To any sound experts - buzzing powered (2.1) PC speakers - tried everything - problem with earth?

[madsamjp]

I'm not sure if Tom's Hardware is the best place to post about an issue such as this. If not then please do recommend any alternative sites. Ok, so I'm tearing my hair out here trying to find a solution and I'd really appreciate some expert advice I have a pair of inexpensive Logitech 2.1 speakers. They are good enough for my setup and produce adequate sound quality. However, I notice that when I plug them into my PC (built myself) I get a quiet but annoyingly obvious humming sound coming from the satellites and the sub. It sounds very much like a ground loop buzz. So I've done some basic troubleshooting and this is what I've found:

• 
  -The buzzing stops when I unplug the audio jack from the PC
  -The buzzing persists when the audio jack is plugged in but the PC is powered down
  -The buzzing sound stops when connected to the PC with the PC AND the monitor unplugged from the mains (buzzing continues when monitor only is connected to mains)
  -The buzzing doesn't occur when I plug the speakers into my phone or MacBook Pro
  -This model of speakers (Z 313) is double insulated, meaning they aren't earthed
  -There is no buzzing sound when I connect (passive) headphones to my PC
  -The buzzing sound doesn't change when I adjust the volume on Windows or the speaker unit (constant amplitude regardless if volume is 0% or 100%)

So this is what I've managed to deduce from the information so far: despite being relatively inexpensive, the speakers themselves aren't faulty or the problem (they work fine connected to other devices), there isn't any external electromagnetic interference (i.e from GPU or monitor as they work fine connected other devices), my motherboard, on-board soundcard, and PSU isn't faulty (headphones work fine), and this cannot be a ground loop issue coz the speakers aren't grounded (I'm unsure about this, please correct me if I'm wrong. I opened the plug and there isn't an earth cable connected).

So I'm fairly sure it's some sort of interference with the earth in my building and not an issue with the speakers or the PC. I'd also like to note the buzzing sound persists when I plug them into the audio jack located on the back of monitor (just to try a different AC device for comparison). I completely disconnected the monitor from the PC but left it connected to the mains supply - still buzzing.

So my question is: does anyone have any ideas as to what the issue might be from the information I've provided? If it does indeed sound like some sort of interference from the earth, is there an (inexpensive) way to test for this? How would I go about doing this?

Thanks for reading and thanks in advance for any answers

 

[madsamjp]

Would a ground loop isolator help? I mean, it's a small 1:1 transformer right? So it would effectively sever the direct connection between the powered speakers and the PC thus isolating the sound signal from the earth - much like when plugged into a phone? That is of course if it is an earthing problem. How dangerous would it be to temporarily disconnect the earth pin on my PC and monitor to use as a diagnostic?

 

[americanaudiophile]

Yup that's what you need. This is the best one but not cheap
http/www.jensen-transformers.com/product/ci-2rr/
Easier than trying to find the AC ground loops. There might be more than one.
Yes you can disconnect the earth pin to check it.
http/web.mit.edu/~jhawk/tmp/p/EST016_Ground_Loops_handout.pdf