The View Up Here

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Posts Tagged ‘Phantom Power’

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.

Neewer NW-800 Bling

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.

Neewer NW-800 Windscreen

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.

XLR to 3.5mm Cable

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?

Um…

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.

Neewer NW-800 Disassembly: Assembled

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.)

Neewer NW-800 Disassembly: Shell Removed

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.

Neewer NW-800 Disassembly: Headbasket Removed

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.

Neewer NW-800 Disassembly: Circuit Board Closeup

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:

2D CAD - NW-800 Cavity and Board Outline

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.

Tom

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DIY Microphone: EM172 Capsule and XLR Plug

Posted by Tom Benedict on 05/03/2016

This is the last in a four part series about powering the Primo EM172 microphone capsule. Part 1 outlined the problem of how to provide 5-10v to the capsule and predicted some results. Part 2 shared some results and pointed out that the gain differences between inputs on my recorder invalidated my predictions. Part 3 discussed my reasons for going with XLR connectors on all my microphones, and some of the details of that. This last part puts it all together into a step-by-step DIY for building microphones with Primo EM172 capsules, powered by 48v phantom power on an XLR plug.

If you need to build a microphone based around the EM172 capsule that plugs into the 1/8″ mic jack on your recorder, or a laptop, tablet, whatever, there are already several excellent tutorials out there. Rather than adapt this one to your needs, refer to one of the existing tutorials. The two I used when I first started building EM172 microphones were the ones on Zach Poff’s page and the one on Wild Mountain Echoes.

In this DIY I’m going to assume you already have a plan for making a mic body. I made mine out of Delrin bar stock on a lathe. Others have used Sharpie pen caps, which also provide a nice clip for clipping the mic to things (see the tutorial on Wild Mountain Echoes), PVC pipe, brass tubing, etc. When mounting the mic in the mic body, make sure the front of the capsule is flush with or slightly proud of the mic body. Don’t recess it. I made that mistake with my first set of mics and wound up with mics that sounded like they were inside a sewer pipe. If in doubt experiment by wiring up the mic completely, plugging it in, and listening to it as you slide it in and out of the mic body you plan to use. After all, this is DIY. Experimentation is part of the deal.

Primo BT-EM172 to P48 XLR Wiring

Credit for the circuit goes entirely to David McGriffy, and credit for the component choice goes entirely to David McGriffy and Ricardo Lee. Ricardo Lee’s writeup, SimpleP48wm61, goes into the theory of the circuit and the reasons for the component choices in depth. It’s the real reference for this. (In order to use that link to download Ricardo’s file, you may need to be a member of the micbuilders group on Yahoo!. If you’re doing this DIY you’re a mic builder, so it’s not a stretch.)

EDIT: A couple of weeks ago Akira So brought to my attention that I had the capacitor poloarity reversed from how David McGriffy and Ricardo Lee have it in SimpleP48. I’ve since corrected the schematic here. Credit where credit’s due.

EDIT: Akira also pointed out that my value for R (120k) resulted in something like 1.3-1.5V at the capsule. I experimented with a number of resistors to see what value of R would produce 7.5V at the capsule on my recorder, and for a Tascam DR-70D, R=40k produces just over 7.5V. When you do this build, you will have to find what works best for your equipment.

EDIT: I also swapped the supplier for the EM172 from Frogloggers to Micbooster (FEL Communications). I haven’t heard from Gene at Frogloggers in a while. Hoping he’s doing ok.

For my build I used the following:

I also used some metal tape (copper in my case, from the local gardening center), heat shrink of various sizes, and the solder I found on the bench in the lab. (My Alphametals solder I’ve been using for the past 20 years isn’t ROHS certified, so I can’t say “use this stuff, it’s great!”)

Not including the tools necessary to fabricate the mic bodies, you’ll also need:

  • Soldering iron (temperature regulated if possible)
  • Source of heat for heat shrink (heat gun, lighter, etc.)
  • Assortment of wire cutters, strippers, fine tip pliers, etc.

Since most of the bodies people use for these require the mic to slide in  from the front end of the housing, we’ll start with the mic capsule.

EM172 Back End

The first step is to strip one end of the cable, trim back the red and white wires to a workable length, and still leave plenty of shield exposed. The red and white wires are then soldered onto the appropriate pads on the capsule.

Warning: The EM172 capsule is sensitive to heat. These two photos were made with a capsule I’d killed using an unregulated soldering iron, which is why the capsule looks a little ugly. If you have access to a regulated iron set your iron no higher than 735C and don’t hold the iron on a pad for more than a few seconds. If you don’t have access to a regulated soldering iron, be sure to get EM172 capsules with stub leads already soldered in place. The tutorial on Wild Mountain Echoes uses capsules with stub leads, so you can see how she did it. Do all your work on the stub leads. Don’t fry your microphones!

EM172 With Wires

Now we build the shielding around the capsule itself. Insulate the sides and back of the capsule with some heat shrink.

Capsule Isolated

Be sure to account for every strand in the shield as you bring it up and over the heat shrink. Wrap with foil tape and trim back the shield so no wires protrude. Be sure no wires cross over the heat shrink and touch the front of the capsule.

Making a Shield

Apply a second layer of heat shrink over the foil tape. I like to apply a short length of colored heat shrink to help me identify which mic is which when I’m running wires and plugging things in out in the field.

Heat Shrunk Ready To Go

At this point go ahead and run the mic cable through your mic body, but don’t mount the capsule just yet. Once you’ve soldered the connector end of the cable, it’s a good idea to test everything to make sure you didn’t make any soldering mistakes, and to make sure the capsule didn’t get damaged during soldering. Strip the other end of the cable, leaving a little more wire to work with than on the capsule end. Thread the wire through the end cap for the XLR connector and set it aside. Since the XLR connector provides its own shield you don’t have to do any metal tape trickery on this end. Gather the wires from the cable’s shield, twist into a bundle, and cover with heat shrink tubing. This is also a good time to apply a length of colored heat shrink to match the capsule end of the cable.

Cable Prepped With Shell

Grab the XLR connector body in a vise or some other holding fixture. If you don’t have a vise, a set of vise-grip pliers with tape over the serrated part of the jaw works well. Just don’t grab it so hard that the connector body is damaged or distorted. Another way to hold these connectors that works great is to have the mating connector screwed into a board. Plug the connector you’re working on into its counterpart and solder to your heart’s content. (I used a vise.)

Trim back the leads on the capacitor and resistor to something reasonable that’ll fit inside the XLR connector. Save the snipped off bits of the leads. One of these works well to bridge from pin 1 to the ground tab.

Resistor and Capacitor

Solder a leftover component lead from pin 1 to the ground tab. Next, solder one end of the resistor to the ground tab as well. Next, solder the (+) end of the capacitor to pin 2. Finally, tie the two free ends of the capacitor and resistor together.

XLR Plug with McGriffy Components

All that’s left is to solder the cable onto the plug. Red goes to pin 3, white goes to the (+) lead of the capacitor as well as the free end of the resistor, and the cable’s shield is soldered to the ground tab. (In this photo the connector is rotated 180 degrees from how it’s drawn in the schematic, but that’s how the solder cups are oriented. Flip it around in your mind and it’ll make sense.)

XLR Plug with Cable

At this point your microphone’s electronics are finished. Put the connector together and screw things tight.

This is a good time to test the mic to make sure nothing went wrong. Plug it into your recorder, turn on phantom 48v power, and dial up the gain. If all went well you should have a low noise microphone ready to be installed in its mic body. If not, go back and check each step to find out what went wrong.

Finished Mic

Have fun recording!

Tom

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