On December 26, 2018 at 13:16, Fins said...
First, for the readers, let’s clarrify that there is actually only one emitter being used. It has dual 3.5mm male plugs. I didn’t know these existed.
That's my fault. I didn't check the model number (chagrin) to realize that this is two feeds to one LED, and it sounded like he was talking about two LEDs. And then, wrong, I went from there....
A crucial thing about the actual product is that the two inputs share a common ground. If the AMX and the Xantech system have no ground interconnection, fine. If their grounds are already connected, probably fine. If there's somehow a voltage difference between the grounds of the two systems, then problem.
Look at and think about, maybe even measure resistance, between the grounds of the two IR outputs. I just remembered, too, Xantech outputs have always been voltage to ground, while a dozen years or so ago, Niles LED driving circuits used open-collector circuitry, so the hot signal going into an LED would be power supply voltage; the "ground" of the LED would be connected to ground through a resistor. If the AMX output is open-collector, connecting the Xantech should turn the AMX LED on HARD. Again, look with your cell phone.
The suggestions to use two LEDs should solve the problem if it's ground-related.