An amplifier without an analog input? I just realized I'm old.
Your pretty much only option is to buy a converter from analog audio to Toslink. That should cost you around 30-40 dollars, including the cables.
Added: The cable certifiedfunds
mentioned can only be used with special 3.5mm connectors that have both electrical and optical outputs. Non-Mac mp3 players are unlikely to have it; you will need a standalone converter like this: