Quick answer: Phantom power is a DC bias, nominally 48 V, applied equally to pins 2 and 3 of an XLR connector with pin 1 as the return. The condenser microphone draws its operating current from that bias, typically 2 to 10 mA, and the cable carries both the DC and the balanced audio on the same pair. A cable with an open signal leg or a shorted shield can remove the bias without showing any external damage.
Condenser Mic Cable and Phantom Power: What the Wire Carries
A touring engineer repatches a condenser mic to another channel and gets nothing but a faint hiss. The same mic worked ten minutes earlier, the desk is fine, and the cable passes audio when a dynamic mic is plugged into it. What failed is the DC path, not the audio path: a cracked joint on pin 3 has left the capsule without bias while the pair still carries enough signal to look alive on a meter. Phantom power is usually described as a preamp feature, but the cable sits inside that circuit and every contact in it matters.
The Bias and the Audio Share One Pair
A condenser capsule needs a polarizing voltage before it can produce any signal at all, and that voltage arrives on the same two wires that later carry the microphone output. Pins 2 and 3 sit at the same DC potential, roughly 48 V with respect to pin 1, so the capsule sees a bias across its own circuitry rather than across the pair. Audio then appears as a small differential voltage riding on that common DC level. Because the bias is common to both legs, a transformer or a differential input at the far end cancels it while passing the difference. This is why a condenser microphone cable is not a power cable in any meaningful sense. It is a balanced pair that happens to carry a DC offset, and its twist, shield and capacitance are chosen for the audio, not for the current.
Where the 48 V Comes From
The supply lives in the preamp or console channel, not in the microphone. A 48 V rail feeds each signal leg through a resistor, and the microphone circuitry draws current from the difference between the two legs while returning it on pin 1. IEC 61938 defines the arrangement, including the current a microphone may draw, so any compliant preamp should power any compliant condenser. The tolerance matters in practice: a supply that measures 48 V with no load can sit well below that once a mic is connected, and the standard allows the voltage at the microphone to fall to 12 V. The rail is regulated but not remote sensed, so cable resistance and contact resistance appear as series loss. Measure at the microphone end under load, not at the supply.
Feed Resistors Set the Source Impedance
Inside the preamp, each leg connects to the 48 V rail through a 6.8 kohm resistor, the value fixed by IEC 61938. The microphone therefore does not see a stiff supply. It sees roughly 6.8 kohm in series with each leg, in parallel with the preamp input. That resistance, not the rail, sets the maximum current, and it also raises the impedance the mic works into, which is why phantom-fed condensers are less sensitive to cable capacitance than their own output impedance suggests. A capsule drawing a large current pulls the node down, and the standard accepts that sag.
Current Draw and Voltage Drop Over Distance
Most condensers draw between 2 and 10 mA, and that current returns through pin 1, so it passes through the shield and through the copper at both ends of the run. At 24 AWG, roughly 26 ohm per 1000 ft, a 250 ft cable adds about 6.5 ohm per leg. At 10 mA the drop is small, but near the low end of the voltage tolerance, and with a connector that has lost tension, a marginal mic can brown out. Measure the voltage at the microphone connector with the mic connected and the channel open.
Connectors and Mics That Should Not See 48 V
Three contacts are the minimum, and they have to be three separate contacts. On a 1/4 inch TRS plug the sleeve doubles as the shield, which is acceptable for a balanced line signal but leaves no independent return once the plug enters a jack that ties the sleeve to ground. On a TS plug the jack shorts the ring contact to the sleeve, so one leg of the pair is grounded at the destination. A 3.5 mm TRRS plug has four contacts but assigns them to mic, ground, left and right in an order that varies between phone makers, which is why a consumer lavalier often refuses to work on a camera without the matching adapter. The plug body itself is not a reliable indicator of what the jack expects.
Why a TS Plug Shorts One Leg
Insert a TS plug into a jack wired for a balanced pair and the ring contact, which normally carries the cold leg, lands on the sleeve and is grounded. The microphone now drives one leg against ground instead of against a second leg. Level drops by roughly 6 dB, common-mode rejection disappears, and any bias present is pulled to ground through the microphone output stage. The mic may still work, which is exactly why the fault goes unnoticed until a quiet passage exposes the hum.
