How to connect an external mic/synth/guitar to the Android/iOS device?
Русская версия здесь

There are several ways.
  • For high quality sound - use some external USB audio interface.
  • Use headphone jack of your device (3.5mm TRRS, 4pin) with adapter (there are many different ones; just search: headset mic splitter adapter).
  • Build your own adapter for headphone jack (3.5mm TRRS, 4pin). The circuit diagram is below. Note that the GND and the MIC pins may be swapped on some non-iOS devices with OMTP standard!