the arduino is power truh usb from a reguler usb phone charger. (note it’s actully a nodemcu esp8266 esp-12e)
how is it recomended to solve this ground problem ?
Personally, I would use an optocoupler, because I have had some time to play with them and understand how they work.
Google for an optocoupler tutorial. One thing I found confusing is the optocouplers have a ‘current ratio’ which is not explained very well. Basically the output current is a multiple of the input. Usually that mutiple is quite small, in the range 50% to 200%. But the Arduino needs nano-amps input current so any current ratio will work for you.
You will need an optocoupler and one resistor to control the current through the input side of the optocoupler. On the output side, you can use a resistor or the INPUT_PULLUP pin mode.
saf[e]ty for my amplifier is super importan[t] so my question is how w
ut[ould] you solve this situation?
The amplifier probably has pretty good protection on that output. Shorting it to ground or even -12V will probably not damage it. The Arduino (or ESP8266) has no built-in protection and must have protection added by you.
ay not use an current limiting resistor to limit the current to like 5 ma will?
Because the top resistor of the voltage divider is a very good current limiting resistor. You don’t need to add any more.