, is not something I’d have much interest in trying to change at this point. It’s fine as it is.

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