Powering the EM172 Capsule – Part 2: Results?
Posted by Tom Benedict on 06/02/2016
In my previous post I wrote about the issue of powering the Primo EM172 microphone capsule. Quick summary: The 1/8″ stereo mic input jack on the Tascam DR-05, the Tascam DR-70D, and the Sony PCM-M10 supply a plug-in-power voltage of around 2.3-2.7v. The EM172 datasheet says it’s designed for 5-10v. My question was how this affects performance, and what my options are for doing something about it. I took a two-headed approach: The first was to buy a 9v microphone battery box from Church Audio. The second was to order all the bits from Mouser Electronics to convert my microphones to XLR plugs and 48v phantom power. Then I tried to calculate how each option should perform. Push comes to shove, no matter what I do I should get better performance out of my microphones.
Church Audio builds equipment to order, so delivery takes a while. (Translation: The battery box isn’t here yet.) The Mouser order came in a couple of days after I placed the order, so that’s where I started.
Very early on I killed one of a pair of EM172 capsules, so I built the survivor as a standalone mono mic. I decided this was a good donor mic for the XLR conversion. I gutted it, cut off the 1/8″ connector, and rebuilt it for phantom power according to Richard Lee’s write-up of David McGriffy’s circuit. In the process I learned one of the drawbacks of using a pressed/glued in housing: The only way to get the mic out of the housing is to destroy the housing.
(I’ve now done this to three microphones, and made three new housings for them. Want to know what the new housings look like? They look exactly like this one! And the next time I re-wire one of them I’ll have to saw it apart, too! Part of me wishes I’d made the new ones with screws, but these are so easy to make I just can’t justify it.)
Here’s a list of the differences between how I’ve been building mics up to this point and what I did for this one:
- The mic is wired differentially, and is now shielded end-to-end. I followed Richard Lee’s idea of heat-shrinking the mic capsule, wrapping that with metal tape that contacts the cable shield, then heat-shrinking that to insulate it. The shield is also tied to the XLR plug’s shell, so it’s living in its own little Faraday cage from the diaphragm to the plug.
- I sized the resistor in David McGriffy’s circuit so my DR-70D would provide 9-10v plug-in-power to the mic. (It wound up being 9.6v.) This puts it in the S/N sweet spot, according to Primo.
- It’s got an XLR plug instead of a 1/8″ plug, so I can only use it on my DR-70D now. (No more plugging it into my DR-05. Bummer.) But the DR-70D has four XLR inputs so I can use four of these at once.
As an initial test I plugged it into one of the XLR inputs on my DR-70D and plugged my twin EM172 lavaliers into the 1/8″ jack. Using the built-in plug-in-power for the lavaliers I set all channels to the same gain settings and gave it a listen. The XLR mic sounded great! The 1/8″ mics were almost inaudible. Next I pointed both mics at a compressor in one of our labs at work so I could measure the difference in signal. With the gains set the same I got -18dBFS on the XLR converted mic and -39dBFS on the 1/8″ jack mic. I was only supposed to get a 0.9dB boost in sensitivity out of this, so clearly I missed something in my calculations.
That’s when I learned that not only do the two types of inputs on the DR-70D supply different voltages and have different input impedances, they also have different gains! The 1/8″ input provides gains of +3dB, +11dB, +26dB, and +38dB. The XLR input provides gains of +21dB, +36dB, +51dB, and +63dB. So not only was I not comparing apples and oranges, I’m not even sure I was comparing two fruits!
At that point I tossed all my quantitative testing out the window. In the end all that matters is what the darned thing sounds like! I made a couple of side-by-side recordings with the gains set so the two tracks would have the same levels, but eventually gave up. The S/N was so much better with the converted microphone, there was no question which one won.
I have enough parts to convert the rest of my microphones, but I’m still holding out for the Church Audio battery box. The DR-70D is really too heavy to fly from a kite, and I still have aspirations of recording aerial soundscapes. So I have to leave at least some of my microphones on 1/8″ connectors. Meanwhile I’m looking at my SASS and trying to figure out where the XLR jacks will go.