Back last century working for Peavey I designed an active direct box (The EDB-1). I used a bifet opamp front end (TL072). I grabbed power from the +48V phantom supply, and I used an internal 9V battery.
You need to draw current from both pin2 and pin 3 common mode to not degrade the impedance balance and maintain good CMRR.
JR
PS: I used an LED but don't ask me to cite the schematic from memory, that was last century
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