Phantom power delivers DC polarizing voltage to a condenser mic over the two balanced signal conductors

Condenser microphones need a DC voltage to polarize the capsule and run their built-in impedance-converting amplifier. Phantom power supplies this from the console or preamp by placing the same positive DC voltage (commonly 48 V, though the standard permits a range) equally on both signal conductors of a balanced line, with the shield as return. Because the voltage is identical (common-mode) on hot and cold, it is invisible to the differential input and to balanced dynamic mics, which ignore it. One cable thus carries both audio and the mic’s power. Cautions: unbalanced connections and some ribbon mics can be damaged by phantom power, so switch it off before patching such devices, and mute channels when toggling phantom to avoid loud transients.

Examples

An AKG C414 condenser plugged into a channel with the 48 V phantom switch on receives the same 48 V on both pins 2 and 3 of the XLR. A balanced dynamic SM58 on the same phantom-enabled channel is unaffected because the common-mode voltage cancels at the differential input.

Assessment

Why can a balanced dynamic microphone share a phantom-powered input without harm, while an unbalanced source might be damaged? At what point in patching should phantom power be switched off?