1/8 plug to rca cable problem

I am trying to run 1/8 from computer with an adapter, it is an adapter that is 1/8 - female rca from Radio Shack. the problem is when I attach it to my Counterpoint SA 2000 preamp I am getting a hum, I am guessing the adapter is using common ground that is causing this. Any suggestions on another cable to use, 1/8 - male rca ???

Thanks