Good Info: How To Connect your Computer to your Stereo...

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Tips
This process can be significantly simplified by purchasing a cable of sufficient length that has a male 1/8" mini jack (headphones style) connector on one end and two male RCA connectors on the other end. This decreases the number of components required and also saves you a couple bucks.

Warnings
Make sure to start with the lowest volume on both systems or you could damage your speakers.
While it is not necessary on modern systems, to be safe, turn off the computer and stereo until you're done connecting the cables.

Things You'll Need

RCA Cable