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microphone circuit noise?

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Full Member level 3
Jun 19, 2001
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When I remove the microphone completely, I measure the noise at the input terminal of the OPAMP, the measurement is 1mv. And when the mic is on, it only adds 2-3 mV , therefore the noise is big for my circuit. I hear a lot of static.

How can I design the mic amp to have higher SNR? how many stage do I need?
I guess I need a good power regulator also.


Try to be more specific. Maybe upload the schematics of the input stage.
What kind of microphone you are using (dynamic)? What is input impedance of prestage?


The mic is an electret microphone.

The input stage is just a non-inverting amplifier with 100-200 gain. And I see a lot of circuit noise, even the mics are removed. (1mV ac noise).

should I do 2 stage? what is the logic behind using 2 stage? first one differential?
Where does the noise come from? from power supply? My ground is separated from digital ground.

Elektret mic has usually low output impedance because the buildin preamplifier. Power supply for mic must be well filtered.
When measuring noise input of the amplifier must be terminated correctly actually with output resistance of the mic. Input must not be opencircuited.

Make sure 1mV noise is not generated by osciloscope wires!!
If it's not, noise can come from a bad ground routing, you can put external wires from all your circuit grounds to ground of power capacitor, this can strongly reduce noise, same for VCC.

Incrementing number of stages, you will amplify the noise, not eliminated, you must put your attention in eliminating noise at first input of your circuit.

Normaly, electrek microphones has higher level of signal, maybe your microphone can be wrong.


power supply

I know the power supply must be one problem. anyway to put some components arround a regulator and reduce the noise by 40dB? I saw a circuit that uses an OPAMP and a PNP to construct a LPF, but wonder if I can do it by tweaking exising LDO.

my circuit is a simple electret mic biased with a 3K resistor at 3.3 V. followed by non-inverting OPAMP amp. will a differential setup help?

I will try with a battery, if that solves the problem, then it must be the power supply, if not, maybe the ground loop is dirty.

I wonder if there is existing circuit that implement this, this must be very common.


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