I recently bought a pair of KRK Rokit 5 G3 speakers. Its a pretty big upgrade from my previous speakers. Currently, I have them connected using a RCA to 3.5mm cable. When I have it connected to my phone, it sounds great. But when I try to connect it to my desktop, there is this horrible hiss sound. Even when i'm not playing anything, the sound remains there. Do I need to get an audio interface to get rid of the horrible sound? (BTW, There is no hiss when I have it connected to my phone so I know it was something to do with my desktop.)
That horrible hiss sound is system noise. Realtek audio inputs on PC's and laptops are cheap and the circuits are not well shielded. So noise from the electronic components finds its way in to the audio path. Either buy an interface or a better soundcard like a soundblaster if it's playback.
If you are indeed using these monitor speakers as studio monitors, then you should 100% get a properly shielded, balanced, professional-leveled system. You won't have to pay much if you don't need any inputs or many outputs. Studio monitors like KRKs aren't meant to be used with on-board sound, especially not the output from a laptop.