SPDIF Output to Amp

Ok here is my setup:
- My SPDIF Output looks like a normal headphone jack
- A40 MixAmp, which I can connect to with either RCA (Red and White) or Coax (Orange) or TOSLink.
- A pair of Sennheiser HD555

My computer doesn't have Coax as a digital output nor TOSLink, so I'm left with this weird SPDIF Output.

I've tried putting a 3.5mm stereo plug to Red and white. It works in the headphone jack, but not in the SPDIF output. All I get is weird sounds and hissing. (I did change my computer settings so it uses the digital output). All I could find on the internet was that I needed a mono jack instead of a stereo jack. First, I don't know if it's true and second I can't find any cable 3.5mm mono male to RCA male.

Any help would be greatly appreciated.

Thanks in advance,
WarpedMonkey