Trying to track down feedback

Ok i was running my equipment thru a general setup test when I had something happen that I haven't encountered before *mind you I'm relatively new to all this so*

I have my unpowered mixer hooked up to an amplifier which in turn connects to two speakers with 1/4" inputs.

When i connected my laptop's sound to the mixer thru rca inputs via a simple USB sound card, I got a loud humming noise.

Then I plugged in my backup 1/8 to 2 rca inputs cord into my mixer and plugged it into the line out jack on my computer. No hum.

When i plugged the usb card back in, same thing. But then i plugged an extra set of rca cables into the 'in' jacks on the usb. Hum stopped instantly.

Any thoughts?

-James

Thanks Jeff problem cleared up instantly when I decided to throw away all my old cables and get brand new, much better quality cables. Cost me an arm and a leg, but i figure these new ones will last way longer and probably eliminate guesswork.

-James

update:actually i had purchased the cables before I posted the problem I just wasn't sure if I needed to use/open them.. heheh