diy electret mics

DIY stereo electret mics with Primo EM-172 capsules

When I attended the meeting of the Nature Sounds Society in June 2014, I was able to see first-hand some of the exciting and effective stereo mic arrays made from tiny electret mics.  Dan Dugan provided a variety of microphones from Telinga, which are based on Primo EM-23 or EM-172 capsules, and both sounded much quieter than the Audio Technica AT2022 microphone I had been using.  After some research when I got home, with some helpful advice from Steve Sargeant (who occasionally makes a modified Jecklin array with EM-173 capsules available on Ebay), and I decided to take the plunge and attempt to make some myself.

 EM-172 capsules

I decided to stick with the EM-172 capsules due to their relatively inexpensive cost, low self-noise, and sensitivity.  Their specs can be found here.  These are the same capsules used by a number of nature recordists.  It is important to note that they need 3-10 v power.  My Sony PCM-M10 only supplies 2.5v (as do many of the small recorders).  According to Steve Sargeant, the capsules work better with at least 5v of power, which results in lower input noise and greater dynamic range.  But the capsules are still very impressive with only 2.5v.

I purchased a set of capsules and a kit from FEL Communications, and the necessary soldering gear (some links at the bottom of the page).  The results were less than stellar, which I attributed to my limited skills in soldering.  I also realized that the two most difficult parts of assembling the mics were soldering the plug and soldering the wires to the capsules.  I decided to start over, this time ordering capsules from Frogloggers.  These capsules came with wires attached (they are also available this way from FEL Communications).  I bypassed the plug soldering by buying a “y” adapter RCA cable, which included a figure of 8 wire already attached to a plug.  Below I go through steps I used to construct my mics.

Building the mics

Needed equipment:

Needed equipment
RCA “y” adapter cable, heat shrink, EM 172 capsules and a Sharpie(R) pen cap. I’m not advocating this brand of heat shrink, it’s just for illustration. This tape shrinks by half, a tape that shrinks by 4x might work better.

Plus some basic soldering equipment:

soldering equipment
Soldering iron (25w), soldering iron holder, helping hands, and rosin-core solder.

Start by figuring out how long of a cable you want, and snipping off the RCA plugs at that length.   Split the two wires at least 6 inches.  Strip about 1/2” from each end.

Cable with connectors removed and ends stripped
RCA cable with the RCA connectors cut off, and the ends of the wire stripped.

Carefully separate the ground wire from the insulated signal wire.  Twist both tightly.

ground and hot separated and twisted
The ground wire separated from the signal wire and twisted tightly. The signal wire was stripped of its coating, and twisted tightly.

Tin each wire with solder.  Strip about 1/4” to 1/2” of insulation from each of the wires attached to the capsule.  Note that the wires are thin and fragile – handle with care.  The wires are probably already tinned, but if not, tin them, too.

Wire tinned
Cable wires after tinning. Solder should actually extend closer to the cable housing.

Bend each wire into a tight hook.  BEFORE you do this, make sure you have the heat shrink in place!  You’ll want heat shrink for each connection, plus a larger piece to cover the whole thing.  If you are going to slide the capsule inside a holder, you might also need to get that on the wire before soldering as well.

Wire ends bents in u-shape
Bent ends on the wires attached to the capsule. Note that the heat shrink is already on the wires.

Place the wires from the cable and the capsule in the helping hands, and carefully hook the ends of the red to red (or white) and black to ground wires together.

Hooking cable to electret wires
Capsule and cable wires hooked together.

Carefully solder the wires together. Do a better job than I did.

Wires soldered together
Capsule and cable wires soldered together.

Slide the heat shrink over the connections.  The ground wire CANNOT touch the hot wire or it will short.  If you have problems with the heat shrink covering everything, try using some Liquid Tape (see below) to cover any exposed wire.

Connection covered with heat shrink tape
One capsule done, with heat shrink in place.

If you are going to use the mics as binaural mics, make sure the heat shrink includes the base of the capsule, overlapping the sides, and you’re done.  If you are going to seal the capsules inside a holder, then the heat shrink as pictured is ok.

Last step, seal the mics inside the holder.  This can be done with hot glue, but you have to be very careful of the temperature as the capsules are really sensitive to heat.  Better is a silicone sealant like Permatex RTV sealant (see below).  Be very careful to not get any sealant on the front of the capsule.  You might want to cover the front with a small piece of electrical tape when sealing.

