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I want to use a CMC-5042PF-AC microphone (datasheet: http://www.cui.com/product/resource/digikeypdf/cmc-5042pf-ac.pdf) to send the type of audio signal a component would expect on an RCA line (line level I am guessing). It's going to go to a wireless video/audio transmitter, which would normally except a line level signal form an audio source such as a security camera.

Here is the schematic for the mic:

Mic Schematic

I am connecting it to 5V power. From what I understand, the R1 resistor would determine the sound level, so I can play around with that to figure out what works best. But what about the capacitor? I can't find in the datasheet what kind of capacitor to use. I have seen people in other schematics use a 1uF cap. Is that what I should do? Or is there a formula I need to use to figure out the cap I need. Also, does it matter if it's a ceramic cap?

Thanks for any hints.

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  • \$\begingroup\$ It's just the normal RC frequency formula. \$\endgroup\$ – Ignacio Vazquez-Abrams May 3 '16 at 4:03
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R1 (RL in your circuit) determines the microphone gain and the output impedance of the microphone. Basically the output impedance is RL. If RL is (say) 2k2 then the amplifier that C connects ought to have an input impedance (resistance) => 22k. If it is 22k then C and the 22k form a high pass filter limiting the low frequencies and, at a frequency of \$\dfrac{1}{2\pi RC}\$, lower frequencies progressively attenuate more and more.

For 1uF and 22k the frequency is 7.23 Hz i.e. OK for audio - frequencies lower than this will get progressively more attenuated. A 100 nF capacitor would be fine when using the microphone for speech because the cut-off frequency would be 72 Hz.

I'd use a ceramic capacitor - probably X7R type.

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Thanks to the comment I found a calculator online:

http://www.v-cap.com/coupling-capacitor-calculator.php

It uses RC frequency formula to calculate the optimal cap based on impedance and frequency. In my case it was 4.7uF.

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