These are phantom powered electret microphones that are built into male Neutrick XLR connectors.
Here’s a schematic…

The transistors I used were 2N5087. The microphone capsules were WM-61A, available from Digi-Key. Note that the Two Wire Linkwitz modification must be done to the WM-61A capsule, as shown here…
http://www.mp3forkidz.com/mic/mod.html

A couple of tips about doing the Linkwitz mod on the WM-61A…

The capsules are very small and easy to destroy in the process of attempting the mod. So, buy more than one.

An exacto knife, or other small razor-like blade, works well for cutting the trace between Term 2 and the capsule perimeter (ground).

The capsule’s case is made of aluminum, so it’s very nearly impossible to get solder to stick to it. But the back of the capsule is actually a very small circular circuit board with a ground trace running around the perimeter. The lip of the case covers that ground trace. It’s possible to carefully cut a small notch in the lip of the case and uncover a small section of the ground trace.

I found it helpful and time-saving to simultaneously attach the wire at the same time you make the solder connection between Term 1 and ground, rather than soldering the connection first and then going back to attach the wire in a second step.

Here’s a PCB layout…

Part of the inside of the Neutrik connectors had to be cut away to make room for the PCB, as you can see in the next pic…

Here’s the PCB…

And, here’s the complete interior assembly…

The rubber boot didn’t quite fit all the way back onto the XLR connector when I reassembled them. So, as you’ll notice in the picture at the top of this page, I used some black electrical tape to secure the boot back on to the XLR connector.

So, how do they sound?

I’ve used them to capture overall room sounds and they sound pretty damn good to my ears. Maybe not the best for vocals, but I’m thinking they could be great for acoustic guitar, percussion instruments and maybe drum overheads.