MP-V57 Upgrade boards
MP-V57 Microphone board set
Sunday, December 8, 2013
There are a lot of low-end Chinese condenser mics built on very similar chassis which use a pair of rectangular or wedge shaped circuit boards mounted to a pair of rails running from the capsule head to the cable connector housing. Lately most factories have mechanized as wages have risen, and it’s cheaper now to have a machine build a surface mount PCB than to have a girl solder through-hole boards by hand. Modifying SMT boards at home by hobbyists is not easy, so microphone-parts.com has developed replacement printed circuit boards.
The MP-V57 kit includes a mic amplifier board and a bias supply board along with all parts needed to construct the circuit. These PCBs fit the MCA SP-1, MXL 2006, V63M, 550, 910, and probably a lot more, including the new surface-mount versions of these mics. The circuit is Jim Williams’ enhanced version of the Schoeps impedance converter. Parts are selected and matched before shipment, so you get everything ready to solder. It really makes upgrading any of these mics quick and easy. Add your choice of capsule, and you can upgrade a pair of mics in a Sunday afternoon.
Matt sent me a prototype kit to check out, which went together without a hitch. The manual wasn’t ready, so I just matched parts to the legends silk screened on the boards. The boards are double sided with plate through holes and a ground plane on one side. There is a switch to select cardioid or omni pattern if your capsule permits, and a trimmer to fine-tune capsule bias.
So here are pictures taken during the build using a $50 eBay karaoke mic chassis and a M-P RK-87 capsule. An optional EQ capacitor is included to flatten the top end of ’67 and ’87 capsules.
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.
