Phantom Power supply for Electret mic
You have roughly the right idea, but you should use a resistor divider instead of a single resistor to bring down the voltage.  With your setup, the microphone is still subjected to high voltage, depending on how much current it draws.

You say you want 2.5 V, but not at what impedance the mic wants to see.  I'll use a target of 3 k&Omega;, which most electrets will be happy with.  You need to check the datasheet of your specific mic.



The two relevant formulas are what voltage is applied to the mic and the impedance presented to the mic:

&nbsp;&nbsp;Vmic = (R2 / (R1 + R2)) PWR = 2.5 V

&nbsp;&nbsp;Vmic impedance = R1 // R2 = 3 k&Omega;

Now you have two unknowns and two independent equations.  Solve for R1 and R2.