Electronic – Connecting Electret Mic to MT8870 Decoder Module

The Op's question reminds of a tricky question paper :) . What I construde is;

• 1) 3.5mm Jack= O/P (looks like an I/p).
• 2) He wants to insert electret microphone into this jack .

But If he is listening to something with MP3 then the Jack is not an output device , he probably has an additional speaker somewhere ?? .If this is true then Jack is an I/P connector. The electret mic cannot be inserted directly, you need biasing as below .

It would be better to add a pre-amplification stage as below;

from: here

When you plug into the microphone , slowly adjust the gain of your opamp and O/P volume of your amplifier such that feedback is eleminated.

Line level output (.9Vpp) is standard (https://en.wikipedia.org/wiki/Line_level), but I have found that the output level on phones can vary across manufacturers. Line level is probably your best starting point though.

A voltage divider (2 resistors) would attenuate the level coming from the phone if you need it - though you also have the volume control buttons on the phone to contend with.

Another item you need to consider is the DC bias voltage coming from your module. From the schematic, it looks like a 2 pin ECM was connected. In this case, the DC bias is on the Mic + line, with the AC voltage coming back to the module from the transducer. Since you're replacing the ECM with a phone output, you should be sure to remove the DC component from that Mic + line. Depending on where the ADC is internally in your module, removing Rx might not help (you should measure to double check). If not, you may need to add a series capacitor to the Mic + line for DC blocking.