BM-800 Microphone Conversion Part 1
Posted by Tom Benedict on 09/10/2016
Several posts ago I mentioned a plan to build an MS mic by following an Instructable written by Jules Ryckebusch. Jules used a BM-800 microphone as a donor mic and replaced its guts with two Pimped Alice circuits and three cardioid capsules. After several Ebay vendors whose listings indicated they would ship to Hawaii later changed their story and said they wouldn’t, I finally picked up a BM-800 microphone off of Amazon. The one I got is a Neewer NW-800. (I liked its shock mount better than the other one I found.) It arrived, and I started poking and prodding at it.
Along the way I discovered another reason to use a windscreen on a microphone. This thing is bling central. To be fair some of the other BM-800 mics I found on Ebay didn’t have nearly as much… presence… but this is the one I could get.
Unless I’m recording birds that are drawn to shiny objects the windscreen will probably become a permanent fixture on this mic, just to keep me from going blind.
Before tearing into the thing I decided to try it as-is. On the face of it it’s a phantom powered cardioid condenser mic. This means plugging it into a device that doesn’t provide some kind of power (aka my laptop, my phone, even my kids’ desktop computer) won’t work.
The mic has a male XLR jack at the back, and came with an XLR-to-3.5mm cable. 3.5mm inputs that provide power typically provide plug-in-power (2.3V to 5V, depending on the device). XLR inputs provide phantom power (typically 12V, 24V, or 48V). That discrepancy made me a little leery of just plugging this into whatever and cranking volts through it. I started by ringing out the cable to see what it was actually doing.
Up to this point all the XLR plugs I’ve dealt with have been for balanced signals. That is to say that one pin of the 3-pin XLR is ground (pin 1), another is the positive signal (pin 2), and the third is the inverse or negative signal (pin 3).
3.5mm inputs typically use a TRS connector and unbalanced signals. In the case of the 3.5mm stereo input on my recorders the tip is the left positive channel, the ring is the right positive channel, and the sleeve is ground.
The cable supplied with the BM-800 ties XLR pins 1 and 3 together and routes them to the sleeve of the 3.5mm plug, and routes XLR pin 2 to both the tip and ring of the 3.5mm plug. This effectively turns the balanced output of the mic into two channel mono unbalanced output on the 3.5mm plug, meaning it should be able to be plugged into any 3.5mm stereo input and drive both left and right channels with the same signal. Neat!
What this also means is that as long as the mic can run on a wide range of voltages, the plug-in-power on any recorder should be able to drive this thing. So should the battery box I got from Church Audio. Or by removing the XLR-to-3.5mm cable and plugging in an XLR-to-XLR cable, I should be able to power it with phantom power (12V or 48v – the only two options on my recorder) and use it as a single channel balanced input.
Still leery of running such a wide range of voltages through it, I tried all three configurations anyway. I’m planning to gut this mic, after all, so if I burned it out the loss would be minimal. To my surprise all three worked! The plug-in-power on my DR-70D puts out a little under 3V, and my battery box from Church Audio puts out a little over 9V with a fresh battery. The 48V phantom power on the XLR inputs on the DR-70D put out right around 48V. I noticed a gain difference between the PiP and battery box, but because of the different gains on the XLR vs. 3.5mm inputs on the DR-70D I wasn’t able to tell if the additional voltage was doing anything to the mic itself. (My guess is it doesn’t. To survive that wide a range of voltages I’m guessing the mic has a voltage regulator on board. Past a certain point it’s just dissipating as heat.)
So how does it sound?
How to put this…
I’ve seen the shock mount it came with listed for more than what I paid for the mic. I don’t think this is too far out of line with how it sounds. It’s not bad, mind you. It’s just not anything I’d write home about. A little creative EQing would probably make it a decent podcast microphone. But as for making ambient nature recordings? Mmmm… no.
So without further ado I tore into it to see what I was going to have to deal with.
The first step in disassembling the microphone is to unscrew the butt cap. This also releases the shell, which simply slips off to expose the circuit board. The shell is keyed to a tab just under the headbasket which fixes the orientation of the logo on the mic. This is important since the mic is a side-entry rather than end-entry, meaning sound must enter from the side and not the end. Added to that, it’s a directional microphone so it’s only sensitive on one side. Can you guess which side? (Answer: The one with the logo.)
Some nice features on the inside of the thing: First, there’s a ton of room. Second, there’s a nice frame with mounting holes tapped for M2.5 screws. (More about those in a sec.) The only weird part is the taper on the frame and the circuit board. I like the look of the tapered board, so I decided to taper the boards for my Alice conversion, too, and put mounting holes in the boards to make use of the holes in the NW-800 frame.
Two M2.5 flat head Phillips screws hold the headbasket in place. They’re located just under the headbasket, above the circuit board. Once the screws are removed the headbasket lifts off, exposing the capsule.
Despite the appearance, the capsule in this mic is the same size as the EM-172 and the EM-184 capsules from Primo: 10mm diameter. At this point I was sorely tempted to gut the mic, drop an EM-184 capsule in the mic saddle, and call it quits. But the whole purpose of this exercise is to move beyond Primo all-in-one capsules and try my hand at building more complicated (and better performing!) microphones.
All of this starts with the circuit board.
Simple stuff first: The screws are M2.5, spaced 30mm apart. They’re biased a couple of millimeters above the centerline of the cavity. If you’re planning to make a rectangular circuit board to fit inside this mic, that’s probably all you’ll need. (The tube with the logo has vertical walls, so a rectangular board will fit fine.)
Since I wanted to make a tapered board I measured the whole cavity and threw it into CAD. At some point I’ll draw it in 3D, but for now a 2D representation is plenty for me to design the new board outlines. I’m building the Alice boards using through-hole components, so I needed a little more real estate than the original board provided. The 2D drawing of the cavity and the new board outline looks like this:
I sent the boards out for fab and ordered enough components from Mouser to build out three of them. One is destined to receive the TSB-2555B capsule I ordered from JLI. The other two will eventually be used to build a copy of Jules’s MS mic using three TSB-165A capsules, but that’s a project for another time. Once all the bits arrive I’ll write the second half of this article, which will cover the construction of the TSB-2555B mic.