My daughter connected her desktop computer to her Stereo Amplifier just 2 feet away from Computer. She connects audio cable from headphone jack output of the soundcard and connects to Aux Input of stereo amplifier. But the 60hz sound or hum sound was also amplified together with the music.
Since , I know that the computer was using 3 prong ac plug and the ac outlet was properly grounded. But the stereo have 2 prong ac plug. This means ground casing of Stereo was floating . So, I connect 1 meter awg #14 wire from metal casing of stereo and casing of computer. So that both casing will have 0V potential difference. Humming sound therefore was reduced to minimum.