iMac optical output and toslink adapter problem

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I have an intel iMac (white 17" model) with optical outputs. I have a 15 feet toslink cable and a 3 feet toslink cable with an adapter for the minijack's iMac output.

The 15 feet cable works well without the adapter and doesn't work with the adapter. The 3 feet cable works with or without the adapter. Why the adapter doesn't work on my long cable???

Does anybody uses and adapter that works???