Headphone Audio Tap for Boom Operator or Live Performance in ear Monitoring.
Pros: Ability to tap multiple audio sources (balanced line, mic and I've even used unbalanced guitar 1/4") transparently for headphone monitoring. Phantom power will pass through uninterrupted to your microphone. Two headphone channels for two people. Two concurrent sources that can be individually adjusted. 9v battery or external which I tried with a 12v battery brick.
Cons: The headphone out has some typical 1990's Shure headphone background noise/hiss but that is not introduced into the audio line you are tapping into.