Getting opposite expected voltage with electret microphone

audioelectretmicrophone

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.

schematic

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?

Best Answer

If you look here you will see that the electret microphone secretly contains a FET ! It must because without it it is almost impossible to use such a microphone as it is a very small capacitor.

The circuit that you have should be OK but you can try lowering that 15 kohm resistor to 5.6 kohms. Don't worry too much about the DC level.

Do use a capacitor (1 uF) to connect to your audio input (unless that already has a coupling capacitor).

Due to the FET, measuring the DC resistance of the electret microphone is meaningless. In my experience they are also quite hard to break so my guess is that yours will still work.