#16 Cheap chinese BM700 / BM800 microphone modding / upgrade, part 2 (CK12 & K47 capsules)

<Continuation from part 1>

After quite a while, i "finally" got around to modding the other three BM800 mics i had. Just for the hell of it, i decided to make a matched triplet (to use as, say, three overheads on a big drum set).

I went with more of my modded-Schoeps boards, and matched components between them, as closely as i could (within reason, at least). Resistors were no big deal, and neither were the capacitors; one of those cheapo chinese ATMega328-based "component testers" helped with matching the JFETs and PNP transistors. No, i didn't necessarily care about the absolute values, just that they're the same (or as close as reasonably possible).

Construction went pretty much the same as my first effort, no real rocket surgery, no drama or anything. Granted, the cutouts in the body-tubes might not be as accurate as factory-made, but hey - i'd like to see you do better with a dremel and a set of needle-files. Good enough for me, i've always been a function-over-form sorta guy, so there...

One thing i admit i skipped, were the 1M trimmers for precision biasing of the JFETs, but that's a detail i can very well do without, at least for the time being. Maybe if i feel like "guilding the lily" at some later date...

A couple weeks after completing them, just for the hell of it (or rather, out of some measure of curiosity) i decided to more-or-less "measure" them. Unfortunately, due to the way REW (RoomEq Wizard) is designed, despite my hopes, i didn't manage to get any relative measurements (using my EMX-7150 calibrated measurement mic as a reference). On the bright side though, i could at least use the reference mic sweep as an "eye-metric baseline", shall we say, to roughly gauge where the mics under test sit.

For the measurements, i used one of my Tannoy Precision 6D monitors as a signal source; orange trace = EMX-7150; other traces = the three "BM812" mics. Orange ("reference") trace is offset for clarity. The 6-7kHz dip is partially from the speaker, but it may well be accentuated by some measurement artifacts or whatever. Inconsequential for the purposes of this little experiment.

Right off the bat, you have to admit - especially considering these were ~30$ capsules randomly selected from a batch of 150-200 (or however many they were) straight from the factory in China, the matching between them is nothing short of impressive! Bass response (under 100Hz) maaaaybe 1-2dB under the reference, a couple extra dB in the mids and highs, but all in all, not shabby, if i may say so myself.

A possibly more relevant view - the three "CK12"-equipped mics, using the EMX-7150 as the calibration reference. Traces offset for clarity; lowest one using 1/3rd octave smoothing, the other two using 1/12 octave smoothing. Still not bad, eh?

For (extra) kicks, i also swapped out the franken-K67 from the first BM800 for a chinese K47-type capsule (from the same source / same group-buy), for two reasons. First, i didn't wanna bother with tacking on an HF-attenuation network; second, because that K67 will go into my third AKG Perception 200... as soon as i can bring myself to "carve out" the window for the pattern-select switch, into that beefy cast-brass body.

So, let's see - 30$ or so for a capsule, under 20$ for the donor mics, maybe 10$ for the set of 3 PCB's, let's exaggerate and say another 10$ for the components; so about 70$ plus some elbow-grease for a mic that, dare i say it, might as well compete with ones at least 4-5 times the price (here's lookin' at you, Roswell Mini-K47 - which is cardioid-only, by the way).

Not that i'm patting myself on the back, but i'm pretty damn happy with this little endeavour. I've never been one to just throw money at a problem - anyone can just go out and BUY a mic, y'know? And then there's the "added bonus" of ending up with something you made yourself. That's ever so slightly special, right there...