Phantom Power Calculator — Current & Voltage Drop

Phantom power (typically 48V DC) provides the polarising voltage for condenser microphone capsules and powers their internal preamplifier circuitry. It is sent from the preamp or mixing desk along the same XLR cable that carries the audio signal, using pins 2 and 3 equally referenced to pin 1 (ground).

Standard dynamic microphones are not affected by phantom power because it appears as equal voltage on both signal pins, cancelling out across the balanced coil. However, unbalanced dynamic microphones or those with faulty cables can be damaged. Ribbon microphones (passive) can be destroyed by phantom power if the cable is faulty or hot-plugged.

Barely. Cable resistance has a negligible effect on phantom DC: even 200 metres of thin AWG26 cable adds only about 54Ω of loop resistance — roughly 0.2V of drop at a typical 4mA draw — which is tiny compared with the 3.4kΩ effective feed resistance inside the preamp. Where long runs do matter is signal quality: cable capacitance causes high-frequency loss in the audio signal, which is a separate issue from phantom voltage.

Most small-diaphragm condensers draw 2-5mA. Large-diaphragm condensers typically draw 4-8mA. Active DI boxes draw 5-15mA. IEC 61938 specifies a maximum of 10mA per microphone (delivering up to about 170mW), fed through two matched 6.81kohm resistors. Those two resistors act in parallel for the DC phantom current — an effective 3.4kohm — so the short-circuit current is limited to roughly 14mA. Most mics draw 4mA or less.