# Getting opposite expected voltage with electret microphone

I am trying to build an electret microphone amplifier circuit run from a 9V battery. The mic doesn't have a part number on it, so I have been making assumptions based on similar datasheets. Most have impedance measured at 2.2k, but I have measured mine as 1.2k. I'm assuming it will draw the same max current, 0.5 mA, and chose a resistor value of 15k. With 0.5 mA running through it, theoretically there should be a 7.5V drop across the resistor, giving a Vmic of 1.5V and powering it.

simulate this circuit – Schematic created using CircuitLab

However, when I built this up, I was getting 7.5V across the Microphone, and 1.5V across the resistor, which I cannot figure out. I'm guessing it's not a voltage divider, and the mic is acting more like a cap. Is it not an electret microphone, or am I missing something?

• You may well have killed it. Next time use a lower voltage so the applied potential is less variable. And couple your audio out through a capacitor. Dec 14, 2015 at 20:20
• see, for instance, wildlife-sound.org/equipment/newcomersguide/pip.html figure 3. Dec 14, 2015 at 20:25
• I'm thinking it must be blown, since that's the only reason I can think the voltage would be like that. Seems odd though, since most datasheets claim the source voltage can go as high as 10Vdc. Dec 14, 2015 at 21:01
• Proof of pudding, etc. - please stick in a coupling cap, and then see if you get sound out or not... I suppose the other dumb question would be did you find and follow the often a bit subtle polarity markings on the capsule...? electronics.stackexchange.com/questions/145718/… - but you have no data sheet... Dec 14, 2015 at 23:19