ok so here is a schematic of the whole thing.
it also contains some features i still haven't implemented, but that i would like on the final design, plus i've written some notes here and there to explain things better... 
i really hope this will somehow help understand what's wrong.
p.s. since it's my first creation ever i really appreciate general feedback or comments on it XD
dc42:
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.
i thought i had a switched power socket somewhere at home, but turns out that i don't.
tomorrow i'll buy one and try, and we'll see what happens
sequencer_schem.pdf (2.46 MB)