I placed the capsules inside the caps from Sharpie® permanent markers.  I snipped off the end below the clip and fed the wire (before bending or soldering) through the hole.  I used the “fine” markers, but the “ultrafine” will also work.  I used Permatex RTV to seal the capsules inside the caps.  The arrangement has worked quite well, and has resulted in capsule that I can use as tree ears, with a Jecklin disk, and in a spaced array.  And the clip could also be used to fasten to the front of a button-up shirt so the mics could be used as lavaliers (better in a mono setup, though).

Capsule complete
Completed mics, in their pen cap housings.

The mics could also be mounted in a variety of other materials.

Using them in the field

To use the mics outside, they need some form of wind protection.  I used foam (insulating foam for air conditioners, available at hardware stores) with a cut-out for the capsule and rounded off, and covered that with a thin layer of acoustic foam.  I covered the whole thing with faux fur.  I attached Velcro to the bottom of the fur to reduce wind coming in the bottom.

2014-12-13 16.40.06

To use the mics for soundscape recording, I usually use them in “tree ears”fashion, by wrapping a cord around a tree 6-8 inches in diameter.  I use wooden clothespins to secure the fur-covered mics to the cord.  I have a small camera case that my Sony PCM-M10 fits in; it has a strap to slide over a belt, so I use that to hang the camera case with the recorder on the cord also.

clip-on mics
Clip-on “tree ears” made from EM 172 capsules.

Equipment and supplies

Here are some links to the supplies I used, or reasonable substitutes.  Note that the cables are relatively cheap.  I consider these “testing” cables.  I will likely make another set of mics in the future using higher quality cables with better connectors.

Belkin Audio Y Cable Splitter 1-Mini Plug, 2-RCA Plugs (6 feet)
URBEST®180 pcs Heat Shrink Wire Wrap Cable Sleeve Tubing Sets Assorted Size
Kester Pocket Pack Solder 60/40 0.031 0.50 oz. Tube
SE MZ101B Helping Hand with Magnifying Glass

Some optional supplies/equipment that make things easier:

Solder Sucker- desoldering pump
Eclipse 902-109 5″ Diameter Magnifier Workbench Lamp with Bench Clamp, White
Permatex 80050 Clear RTV Silicone Adhesive Sealant, 3 oz.
Gardner Bender LTB-400 4-Ounce Black Liquid Electrical Tape

 

Using with phantom power instead of PIP

There was some discussion last year on the Micbuilders Yahoo Forum as to how to optimize the power for the EM-172 capsules, which appear to do even better at higher voltages than small recorders can supply via PIP.  If 12v or 48v power is available, here are some instructions to create an XLR version of the EM-172s.  Tom Benedict has created a great tutorial on building the mics for phantom power.

More information

For more information about building the mics, including illustrations of other builds and some soldering instructions, check out the following links:

https://www.zachpoff.com/diy-resources/low-noise-binaural-mics-primo-em172/

https://jg1983.co.uk/Blog/diy-miniature-omnis/

Also check out the Micbuilders Yahoo forum (search for EM 172); there is a lot of information there. And please share your experiences. There are many ways these capsules can be put together and used, and we could all learn from other experiences.

Last modified June 2016.

8 thoughts on “DIY stereo electret mics with Primo EM-172 capsules

  1. Will do. I’m planning to explore this, so I’ll happily share whatever I learn.

    I started with a Tascam DR-05, and recently bought a Tascam DR-70D. The 70D has better amplifiers than the 05, and adds the XLR inputs the 05 lacks. But so far I’m running all my EM-172 mics straight in on the 1/8″ jack on both recorders. The 05 had a slightly higher PiP voltage, but they were both under 3V, and certainly under 5V. I’d need to re-measure to confirm the numbers I quoted, though.

    You’re the second person I’ve run across who has wired two EM172s together for each channel. The other was Vicki Powys with her foam SASS array. When I built my SASS I only used a single mic per channel. It was only afterward that I saw why she’d built it using two. Unfortunately because of the way I built mine I can’t go back and retrofit it without doing a lot of re-work. So you can understand my relief when you said you can’t tell much of a difference! It may start to matter with higher supply voltages, but that’s just one more thing to test.

    Thanks for the pointer to Church Audio. I think I’m going to order one of their battery boxes and give it a go.

