> might be cheaper alternatives in the works to the ground loop isolation certainty which transformers provide.
As JDB asked: "What kind of voltage will be seen across the barrier? DC? A few volts, or a few kV?"
Inside one studio you better have less than a Volt or so ground-difference. Plain opamp-y differential inputs can do well for a few pennies.
In my old joint, from stage to booth there was 1V-6V ground potential. I tested both a 2-opamp diff-in and a transformer. Both worked fine. I ended up with the transformer as primary input so I could also run a switch on the phantom circuit. Also because I could not explain why the ground-difference was as large as it was, and feared it could get worse.
More than 10V-5V of ground-diff, simple opamp tricks will clip. There's fancier tricks, but if you don't need DC coupled, a transformer is just too easy.
Single-ended LED-optocoupler is, as Abbey says, barely radio quality. It can be improved with complication; hiss is still problematic. I fiddled with it long ago and lost interest.
