I built this to Aion’s specs using Vishay RN55D resistors, Panasonic electrolytic and film caps, and C0G MLCC for the picofarad caps. I did not measure the transistors but chose to instead systematically choose them by ear. This is partly because, though I’ve built at least a dozen DIY audio projects, I know very little about circuits and can’t even read a simple breadboard diagram to build the testing circuit (I should get on that). But the better reason is that I’d rather trust my ears, and I humbly suggest other builders try what I tried, even if you have the skills I lack. I purchased 5 of each JFET as Aion suggests, and I set out to audition the pedal with all 25 possible combinations.
This sounds like tedious work, but it probably goes a lot faster than you’d expect. In my case, I didn’t even need to try all 25 combinations. I distinguished my transistors by keeping each one in its own small zip bag (leftover from Small Bear orders), labeling each set of bags “1” through “5”. Using sockets, I kept one 2N5485 in place and tried each of the five 2N5484s against it in turn, playing a short series of chords and riffs, both clean and dirty, trying various knob and switch positions. I jotted down quick notes with each test. The fourth 5484 I tried was a clear standout; for some reason that one clearly had some magic the others were missing…only one of them sounded “bad” but the rest were either harsh or nondescript. The fourth one did exactly what I hoped, added some kind of intangible, musical, harmonic deliciousness that was subtle, but I definitely missed it when I turned it off. OK, promising. I then put in a different 5485, and again cycled all the 5484s against it in the same order, taking notes. Once again, 5484 #4 sounded easily the best. At that point, since the 5484 seemed to keep its good properties against multiple 5485s, I decided that must be the one…OK, the 5484 was chosen. I then kept the magic 5484 in place and auditioned the five 5485s against it. There was indeed a clear standout pairing, with one of the 5485s sounding best — the one I tried initially, actually — and that was that, the transistors were chosen. The whole process took about 25 minutes (probably not much longer than building a test circuit and testing 10 transistors); I have no idea how the chosen JFETs measure, and I don’t care.
The pedal sounds really great, and instantly became an almost-always-on pedal for me. I decided to forgo artwork and labeling and keep it a humble gray box with blue knob/LED. I had a 125B enclosure on hand so I used that. I tried it both at the end of my pedal chain, and the beginning (well, after tuner and fuzz). Both sounded good but I prefer it at the beginning. One of the simplest/cheapest builds that Aion offers and well worth it. And again, I really think people should commit to auditioning transistor pairings by ear. With any luck it won’t take too long, and you won’t be stuck wondering, “They measured ‘correctly,’ but are they really the ones I think sound best?”