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I'd like to connect the audio output of an Electret Microphone to the audio input of an MT8870 decoder module, which uses a 3.5mm stereo cable.

I plan to power the mic at GND and VCC pins and connect the AUD signal to the right and left channel of one of the jack plugs on the stereo cable.

The mic has gain of ~60V/V and I'm not sure if this would be ok or too loud for the input of the MT8870. Can you please advise? Thanks.

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I plan to power the mic at GND and VCC pins and connect the AUD signal to the right and left channel of one of the jack plugs on the stereo cable.

First things first; it's not a stereo input; it's a differential input that the MT8870 uses but, it can be wired single ended.

I'd interface like this: -

enter image description here

The attenuator inside the red square is something I've added that you can easily make (and adjust). The two resistors could be a potentiometer (circa 10 kohm) and the capacitor will be fine with a value 100 nF for DTMF signal frequencies.

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  • \$\begingroup\$ Thank you very much Andy aka. Can you tell me if I'm understanding the attenuator correctly: The mic's AUD is connected to the 100nF capacitor, which in turn is connected to a voltage divider that goes to common gnd. The output of the voltage divider is connected to the left and right channels of the jack plug. Or, if I use a 10k pot - The mic's AUD is connected to the 100nF capacitor, which in turn is connected to pot's Vin terminal (the other terminal goes to common gnd). Vout (the wiper) of the pot is connected to the left and right channels of the jack plug. Is this correct? \$\endgroup\$ – somers Feb 12 '18 at 15:57
  • \$\begingroup\$ Perfectly worded. \$\endgroup\$ – Andy aka Feb 12 '18 at 16:10

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