Noise problem when connected to Macbook

[nlecaude]

Hello,
I made a system with the Teensy 3 where it recieves midi data to control regular rgb led strips using pwm.
The teensy is plugged to rgb amplfiers using an uln2003 because they only work at 12v.
The computer where the teensy is plugged sends midi to the teensy and I also use the audio line out of the computer at the same time to send sound through an amp and speakers. Whenever I use the audio out of the computer with the teensy plugged in I get a high pitched noise through the speakers. I'm using the AGND pin on the teensy and I read in another thread that it's not a good idea to mix this with digital io. Could this be the reason I am hearing a high pitched noise ?
thanks !

 

[PaulStoffregen]

Usually these types of problems are called "ground loops".

Basically, when there's multiple ground paths, current unrelated to the analog signal flows through the analog cable ground wire. Because the wire has some resistance and inductance, a small voltage is created, which the amplifier in the speakers sees as a signal.

Ground loop problems can be tricky to resolve. Usually a good troubleshooting technique, if possible, involves running various parts of your system from batteries or an isolated power supply. You're looking for the place where current is unexpectedly flowing between devices, because they share a common ground.