Ribbon Mics and the Cost of a Wrong Switch
A ribbon element is a few microns thick and sits between the magnet poles with almost no compliance. Applying 48 V across a ribbon mic wired for phantom can tear the ribbon or leave it slack, and the damage is not always immediate, which makes the failure hard to trace. Most ribbon mics are passive and need no bias at all. Label the phantom switch, confirm the mic model before patching, and use a preamp that applies bias only to the XLR input you have selected.
Faults That Kill Phantom Quietly
Phantom failures rarely announce themselves. The mic goes quiet and the desk shows no signal. The most common cause is an open pin 2 or pin 3: a solder joint cracked inside a connector shell, a strand broken where the cable flexes at the boot, or a patch panel contact that lost tension. With one leg open, no bias reaches the capsule, and the differential input also loses its common-mode rejection. A shield shorted to pin 2 or pin 3 is worse, because the pair still passes audio while the bias is pulled toward ground through the shield. A short between pin 2 and pin 3 removes the differential signal entirely. Test with the cable disconnected: pin to pin continuity, then pin to shield, expecting an open circuit.
Phantom power values to verify on a condenser cable
| Parameter | Value | Why it matters | Check |
|---|---|---|---|
| Phantom voltage | 48 V nominal, 12 V minimum | Sets capsule headroom | Measure pins 2 and 3 |
| Feed resistance | 6.8 kohm per leg typical | Limits current, raises source impedance | Compare both legs |
| Typical current | 2 to 10 mA per mic | Decides voltage drop on long runs | Read current at preamp |
| Insulation resistance | Above 100 Mohm typical | Leakage steals bias voltage | Megohmmeter, mic disconnected |
| XLR connector | Rated per IEC 61076-2-103 | Three contacts, locking shell | Inspect pin 1 seat |
| Failure symptom | Mic dead, no signal | Open pin 2 or 3 | Continuity end to end |
Real-World Application Scenarios
In a 900 seat church installation, six condenser podium mics sit 120 ft from the rack, so the integrator measures 48 V at each microphone rather than at the supply, because one corroded pin 1 contact can silence the whole platform. On a touring stage, a vocal condenser runs 25 ft to a stage box, and the monitor engineer keeps a spare cable terminated and tested, since an intermittent pin 3 usually appears during the first song. In a project studio, a passive ribbon mic on a guitar cabinet stays patched to a preamp with phantom switched off and a label on the switch, because one accidental application can stretch the ribbon.
Related Guides on This Site
Three references are worth reading before you order: a buying checklist for a condenser microphone cable covers shield and capacitance targets, a balanced XLR cable overview explains how the differential pair rejects the common bias, and the XLR pin configuration reference shows where pins 1, 2 and 3 land in each gender.
Frequently Asked Questions (FAQ)
Q: Does phantom power travel on all three XLR pins?
A: It travels as a common voltage on pins 2 and 3, with pin 1 as the return. Both signal legs sit at the same DC potential, so the microphone sees bias rather than a differential offset. That is why the audio pair can carry the DC without coupling capacitors inside the cable.
Q: Can I power a condenser through an XLR to TRS cable?
A: Only if the TRS input supplies phantom, which is rare outside dedicated mic preamps. Most TRS jacks on interfaces and mixers are line inputs with no bias. The cable passes audio from a self-powered mic, but a true condenser stays silent until it receives 48 V on pins 2 and 3.
Q: How much current does a phantom-powered mic draw?
A: Most draw 2 to 10 mA, and the standard permits up to 10 mA per input. Two mics on one supply share its current budget, so a small interface powering four condensers may sit near its limit. Check the supply rating before adding mics, then measure voltage at the microphone end.
Q: Will a 300 ft cable starve a condenser of phantom power?
A: Rarely, but it depends on conductor gauge and current draw. At 24 AWG, 300 ft adds about 8 ohm per leg, which is small against the 6.8 kohm feed resistors. The larger risk is a poor contact or a partly broken strand, which adds resistance at one point and can drop several volts.
Q: Why does the same mic work on one channel and not another?
A: Compare the channels with the same cable and the same mic. If the mic works on channel two, the fault is in the first channel supply, its connector or its pin 3 contact. If it fails on both, test the cable for continuity between pins 2, 3 and 1 at each end.
Q: Is it safe to plug a condenser in with phantom already on?
A: Usually yes for a transformer-coupled or well-protected mic, because the bias appears on both legs at once and the pins mate with pin 1 first. Cheap unbalanced adapters and ribbon mics are the exceptions. Power down before patching anything whose wiring you have not verified.