A local convention organization has a bunch of lavalier microphones that aren't really suitable for the panel sessions they were trying to use them in, as they're omidirectional and tend to pick up way too much room noise and cause feedback.  They also need a steady supply of small and annoying RW44 batteries, and the built-in holders for same are VERY flakey and cause their own noise.  What can we say, they were cheap at the time.  Since then we've moved to using normal cardioid mics on small desk stands or even boundary mics designed for conference settings, which when aimed at panelists properly work so much better and reject the noise from the rest of the room.  But now, what to do with all these now-disused lavs?

One of our tech fold determined that the mics themselves are made with fairly common Panasonic WM-61 capsules, which aren't actually too bad, so he had an idea: perhaps it would be possible to build (maybe 3D print) some kind of little enclosure for the lav housing itself, to mask away sound from the wrong direction, and then come up with a little circuit to run it on phantom power from a sound board.  It was suggested that I bring a couple of the old mics home to play with, so I did, and fell down a bit of a rathole on it all.  Thus, of course, a webpage about it.

Here's the lavalier product in question: the Polsen OLM-10.  The battery and super-simple driver circuit go in the little bump-in-the-cord. 

Measurements in the stock setup show about 1.3 volts across the capsule (from a 1.5V battery), and about 0.2 mA trickling through it, varying as it hears sound.  The output is very low, aka "mic level" -- the convention organizations's boards adapted to XLR inputs and needed the trims turned up all the way to get any usable signal out of these things at all.  Next question, therefore, was how to go about using 48V phantom power and not totally cooking the capsule the instant it was plugged in.  I researched through a wide variety of suggested circuits found online, and wanted to try to stay on the simple and passive side without any active devices needed for my initial experiments.

Next was to throw some stuff together on a proto-board, and eventually feed it from an XLR input on the mixer.  Initial testing of this was with a variable bench supply instead, and had a scope and meters hung off it to monitor voltage and capsule current and its output while I tried component values to get in the ballpark.  Of course hanging clip-leads out of an XLR breakout is bound to hum a little, so once I became bold enough to actually plug that mess into phanton instead I didn't expect things to be totally clean here.  I just wanted some proof of concept. 

Now, a little explanation is needed here.  There was also the question of sound directionality, which I tried to address by burying the mic capsule in a container and wrapped in a big surrounding wad of foam, with the element peeking out a small hole.  The first thing I found for containment in "the hoard" of various saved stuff [Because, y'know, New Englanders save everything as "might be useful"] was a holder for dog poop bags that people hang off their leashes.  But it had a closed back [thus the cable going in the front] and in this setup, did provide a *little* directionality and side/back suppression, but only in the high and upper midrange.  An omni element really does do its damndest to be omni.

Even with the careful efforts to not cook capsules, I did manage to kill one of them -- it stopped working and appeared to be internally shorted.  This happened after I'd cleaned up the early test circuit a little and simply plugged it back into phantom power, and I think we can see why, notably on the single-ended circuit where it happened: when pin 2 suddenly supplies power, a 48V pulse reaches the capsule as the capacitor charges, and what likely happened was that during test-circuit cleanup, that cap had discharged all the way.  Zap. That was *before* the protective zener got added across the capsule, to at least try to divert that transient around it.  We couldn't just add a high resistance in series with 2, as that would severely cut down output, and most of the circuits I'd found on the net also had the + side of the capsule capacitor-coupled straight into the XLR.

Even the pseudo-differential one carries the same risk, if, say, pins 1 and 3 happen to connect first and 2 floats for an instant (easy to do, if the plug goes in slightly crooked or its contacts are misaligned) the fairly low resistance to ground isn't enough to limit the inrush.  So from that moment on during further testing, I made sure the zener was always across the capsule.  Now I hear objections "...but zeners are noisy!" -- the idea here is that during operation, it's never passing current since the capsule voltage is down around 2 volts (stated, I might add, as its "typical operating range").  There *are* other circuits I found that use a zener *in* conduction to generate a reduced power-supply voltage, so if that also worked out for people then maybe it isn't such an issue.

So since one of the two mics I'd brought home was now dead, I got curious about what's in the lav head and possibly what the failure mode was.  The internal circuit of the WM-61 or WM-61A capsule is ridiculously simple, just the electret element and a JFET.  It also behaved on an ohmmeter like there might have been a body diode in the backward direction to guard against mis-powering.  And, after application of a bit more force than my fingers could do, it turned out that the top of the head *does* unscrew.

Keep in mind that this whole capsule is only 6 mm in diameter, so this is some pretty up-close photography of tiny parts.

Also note that the parts along the top here are *not* in the correct order...

So while all this playing around was fun and the "poop bag" containment was hilarious, the real question is would it actually be worthwhile to try and re-use the convention's old microphones to build some better directional, phantom-powered sort of units?  Doesn't really seem so, as there is plenty of likely better gear on the market whose cost would be well under the equivalent time and materials we'd have to spend on such a project.  Likely with far better designs of its electronics, too.  However, I won't shut the door completely on these yet, as some on our crew know way more about sound and gear design than I do.  There may be some clever ways to optimize some kind of retrofit.  As these mic elements themselves actually sound fairly decent, perhaps they could be more simply adapted to, say, wireless body packs that can supply electret power internally?  But now we're talking a whole different scenario where lavaliers actually make more sense.