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.
so i tried to do what you suggested to see if it was a ground loop, but turns out that when the socket is off, the problem on the synth disappeares. so it's not a ground loop.
i was also thinking that if we were not to find a solution, i could run the whole sequencer from batteries. i already tried to power the arduino using a 9v battery and it gave no problems at all.
i would prefer the battery to be rechargeable trough wallwart, without having to take it out every time.
and i wouldn't use the sequencer when it's charging, so it wouldn't create any problems synthwise...i could live with that.
what do you guys think?is it doable? is it worth it?
A few comments on the schematic:-
1) You will never read anything from the rotatory encoder because the arduino inputs go directly to ground.
2) There is no pull up on A15 so you will not be able to read that switch state once it is not connected to ground.
3) S18 appears to connect the CV to the Gate out, or Gate Out1 - is this correct?
4) Not sure why R55 is fitted, it doesn't seem to be doing much.
5) You can get + / - 12V if you have an AC output wall wart and half wave rectify the signal for each rail. Although this seems to be a lot of fuss for just one op amp. Maybe you could bias the op amp's ground to 2.5V but I haven't looked at the data sheet to see if that amp can work off +/- 2.5V or a 5V single rail.
thanks for your comments. i fixed the problems on the schematic (i actually wired the encoder right on the arduino, and it works great!)
and sorry for the late answer