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I would like to make a circuit that takes an electret microphone as an input and the output will be the sound recorded by the microphone with amplification.

I've built the circuit with one amplifier (Noisy Cricket stereo amplifier) connected to the microphone but nothing on the headphone. When I disconnect the microphone and I replace it by an audio output (from my phone, for example) the headphone works pretty well.

Any one can help me to understand why it doesn't work with the electret microphone?

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  • \$\begingroup\$ The electret microphone gives a smaller signal than an audio output. How did you connect the microphone? \$\endgroup\$
    – ocrdu
    Jun 15, 2023 at 8:28

3 Answers 3

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It's probable that the electret microphone has not been powered up.

Here's how it's done.

1. Two-terminal electret microphone

The 2-terminal electret microphone has an inbuilt FET. The resistor and output coupling capacitor are required to be connected externally.

enter image description here

2. Three-terminal electret microphone

The 3-terminal electret microphone has an inbuilt FET and resistor. Only the output coupling capacitor is required to be connected externally.

enter image description here

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The "Noisy Cricket" is intended for line level inputs, not microphone inputs.

"Line level" means a signal with (approximately) 0.4V peak.

An electret microphone will usually have an output voltage of a few tens of millivolts (0.03V, perhaps.) That is too low for your amplifier to reach its full volume.

The bigger problem is that an electret microphone requires a small DC voltage to work. The "Noisy Cricket" does not provide that voltage to the microphone.

The Wikipedia electret microphone page shows this diagramas an example of how to provide power to an electret microphone:

enter image description here

Typical values would be 2.2k ohm for the resistor and 4.7uF for the capacitor. The voltage (V+) would be anything from 3 to 5 V.

If you add the biasing circuit to your amplifier and microphone then you should get sound out of the speaker, though it may not be very loud.

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  • \$\begingroup\$ "an output voltage of a few tens of milliamperes" ... did you mean millivolts? \$\endgroup\$
    – jonathanjo
    Jun 15, 2023 at 9:26
  • \$\begingroup\$ @jonathanjo: Fixed. \$\endgroup\$
    – JRE
    Jun 15, 2023 at 9:34
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Not much is said about the inputs for that amplifier board. So the only assumption is line level.

Note: Line level may mean different things depending on context. Pros tend to use \$\text{dBu}\$, which doesn't specify the load being driven, while consumer equipment uses \$\text{dBV}\$ (with cap-case V), which implies a default \$600\:\Omega\$ load (memory serving.) Probably doesn't much matter here. Just recollection.

This likely means that board doesn't provide the necessary power source needed by most electret microphones. If I'm right, then you need a pre-amplifier for the electret. Assuming you are using \$+5\:\text{V}\$ at this time, then perhaps this:

schematic

simulate this circuit – Schematic created using CircuitLab

Lacking more detail about the electret itself, this bootstrapped design should be somewhere in the right ballpark, I think.

If you do try it, let me know how it goes.

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  • \$\begingroup\$ If one is limited to E12-valued resistors, would it be possible to replace \$R_3 = 7.5 \: \text{k}\Omega\$ with a series connection of a \$2.7 \: \text{k}\Omega\$ and a \$4.7 \: \text{k}\Omega\$ resistor, or would the addition of the auxiliary resistor alter the function of the circuit? \$\endgroup\$
    – Carl
    Jul 3, 2023 at 11:31
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    \$\begingroup\$ @Carl The sum of those two is very close. The circuit isn't that sensitive. So the substitution is fine. \$\endgroup\$ Jul 3, 2023 at 11:50

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