Microphone-Parts.com has designed a pair of circuit boards to fit a large number of low end Chinese microphones. They'll fit the MXL 2006, V250, 910, and V63M, and MCA SP-1 among others. They also fit the “karaoke” mic body I had on hand.

The original capsule from the karaoke mic.

An electret cardioid about 2/3 inch dia.

The RK-87 dual-backplate capsule

Bias supply PCB

Audio PCB

Ground planes

The biggest hassle building this mic came when I tried to pull the inner two layers of screen out of the head basket. It turns out in this mic there was more solder holding the layers of screen together than there was holding the outer layer to the frame. The inner layers had to be clipped out wire by wire, but once started, there is no going back, and it took a couple of hours.

The prototype boards were a little wide for this smallish body and had to be filed down a couple of millimeters, easily done before installing components. Production boards are a bit smaller and fit some of the slimmer mic bodies like these and the MCA SP-1without trimming. Perfect.

The final mic. A refined ’87-ish sounding gadget with nothing left of the original but the body.

Accessories included with mic.

Measurements from 10Hz -48KHz show electronic noise is well below thermal noise of air molecules bouncing around inside the RK-87 capsule. Frequency response is -1dB @ 20 & 20KHz.

Violet = noise with capsule connected, very quiet room at night, holding my breath.

Yellow = noise with capsule replaced by 68pF NPO capacitor, C14=C15=470pF.

Cyan = noise with experimental variable EQ circuit (look for upcoming article).

Green = noise floor of TASCAM US144 mkII with 220Ω shorting plug in mic input at full gain.

Performance is equal to the other Jim Williams upgrade kits, quiet and very low distortion. This is no longer a toy. Under $300, it performs like an expensive boutique mic, and you get to choose the capsule and fine tune the response to taste. A handy kit indeed.