Static from computer to amp, ground seems OK?

[oberlin]

I have a little Fender Frontman 15G practice amp with a stereo RCA aux in port. I'd like to be able to play along with youtube videos. When I connect the aux in to my phone, no problems, sound comes through clearly. When I plug it in to my desktop computer, I get the sound but a loud background hum as well.

I tried it with the headphone jack of a stereo system as well, and got a similar buzz. Seems that anything that's battery powered works fine, anything on AC generates the hum.

I don't know much about electronics, but I did try to check for grounding problems. I used a multimeter to check for connectivity between the chassis of my computer and the amp enclosure while both were plugged in, and found what seems to be a solid connection. I'm guessing that means that it's not a grounding problem?

I also double checked that signal ground and electrical ground were connected in the amp, and they were (connectivity between ground plug, the guitar input jack, and the sleeve on the RCA aux in jacks). I read on the web that these Frontman 15g amps can have that problem. However, this seems to be a newer model without that problem. I also think it's a newer model because the schematics available on the web for this amp don't have an aux in jack, whereas the 15Gs in the stores now do.

I'm wondering whether it's some kind of line noise and I should buy some sort of filtering power bar? That doesn't seem quite right, though, because there's no hum when the amp is just plugged into a guitar, only another mains-powered device.

Any insight would be appreciated!

 

[americanaudiophile]

A ground loop is when you have 2 or more ground connections between equipment not a lack of ground connections. You will have a common ground connecting the audio out of the PC and another the AC grounds are also common. Try using a 3-2 prong ground lifter on the amp. If this works you can substitute a 2 prong AC cord. You could also use an audio ground loop isolator between them. This is better practice from a safety viewpoint.