BM-800 With Cardioid, Omni, & Figure-8 Pickup
3-Pattern BM-800 Mic
Monday, January 31, 2022
This time, we build a BM-800 with two 25mm CM012 electret capsules back-to-back and a switch to select Omni, Cardioid, or Figure-8 directional patterns. One capsule faces front, one faces rearward. Since the capsules are electrets, the added circuit consists of a DPDT On-Off-On 3 position switch to select both capsules in phase for omni, the front capsule alone for cardioid, or both capsules out of phase for fig-8.
The problems in construction were all related to mounting the capsules and switch. After gluing the capsules to the support saddle, the assembly was too wide to fit through the hole in the headbasket mounting flange, and much filing, trying, and swearing resulted. Likewise, after gluing the switch to the frame, it was discovered the switch covered the hole for the front headbasket retaining screw. Again, the jeweler’s files were broken out and the hole elongated just enough to slither the screw into place. A couple of years ago when I first toyed with the idea, I based it on JLI TSB-2555B capsules, which are significantly thinner, and would fit through the bottom of the basket. “Measure twice, cut once,” “look before you leap,” and such warnings before you follow my example.
Test fitting capsules
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Gluing capsule #1 to the saddle
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Solder wires to both capsules before gluing.
The capsules are spaced 3 quarters
(25¢ coins) apart.
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The DPDT On-Off-On switch is glued to the frame with handle protruding thru a slot in the body sleeve. ====>
Switch wiring. Front capsule (red & white) connects straight to input of electronics. Rear capsule (blue & green) connects thru polarity reverse switch.
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Electronic mods are the same as described in previous BM800 mods, consisting of an added 220µF/25V electrolytic capacitor to filter the output of the voltage regulator, and a 22pF capacitor from drain to gate of the FET to reduce gain and distortion, and raise the maximum sound pressure level before overload. A blue LED was added in series with the 22K resistor in the voltage regulator just for fun and to show when phantom power is on.
The finished mic sounds like other mics using this CM-012 capsule, with a bit of presence lift in the upper mids. The patterns of this pair of capsules are well matched, with a deep null to the sides and top of the mic in fig-8, and uniform omnidirectional pickup.
In all, a useful and versatile mic for little money, but not a quick build, with a lot of filing to get pieces to fit. If I build another one, I’ll avoid some of the pain. For instance, I found the grille screens are glued, not soldered in place, and the top screen can be popped out to access the capsules from above. And perhaps drill and tap alternate headbasket attachment screws before putting the switch in place.
