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designing a basic mic

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Enzy

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Good Day,

was wondering if hobbyist would make a mic circuit for themselves?

does a mic require a circuit, is a mic just a speaker that would pass through mixers that have built in amplifiers for it?
 

Audioguru

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A microphone has a tiny output level of about 10mV when 10cm away from your mouth and you are talking at an ordinary conversation level. A preamplifier with a gain of about 100 to 300 is needed for the preamp to drive a recorder or a power amplifier.

There are a few different types of microphones:
1) Dynamic, has a coil and magnet like a little loudspeaker.
2) Electret condenser, that has a Jfet impedance converter inside that must be powered with about 400uA at a few volts.

A hobbyist can connect a mic to the input of an LM386 little power amplifier set to a gain of 200 as shown on its datasheet. The cable between the mic and preamp or LM386 must be shielded audio cable to shield away mains electricity hum and other interference.
 

BradtheRad

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You can talk into an ordinary speaker and it acts as a microphone.

If you wish to experiment you can place a coil of wire next to a magnetized plate (or magnet attached to sheet steel). Sound makes the plate vibrate, which in turn induces AC current in the coil.

Another home-brew mic is made by filling space between two metal plates with carbon granules. Apply small voltage. Vibrations of the plate cause the carbon to shift and change its electrical response, thus producing a signal.
 

Audioguru

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You can talk into an ordinary speaker and it acts as a microphone.
I think a speaker makes a poor microphone because there is nothing to damp its resonance so the sound is very boomy. The input impedance of a mic preamp is much, much higher than the 0.08 ohms or less output impedance of a modern amplifier which damps the resonance.
 

BradtheRad

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I think a speaker makes a poor microphone.
You're right of course. It's just a fun thing to try if we like to see the principle at work (moving coil past a magnet generates electrical signal).

Also to observe how you can create a waveform going toward positive or negative polarity, depending on what direction you puff or tap on the cone.
 

c_mitra

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I think a speaker makes a poor microphone because there is nothing to damp its resonance...
True, but.

Smaller the speaker, the natural frequency goes up. 1" speaker (or smaller) with a 16 or 32 ohm coil is quite usable as a mike. Good thing is that they do not need power but the output is smaller.

As you have mentioned, dynamic microphones are basically speakers in reverse.
 

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