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I have two balanced outs of an interface I want to use as L and R channels on a pair of headphones. I know the single stereo jack won't carry a balanced signal to the headphones, but I want to make sure I don't damage any of the equipment or get weird effects; I am a bit paranoid after reading about monoing stereo signals: http://www.rane.com/note109.html
Can I just wire sig+ and sig- from each channel to respective lead on the headphone jack? Do I need any resistors anywhere?
The first problem is that the balanced outputs won't provide enough signal to drive a pair of headphones unless they are 400 ohms or so. But most headphones these days are somewhere between 32 and 80 ohms and the balanced output will struggle to feed them properly.
As to how best to combine them to feed a pair of headphones it depends how the output circuitry is configured.
Take the +ve from each output and wire to the tip and ring of the headphone socket. Then take the screen from the output and wire to the sleeve of the headphones so you leave the -ve unconnected.
Do as option 1 but take the -ve of both outputs and add to the sleeve of the headphones
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The safest, simplest, and most flexible option would simply be to get an inexpensive headphone amp that takes line level audio inputs. I also always tend to consider the soldering solution first, but often realize it's ultimately more pain than gain.