Passthrough not activated on Linux




I'm using OpenPHT on Linux Mint 18 with an audio output through HDMI but I can't activated the passthrough (it doesn't appeared in the audio ouput menu).
I'm using a HDMI -> HDMI Splitter -> Optical -> A/V Amplifier solution, because my PC doesn't have a Optical Output. The audio device is set to PULSE:Default
Have you an idea?