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BM-800 PC Microphone







Search the well known shopping and auction sites for 'condenser microphone' and you'll probably come across the BM-800. It's a rather tacky looking Chinese device that's a bit of a mixed bag. It may be sold on its own or as part of a kit with all the mounting hardware or with a mixer, at prices varying from £10 to way over £100. Whatever you pay, the fact remains that the mic itself is nothing special. Also, the internals can differ depending on when and where you buy them. Some have large diaphram electret capsules, some have small, some have large PCBs, some have tiny PCBs with hardly any components on them. Some need 48V phantom power and some don't, the list goes on. So why would anybody want one? Because they are perfect for DIY mic builders! They have a metal body that easily comes apart by unscrewing the end, a cast chassis with threads for mounting your own board and an integral XLR plug. The one I purchased was part of a kit that came with absolutely everything (except mixer) for £14.95 which was a real bargain. Yes, mine did have the small diaphram electret and the tiny PCB but that didn't matter as they were going to be discarded anyway.


       


The project described here is not a high end studio microphone, it's a general purpose mic for your PC. Though having said that, it is a high output, low noise good quality device. It does not need 48V phantom power as it runs from the 5V available on a standard USB socket, and as it's going to be used close to a PC using just a short run of cable, having an unbalanced output is perfectly fine. In fact, the output level is high enough to feed directly into the line input of a PC. For the electronics, I used exactly the same circuit and layout as my 'One Transistor Electret Mic Preamp' project which fits nicely into the BM-800 body. Click here to visit that page for more details about the schematic. The electret microphone capsule I used was a Primo EM272Z1 from micbooster.com, still quite small at 10mm but so much better than the one it replaced. The stripboard layout below shows the position of the mounting hole and how it's wired to the XLR connector, which as luck would have it, matches pin for pin! Just for reference, the bolts used to secure the microphone cradle, mesh basket and circuit board are all M2.5 x 5mm.



And as for the looks, the BM-800 is available in many different colours for both the body and mesh basket, but I think black ones look the best. I'm not too keen on the gold one I have but it can at least be disguised by fitting a foam windshield.
                        







For the microphone connecting cable I used Van Damme Tour Grade Classic XKE with a Neutrik NC3FXX 3 pin XLR line socket at one end and a Rean NYS322 5 pin DIN plug at the other. The XLR pinouts loosely follow convention with pins 1 and 2 being ground and microphone '+ hot' respectively, except pin 3 is now used for preamp power instead of the usual microphone '- cold' which is not used in this application. The DIN pinouts were chosen for compatibility with the PC interface cable shown below and also a PTT Switch Box which is an optional accessory for radio hams.






The diagram above shows an interface cable used to connect the microphone DIN plug to a PC. The 3.5mm plug goes to the line input socket for audio (tip and ring shorted for mono) and the USB plug goes to a spare USB socket to supply 5 volts DC for the mic (the USB section was obtained from an old phone charging cable that I had knocking around). The DIN socket is housed in a small plastic enclosure with a cable grommet at each end.