I needed to find some quality, low-cost microphones to pair with my basic kit for field recording. Ideally, the mics would be small, with very low self-noise, and capable of assuming different geometries and configurations. Capturing a stereo field requires at least 2 microphones and some form acoustic separation. So, my mics should also be able to adapt to a variety of different strategies for this.
The EM172 from Primo is an omni-directional, electret capsule with high sensitivity and low self-noise. Its small and inexpensive enough to be built into a variety of form factors, and likely durable enough for field work. I ordered a small handful of unmatched capsules from FEL Communications in the UK; they are fantastic to work with, knowledgeable and responsive, and shipping to the US is pretty fast and economical.
The capsule will function fine with 2V – 3V, but a range of 5V – 8V is optimal for a variety of reasons (noise, sensitivity). It also has a built-in FET, so it just requires a powered connection and a typical preamp (a Scarlett 2i2 in my case). The Scarlett provides 48V phantom power thru its XLR connection, and can power the EM172 via a simple additional circuit as detailed by Tom Benedict (also the Micbuilders private group on Yahoo). The comments in Tom Benedict’s blog post contain a lot of information so its best to read through all of them. Zach Poff also has a great blog entry about building out the capsule to a functioning microphone.
An audio interface or recorder will have its own unique characteristics for impedance and voltage (phantom power); its an active component in the overall circuit with the capsule. My Scarlett 2i2 provides 7.0V across a connected capsule with a 47kohm resistor. I’ve reported additional voltages (with my 2i2) for different resistors below. The capsule voltage should be measured with each specific recorder or audio interface to determine the correct resistance required for that circuit. It takes quite a while for the resistance to reach equilibrium, and the lower the voltage the longer it takes. The capacitor is polarized; the negative lead should face the ground contact of the capsule, and the positive lead should connect to the audio interface. A capsule will sit nicely on the end of Neutrik NC3MXX connector, which can also easily house the capacitor and resistor.
Perhaps my favorite aspects of these capsules are their cost and their size, which are quite effective relative to alternatives. I feel better about doing risky things my microphones, because replacing them in the event of a catastrophe is easy and cost-efficient. I can replace an EM172 capsule a 119 times for the cost of a single Sennheiser MKH-series mic; that’s a lot of unintended miscalculation. But perhaps more advantageous is their small size. At roughly 10mm in diameter, they can easily be integrated and/or concealed in a wide variety “form factors” (sculpture, art installations, clothing, headphones, electronic gadgets, etc). And because of their low cost, they can be built into such objects in a dedicated and enduring way.
The following voltages were measured for across the connected capsule for various resistors, with a Scarlett 2i2 providing 47.8V of phantom power on each lead (pins 2 and 3). The maximum operating voltage of the capsule is 10V.
resistor value : voltage
75.0 kohm : 3.1V
60.4 kohm : 5.0V
56.2 kohm : 5.7V
47.0 kohm : 7.0V
36.5 kohm : 8.0V
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