I'm happy to report my mic works too!
TLDR is at the bottom.
I made a lot of mistakes along the way.
As
@Wordsushi said, there were different ways to approach this build, and my choice was to go the cheapest way. So I gathered all components from Aliexpress, with zero certainty on them being the right choice, and hoped for the best.
Initially I made a PCB design because designs at the time were for a different body (HL-95, while I had a BM-800 lying around) and it was cheap. Because I wanted to test that design first, there was a 2.54 mm grid on the PCB, just like a perfboard. After (badly) routing everything, printing the PCB on paper and trying it on top of a perfboard, I realized my first mistake here: routing was done on TWO layers!, which only complicated matters. I tossed the perfboard idea and went straight to PCB manufacturing. So much for the 2.54 mm grid.
Once the PCB arrived, all components were soldered as best as I could (IOW, not great). Because my donor was a BM-800, space was really, really tight (36x63 mm). With the height of the WIMA capacitor and the T-13 transformer, it was impossible to make it fit on the upper side of the BM-800 frame, thus it had to be installed on the underside of it. This presented a problem: the frame itself needed some space on the inside of the PCB, where some capacitors were placed. Lessons learned: center the WIMA capacitor, add a lot more margin to the PCB, and drop the 2.54 mm grid altogether.
I must have mishandled the capsule a lot because I later discovered some fingerprints on it. Much later, while desoldering, I must have pushed the diaphragm accidentally against the backplate as there were some marks (dents) in it. Oh, well. At least it wasn't Arienne's.
First time I tried the build there was absolutely no sound at all. I chalked it up on my poor choice of components (like the 1G resistor, I thought) the handling of the capsule, and the probably burnt transistor as I soldered it. I let that sit for a couple of weeks.
When you guys talked again about the BOM I looked again at my build. With the oscilloscope and REW I found that a 1 KHz sine wave was *very* distorted on the output. No matter what I had on the trimpot, it was really distorted... except sometimes it wouldn't work at all.
I was really excited at that point because it showed signs of life. After fiddling around, the problem became apparent: there was a bad connection in the C11 capacitor, so pushing it a little made it work!, at least with REW signal generator. When somebody mentioned that the JFET _could_ work either way (the Chinese transistor I got had no spec sheet), I tried that with a new transistor, to no avail. Left it again.
I fixed that and then tested with an audio interface. The output was BAD, like B.A.D. -- BAD, really noisy. The baseline for noise was -28.5 dB according to my DAW. Again, I thought about the poor choice of components. Left it again for some days.
Yesterday I decided to bring the transistor back to its place and make new measurements with REW, and decided to solder it with the gate leg lifted (no PCB), straight to the 1G resistor and the capsule connection. It worked!! And it worked amazingly! I am beyond impressed.
TL;DR: Lots of mistakes, but this is a very forgiving build, even with low quality Chinese components.
Here's a list of components (with links to Aliexpress and approximate costs). In total, for just one build (and hundreds of spare components) it's something like US$131.71, more or less.
Thank you, all, who shared your knowledge, your pictures, your ideas, and your suggestions. You guys rock!
And you might want to consider connecting the mounting holes to the ground plane(s).
