Audible hiss when using toslink on my s50 pro +

I have an audible hiss when when no content is playing but only when I am using the Toslink input through my S50 pro+. Content plays normally but when it stops then my speakers have a low level noise that is pretty annoying.

The sound is present even when i lower the volume of the S50 pro+ to 0

The hiss is not present when I am streaming through the S50 pro+ directly.

I use the S50 pro+ as a preamplifier for two channel audio when I am using my LG projector.

Please suggest possible solutions and reasons for the issue.

Setup details:

Nvidia shield connected to projector through HDMI

LG HF85LA projector connected to S50 pro+ through optical

S50 pro+ connected to emotiva Basx A2 power amplifier

Everything powered by the same powerstrip

No antennas in close proximity

S50 pro+ used to attenuate volume