I've got a few electret microphones and I tried to make this circuit:

enter image description here

It didn't amplify my sound, it continuously buzzed. Without mic it did same thing. So I probed mic and I saw 0.000v. Also 0mA (shorted).

I tested 5 microphones sitting next to each other, they all had same results. There was a neodymium magnet about 3-4cm next to them. Are they broken? Or is there something else wrong?

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    \$\begingroup\$ Where did you get that schematic? \$\endgroup\$ – Marla Jul 17 '18 at 21:14
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    \$\begingroup\$ Unless I'm missing something obvious or mystical I can't see how that circuit would ever work. \$\endgroup\$ – brhans Jul 17 '18 at 21:14
  • \$\begingroup\$ @Marla Google images :/ is it normal for the microphones to show 0mV and 0mA? \$\endgroup\$ – sh4dowb Jul 17 '18 at 21:17
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    \$\begingroup\$ I just viewed the "circuits today" article. The circuit is for a carbon microphone. \$\endgroup\$ – Marla Jul 17 '18 at 21:20
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    \$\begingroup\$ Holy crap, that schematic is a real blast from the past. The HEP230 is an old Motorola germanium power transistor -- it probably hasn't been made since at least the 1980s. Completely obsolete. \$\endgroup\$ – duskwuff -inactive- Jul 17 '18 at 22:02

Electret microphones have an internal amplifier that requires a bias voltage at a few mA (or less) current; they have a moderately high impedance. Carbon button microphones are much lower impedance. Note that though a carbon mic is nonpolarized, i.e. it doesn't matter which way it's connected, electrets may require connection of a specific line to positive. Some electrets have a three-wire connection, one for ground, one for V+ and a third for output.

The schematic below should work with most two-terminal electret mics to drive a standard mic input on a PA, but you'd need high-impedance headphones or anther stage of amplification to drive a speaker.

Electret mic amp from http://www.diyaudio.com/forums/instruments-and-amps/188392-buffer-preamp-circuit-electret-mic.html, modified


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