Another pair of mics is built on the same type chassis as before. This time, they have omni, cardioid, and fig-8 patterns and are built around RK-12 capsules.

The bias supply is a little Hartley oscillator with voltage doublers on the output, generating + & - 65 volts. A SPDT On-Off-On switch connects the rear diaphragm to it.

The J305 low-capacity FET, the 1 gigohm gate resistor, and the two source bias resistors are on a teensy scrap of PC board next to the capsule with the gate lead of the FET soldered directly to the tag on the backplate.

If you try this, be careful not to let flux or vapor reach the diaphragm.

This is for expert solderers only.

The rest of the circuit is very much as described in the last article. The circuit board of the karaoke mic which furnished a chassis is cut down to only the output stage, a few necessary parts are swapped and added, and it’s all buttoned up with red paint this time.

More snapshots from the workbench:

Measurements will be forthcoming. I will say that I noticed low level details deep within the music which I haven’t heard with any other mics. My initial reaction is “WOW!”

So they sound like this.