Powering a MEMS Mic For a Wired IEM

[AdrianW]

I often play unplugged (guitar and vocal) in some less than optimal rooms. Because of some hearing loss I've had difficulty hearing myself in those situations. As a solution I've wired and attached a small MEMS microphone overhanging the upper bout of the guitar and used it to monitor myself on a single mono earbud. It works quite well, picking up both guitar and vocal. https://www.sparkfun.com/products/18011 Unlike an electret mic these do NOT require bias plug-in voltage, running on 3v directly to a terminal on the board.

Currently, I'm amplifying the signal with devices intended for mics that use plug-in power, either with a player/recorder (in this case an old minidisc or a Zoom H2n) or a chain running to an electret mic battery box and then to a Behringer P2 headphone amp. While these seem to work they still provide bias voltage. The signal is very hot and requires minimum gain. The battery box is nothing but a resister with a capacitor. The bias voltage and audio share the same line, with the voltage filtered out by the capacitor on output to the amp. My question, I guess, is if this bias voltage to a mic that doesn't need it is a problem or if there is a simple modification I can make to the battery box to block it? Is it as simple as flipping the capacitor? Thanks.

[Hugh Robjohns]

You'll need to check the specs for your specific MEMS capsule, but most of the ones I've seen have a built-in output driver, with the output being internally biased to around half the vcc supply voltage (ie. 1.5V).

So normally the output would be taken through a DC block capacitor of a 1 to 50uF (the exact value depending on the output impedance and required LF corner frequency.

In a standalone design you'd put the +ve side of the cap to the MEMS, with the output feeding a buffer opamp or unbalanced mic pre but, in your case with plug-in power on the output line, you should really use either a non-polarised cap, or two caps back to back (-ve to -ve) with a grounding resistor in the middle.... remembering that the total capacitance of a series pair is half of each cap value.

[AdrianW]

Thank you for this.