Interference in new audio setup - how do I get rid of it? Possible ground loop?

Question

I just bought M-Audio BX5 monitors and connected them to my Creative Sound Blaster X-Fi Titanium sound card with a stereo TRS to an unbalanced TRS (mono) in each speaker.

The sound is fine, but unlike with my previous setup (which used some cheap 40$ Genius active speaker setup with ungrounded prongs) there's a horrible hum in the speakers, resulting (I assume) from electrical interference inside the PC.

But it's weird in the sense that I've only encountered this with cheap motherboards and onboard sound cards, not fairly good dedicated sound cards. I mean the hum is even worse on the dedicated card, but it's also just as bad on an external HyperX DAC connected to a USB hub with different power source (it's connected through a display or USB charger, both have the same interference pattern).

It's the kind of hum that changes based on how the mouse moves and such, which really does sound as internal interference, and I can confirm that the monitors themselves are fine as connecting them to a ungrounded device like a phone clears the sound.

I also tried grounding the speakers directly to the PC case and while that did make it a little bit better, the hum was still very much there and noticeable.

Could this possibly be a ground loop, and if so how would I go about solving that? I tried connecting the speakers to different outlets, to the same extension cord, etc. and it's still just bad.

Thanks for any ideas.

Answer 1

It was caused by the connection between PC and speakers. I used a 3.5mm jack to dual mono (unbalanced) RS Y-cable, which connected the negative and ground in the speaker together, causing the (otherwise well grounded signal from PC) to be "dirty" grounded straight to earth through the speakers, amplifying the interference.

The fix was as simple as putting a piece of plastic tape around the part of the RS connector that connects to ground internally, separating the negative and ground signals (thus making the speaker's ground separate from everything else).

To further improve quality it would be possible to also add a connection between the speakers' ground and the PC's ground/case/whatever, mitigating any other possible interference. But I found out it's unnecessary unless I want to push the amp in the speakers to the limits. And even then it's really audible only in absolute silence.