    Tom

  2. Thanks for writing this. I ran across your post some time back when I first got into sound. I built a couple of microphones around the EM-172 capsules, but haven’t built any with the Sharpie caps yet. (I still think that’s one of the most creative mic bodies I’ve seen. Built in clips!)

    Unfortunately I didn’t read carefully enough back then. I didn’t pay much attention to supply voltage until just recently. One of my recorders puts out 2.8V, and the other 2.7V. I took a look at the charts on the micbuilders group, and it looks like I could get a slight gain by supplying a higher voltage, and a pretty significant reduction in the noise floor (several dB!)

    I can build a battery box to feed my mics, but I’m curious what you’re using, if anything. Like you said, I’m impressed by what I can do with just the PiP supplied by my recorders. But if I can get rid of enough self-noise to record some quieter subjects, that would be pretty darned cool.

    Thanks again! I’m enjoying reading your blog.

    Tom

    • Hi Tom, thanks for the comments and questions. I don’t have a good answer for you. One person I know says a battery box helps, but he also uses EM-173 capsules (not EM-172). I asked the guy at Church Audio (church-audio.com), who makes a simple and elegant 9v battery box, and he said that the battery would add dynamic range (meaning you could record louder sounds without clipping), but would do nothing to help with noise or increase gain. So I admit to being somewhat confused on the issue. The batteries from Church Audio are so inexpensive that it’s probably worth testing anyway. What recorders are you using that put out those voltages? I think my Sony PCM-M10 puts out 2.3v, and I hope to upgrade to a Sony PCM-D100 soon, which is reported to put out closer to 5v. So at the moment, I’m running the mics straight into the recorder. I usually record with mics that have two capsules per mic, which is supposed to increase SNR a little. I don’t notice much difference between those and the pen cap mics (1 capsule per mic), which is probably related to the recorder voltage.

      Sorry I couldn’t be more help. If you come up with anything, please share!

      • I apologize for replying twice, but I’m trying two things out:

        First, I bought the non-ugly version of the Church Audio battery box (thanks again for the pointer!) I’ll give this a try as soon as it arrives and let you know how it goes.

        Second, I ran across a thread on the micbuilder group with a very simple circuit for wiring an EM172 into an XLR input with a 48v phantom supply. The thread ranged across a number of topics, but kept coming back to Richard Lee’s SimpleP48wm51.doc file, and his notes on adapting it to the EM172 capsule. I ordered parts off of Mouser for building a couple of these, and will try this route as well. The idea behind the circuit is to drop the 48v phantom power to the 5-10v the EM172 requires. I’ll let you know how this goes as well.

        Thanks again!

        Tom

          • The battery box from Church Audio still isn’t here, but my guess is that’s because I live out in the middle of the ocean and things do sometimes take a while. Meanwhile I converted one of my mics to phantom power. To make a long story short, all my numbers were off because the recorder I’m using has completely different amplifiers on the 1/8″ jack than on the XLR jacks, and the two have wildly different gain ranges. I was expecting to get a couple of dB improvement in sensitivity, but I wound up with over 20 because of the gain differences between the two amps. I tried testing the PiP mics and the P48 mic side by side, but I kept having to crank back the gain on the P48 mic so they’d have the same levels. That’s when it hit me: If I convert them all to P48 I can record quieter sources! I spent a couple of weekends converting all my other mics over to phantom power.

            I do have one unused EM172 capsule I can use to do side-by-side testing when the Church Audio box arrives, but given how well the phantom power mics are working it may not be high on my priority list. I can say that the circuit for the EM172 in SimpleP48wm61.doc on the micbuilder’s forum works great. It’s one capacitor and one resistor, so the soldering is relatively straightforward. I built mine into the XLR plugs. The resistor value in the document (120k) gave me an unloaded supply voltage of 9.6v, right around the sweet spot of the EM172.

            • Tom, Thanks so much for the update. Very interesting – 20 db gain! I’ll be interested to hear how the CA box does, but the 20 db is going to be hard to beat. I’ll update the page to link to directions for the P48 version. Are you using one capsule per channel?

            • Right now I’m using one capsule per channel. I’ve got two built into a pseudo-SASS and three others built as standalone mics for tree ears. At some point I’ll spring for more capsules and build a pair with two mics per channel, but for now this is working well.

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