What is phantom power and why do condenser microphones need it?

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).

Can phantom power damage dynamic microphones?

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.

Does cable length affect phantom power voltage?

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.

How much current does a typical condenser microphone draw?

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.

Phantom Power Calculator — Free Recording Tool

How We Calculate This

Voltage Drop Calculation

Voltage drop = Current (A) × (Effective feed resistance + Cable resistance)

Phantom power is fed through two matched resistors (6.81kΩ for P48, 1.2kΩ for P24, 680Ω for P12 per IEC 61938), one on pin 2 and one on pin 3. The microphone draws current symmetrically from both signal pins, so for the DC phantom current those two resistors act in parallel — an effective 3.4kΩ for a standard 6.8kΩ pair. The terminal voltage at the microphone is the supply voltage minus the drop across that effective feed resistance and the cable resistance.

Note that the IEC 61938 figure of 48V ±4V (44–52V) is the tolerance on the open-circuit supply voltage, not a requirement at the microphone terminals. Under load the terminal voltage is always lower — at the standard's 10mA optimum a P48 rail delivers only about 14V — and most condensers run reliably from terminal voltages down to roughly 10–12V.

Cable Resistance

Cable resistance = Resistance per metre × Length × 2 (both conductors)

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Last updated: March 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.

Phantom Power Calculator — Current & Voltage Drop