This is a quad-channel version of David McGriffy's Simple P48 circuit as described in Ricardo Lee's "SimpleP48" document on the Yahoo! micbuilders forum. It is designed to work with two-wire electret microphone capsules that include a built-in FET (e.g. Primo EM-172, EM-184, EM-200, EM-204, etc.)…
This is a quad-channel version of David McGriffy’s Simple P48 circuit as described in Ricardo Lee’s “SimpleP48” document on the Yahoo! micbuilders forum. It is designed to work with two-wire electret microphone capsules that include a built-in FET (e.g. Primo EM-172, EM-184, EM-200, EM-204, etc.) Theory of operation can be found in Ricardo Lee’s document and won’t be described here.
JP1-JP4 pin numbers are 1:Gnd 2:Hot on the capsule. JP5-JP8 pin numbers correspond to the same pin numbers on the XLR3M connector on the other end of the cable.
C1-C4 are 47uF audio grade electrolytic capacitors. I use Nichicon Gold but comparable components from Panasonic and others will work just as well.
The values for R1-R4 can be determined by wiring a potentiometer in place of one of the resistors and decreasing the resistance until nominal operating voltage is seen at the capsule. This will vary from capsule to capsule, so I can’t provide exact numbers here.
When mounting the board in the BM-800 body frame, be sure to use screws and washers that will make good contact between the mounting holes and the frame. This is the only connection between pin 1 on the XLR connectors and the body of the microphone. Good contact is necessary to provide RF shielding of the capsules.
I have yet to find an XLR10M core that will fit the cavity at the base of the BM-800 body, so the board includes eight 3.2mm diamter strain relief holes. These should work with all of the Mogami lavalier bulk cables (I use M2697). Thread each cable down through the lower hole, back up through the upper hole, and tie an overhand knot before trimming to length and stripping the outer jacket. I’d still provide some sort of strain relief at the base of the microphone, but this will provide excellent relief on its own.
