tommyrosa:
i just found out that when i power the arduino sequencer using usb from my pc, if the computer is charging (attached to an external power supply) the synth starts going crazy, the same way it would if the arduino itself were powered from an external power supply.
simply disconnecting the laptop from it's external source solves the issue.
That suggests to me that the problem IS a ground loop. Laptop power supplies normally have a connection between the ground pin of the socket and the negative side of the output.
To verify this, try plugging the laptop power supply into a switched power socket. If the problem is caused by a ground loop, then it will occur whenever the laptop power supply is plugged into the mains power socket, whether or not the socket is switched on. But the problem will go away when you unplug the laptop power supply from the mains power socket.
Once you confirm that the problem is a ground loop, here are some things you can try:
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Run the synth and the Arduino from the same mains power socket, by using a 2-way adapter or a socket strip.
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Instead of connecting Arduino ground directly to synth ground through the cable that connects them, put a 100 ohm resistor in series with this ground connection. This will prevent large currents flowing through the ground loop.