Phantom power (commonly called P48) supplies 48 volts to condenser and active ribbon microphones through the same balanced XLR cable that carries the audio. Over long cable runs, engineers sometimes worry that conductor resistance will starve the microphone. This calculator shows exactly how much voltage reaches the capsule, separating the unavoidable drop across the standard feed resistors from the much smaller drop across the cable itself.
How it works
The IEC 61938 standard delivers 48V to the microphone through two 6.81 kΩ feed resistors, one on each signal conductor (pins 2 and 3). Because the microphone draws its supply current through both resistors at once, they act as a single resistor of 6810 / 2 = 3405 Ω in series with the supply.
The supply current I also flows out along one conductor and returns along the other, so it crosses twice the cable length of copper. Copper’s resistance per metre depends on the wire gauge; for example 24 AWG is about 0.084 Ω/m per conductor. The voltage delivered to the microphone is therefore:
V_mic = 48 − I × 3405 − I × (2 × length × R_per_metre)
Example
A condenser drawing 4 mA over a 100 m run of 24 AWG cable:
- Drop across feed resistors:
0.004 A × 3405 Ω ≈ 13.6 V - Round-trip cable resistance:
2 × 100 m × 0.084 Ω/m ≈ 16.8 Ω - Drop across cable:
0.004 A × 16.8 Ω ≈ 0.07 V - Delivered:
48 − 13.6 − 0.07 ≈ 34.3 V
The cable contributes well under a tenth of a volt — the feed resistors, which are part of the standard, account for almost all of the drop.
Notes
This is why phantom-power “voltage” at the capsule is normally in the low-to-mid 30s of volts rather than a full 48 V: the feed resistors are intentionally large. Cable resistance only becomes meaningful for very long, thin runs feeding high-current microphones. If a mic misbehaves on a long line, use a heavier-gauge cable, shorten the run, or check that the supply actually outputs a true 48 V. All calculations run locally in your browser.