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Microphone preamplifier noise

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martintw

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Hello, I am using sim900 GSM module. To the module I would like to connect microphone over 10 meter long cable. If I use basic microphone preamp I hear noise when I make a call. If I disconnect preamp and module there is no noise. I think that is because ground connection. Than I decided to breadboard differental amplifier here is the schematic. It is better but I still hear more noise than speech. Do anyone know why this noise happens when I connect amplifier with the module and how would I get rid of it ? Here is pdf from simcom with some info on how to connect to mic input.

ps At the moment Im not connecting preamp over 10 meter cable I am using only 10 cm.

Thanks
 
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none of your mentioned attachments appear
any specs on your mic? not all mics are the same
any specs on the pre-amp?
does noise appear if you only use the mic and pre-amp?
 

Hello, I used electret microphone. If I connect it directly there is no noise and evryting works fine. Preamplifier that I used is just LM358N preamp for microphones. The noise appears when I connect module and preamp. When I disconnect preamp and module there is no noise on preamp even if I make a call. That is why I breadborded differential amplifier wich is better but noise is still there.

LM358 preamp

lm358_mic_amp1.png


Differential amplifier

42_circuit_1.jpg


PDF

**broken link removed**
 

Your attachments are almost completely hidden since they have faint green text to their links. Please post the actual documents HERE.

You show a fairly noisy video buffer amplifier with no voltage gain, not a low noise microphone preamp with plenty of voltage gain.
You did not describe the noise caused by the circuit. Low frequency hum or high frequency hiss?

The Chinese document you posted makes no sense. It says Line-input but shows an electret mic properly powered with DC, BUT it is driven by a signal generator. Why?
Does the module have a built-in mic preamp with plenty of voltage gain?
Are you using an electret mic?
 

If the microphone works well with direct connection, why are you using a gain of 100 preamplifier? It's simply too much gain.
 

Your preamp uses a noisy old LM358 opamp that is not designed to be used for audio and certainly not as a mic preamp.
It is inverting with an input resistance of only 1k ohms which cuts the signal from the electret mic to only 1/3rd.
It is low power so it produces awful-sounding crossover distortion.

We don't even know if the Chinese module needs a preamp. If it does need a preamp then use a non-inverting circuit with a high input resistance and a low noise audio opamp.
 

Module have built-in preamp with selectable gain, but over 10 m cable the signal gets too weak. I did record a noise and it is attached. Is it normal that this noise appears when I connect negative input MIC-N to gnd ? I tried to connect laptop headphone output to the module and the sound was without noise and clear.

Thanks for your help.
 

Attachments

  • Noise.rar
    286.2 KB · Views: 131

None of the schematics have a wire called "MIC-N" so we do not know what it is.

The sound is digital interference. Your preamp circuit has a 10k resistor R1 connected directly to VCC at the input of the preamp. It picks up digital noise from VCC and the preamp and module amplify it. R1 should be fed from an RC filter.
 

Bread board is not suitable at all , using it will not make solid ground beside the used wires to connect the components will add more noise.
Also use shielded cable for transferring your signal from mic to preamplifier and from the preamplifier to the module
 

The module has two differential inputs called MIC-P and MIC-N. I too tought that the noise is because breadboard and did build preamp on PCB but was the same story. I found another differential amplifier decided to breadboard it and it works the noise is gone finally. The sound was not high quality because of lm358 but the noise that I attached above is gone. Below is the circuit that I used what opamps sould I use for my needs or is there a better differential preamp for microphone ?

Thanks
 

Attachments

  • 4_circuit_1.jpg
    4_circuit_1.jpg
    24.7 KB · Views: 177

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