Using the monitor output my PA Mixer, Yamaha EMX66M, to a Sterling S104H headphone amp via a TRS cable, I am only getting sound in one ear. The mixer is mono, so I assume the monitor out isn't capable of a dual-mono - or maybe my cables were just messed up (need to toy some more). Assuming this behavior is expected because it is a mono output, how can I rectify this?
It's a mono output jack, so needs a mono jack, or preferably TRS with bridge, if you're going to a stereo headphone amp.
A regular TRS > TRS will arrive at the headphone amp as one side of a 'stereo' feed, so will need bridging to provide dual mono. You can buy jack converters that will do this, or with a soldering iron, connect left & right, leaving ground as is.
Additional ref - Stereo and mono cables and jacks? What happens when you cross them?