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# Power electret microphone from 48V DC

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#### Wesli_1

##### New Member
I am trying to get some cheap computer headsets to work with a theatre intercom.

The headsets have electret microphones, but the intercom is designed to work with dynamic microphones. The intercom is a "Clearcom" type (manufactured by someone else but very similar)

I have found instructions on how to do this using a 9V battery to power the electret (**broken link removed**) but I am wondering if it possible to use the 48V DC power supply already present in the intercom system (NB it is NOT phantom power, just plain 48VDC).

These are the issues that I need some help with:

1. How do I keep the voltage on the electret under 10V? I haven't been able to find any info on how to select a load resistor value for an arbitrary voltage.
2. If a high value load resistor is used, the output impedance of the mic may be too high to drive the input of the intercom...
3. How to keep the signal level down to a reasonable level?

A dynamic mic has a low impedance and its preamp input has a low impedance.
an electret mic and the resistor that powers it has a fairly high impedance and its preamp has a high input impedance.

You might need an impedance converter (an emitter-follower) so that you can use an electret mic in your circuit. Try it without the emitter-follower.

Three resistors make a voltage divider to power the mic and the emitter-follower.

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Thanks audioguru

Works with just the voltage divider, R1=47k and R2=2.2k, so output impedance = 47||2.2 = 2.1k (please correct me if this is not correct).

DC Voltage measured over the electret is approx 1.6V - less than expected due to the high impedance of the voltage divider and relatively low impedance of the electret but it still seems to work just fine.

The only problem is audio feedback when the volume knob is turned up more than halfway (very painful the first time) - not sure if this is because the mic signal is too high, or the cheap computer headsets are not designed for isolation between the mic and the earpiece.

And the 10uF electro decoupling capacitor won't fit in the backshell of the XLR connector... might try and find a smaller one, tantalum etc.

A cheap computer headset does not expect feedback to occur because the mic has nothing to do with the ears. The intercom might have some mic audio fed to the ears to make "sidetone" so that you hear how loud you are speaking. But then the poor isolation in the cheap headset causes feedback.

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