TL;DR: If you’re suffering from interference noise in your studio monitors, try using an external DAC. If you STILL keep hearing interference noises, don’t power the DAC from the PC but from a phone charger or something else, your power supply may be causing the interferences.
I recently purchased a pair of JBL LSR305 along with a JBL NanoPatch+ as a beefy volume knob. They sound ridiculously great when I connect them to my phone, but when I first hooked them to my computer (to the motherboard’s audio output) there was a lot of buzzing noise, that heavily (and I mean HEAVILY) increased whenever the graphics card was working hard, such as in games (Overwatch!).
I read a lot about this kind of noises. There are a lot of possible reasons, but the end result is that the analog signal coming from the PC is noisy. The analog signal that is fed to the speakers is built by converting a digital signal generated by the computer software or read from digital sources such as a CD to its analog equivalent. This transformation is done by a Digital-to-analog converter (DAC). If that transformation is done within a noisy environment, such as inside a computer case, the final analog signal may get affected.
A common suggestion is to use an external (as in, physically external) DAC, so that the conversion is done outside the PC case and no interference could ever affect the analog signal, so I bought the cheapest DAC I could find, this PROZOR DAC001. I connected the optical audio in, which is a pure digital signal coming from the PC, and the power cable, which is a 5V DC wire connected to a USB port, and… voilá, the noises were still there.
What. There was no way the digital signal could transport such interferences, so there was only another suspect left: the power cable. I moved it from a USB port to a phone charger and BOOM, no noise!
So there you got it, looks like my power supply is causing interferences inside the case AND through the USBs too. I guess it must be about to die. For the time being I’m gonna keep using the phone charger but I guess I’ll have to swap the power supply sooner or later. Hope it helps.