XLR is a family of circular, latching audio connectors that, in recording, carries balanced microphone and line-level signals between microphones, preamplifiers, audio interfaces, mixers and monitors. The three-pin version, standardized as IEC 61076-2-103 and described for audio use in AES14, is the default microphone connector in professional audio worldwide.
The connector itself is only metal and plastic, but it is designed around a specific wiring convention: two signal conductors plus a shield. That convention makes balanced transmission possible, allows phantom power to reach condenser microphones, and lets engineers run cables of tens of meters without picking up audible hum.
How the XLR connector carries a balanced signal
An XLR cable carries a balanced signal by sending the audio on two conductors with equal impedance to ground, while a third conductor acts as the shield. In the classic microphone case, the source drives pin 2 (hot) and pin 3 (cold), and pin 1 connects to the cable shield and chassis ground.
Interference from power cables, lighting dimmers or radio transmitters induces nearly the same voltage on both conductors, because they are twisted together and share the same path. The receiving input is a differential amplifier that amplifies only the difference between pins 2 and 3, so this common-mode noise cancels. How well it cancels is expressed as the common-mode rejection ratio (CMRR), typically 60 to 90 dB for a good microphone preamplifier at low frequencies.
A common misconception is that balanced operation requires the two legs to carry mirror-image audio. In fact, impedance balance is what matters. Many outputs drive only pin 2 and terminate pin 3 through a matching resistor, and they still reject noise effectively because both legs present the same impedance to the receiver.
The following table shows the standard pin assignment for three-pin audio XLR.
| Pin | Function | Notes |
|---|---|---|
| Pin 1 | Shield and ground | Connects first when mating, preventing pops |
| Pin 2 | Hot, positive polarity | Positive pressure on the microphone gives positive voltage |
| Pin 3 | Cold, negative polarity | Carries the inverted or reference leg |
| Shell | Optional chassis connection | Varies by manufacturer and installation |
The pin 2 hot convention is part of AES14 and is now universal, although some older equipment used pin 3 hot. Mixing the two conventions inverts the polarity of a source, which causes thin, phasey sound when it is combined with another microphone on the same instrument.
XLR variants and pin counts
XLR connectors exist in three to seven pins and more, and the pin count indicates the application rather than the signal quality. Three pins dominate audio recording, while other counts serve stereo, control and lighting tasks.
| Version | Typical use | Remarks |
|---|---|---|
| XLR3 | Microphones, line signals, AES3 digital | The default recording connector |
| XLR4 | Intercom headsets, DC power for cameras | Not interchangeable with XLR3 |
| XLR5 | Stereo microphones, DMX512 lighting | Lighting must not share audio cabling practices |
| XLR7 | Tube microphone power supplies | Carries heater, plate and pattern voltages |
| Mini XLR | Lavalier and bodypack transmitters | Smaller shell, different mechanical standard |
Connectors are also described by gender. The male connector has pins and is found on outputs such as microphones, while the female connector has sockets and is found on inputs. Signal therefore flows from male to female, which makes the direction of a cable easy to identify on a stage or in a studio.
The same XLR3 shell also carries AES3 digital audio, but a digital link expects 110-ohm cable. Standard microphone cable often works over short runs, yet it can cause reflections and dropouts at longer distances.
XLR compared with other audio connectors
XLR is preferred in recording because it combines balanced wiring, a positive lock and ground-first mating in one connector. Other connectors share some of these properties but rarely all of them.
| Connector | Balanced | Locking | Typical role |
|---|---|---|---|
| XLR3 | Yes | Yes | Microphones, line outputs, monitors |
| TRS 6.35 mm | Yes | No | Line inputs, patch bays, headphones |
| TS 6.35 mm | No | No | Instruments and short unbalanced links |
| RCA | No | No | Consumer line-level equipment |
| Combo jack | Yes | Partial | Interface inputs accepting XLR or jack |
Unlike a jack plug, an XLR connector does not short the hot and ground contacts while it is being inserted. This is important when phantom power is active, because 48 V across the wrong contacts during insertion can stress equipment and produce loud clicks.
Using XLR in a recording signal chain
In a typical signal chain, XLR connects the microphone to the preamplifier of an audio interface or mixer, then often carries line-level outputs to studio monitors. The same cable can carry both signal types, but the input it plugs into must match the level, because a microphone signal sits roughly 40 to 60 dB below line level.
Correct gain staging begins at this connection. A microphone plugged into a line-level XLR input will be far too quiet, while a line output plugged into a microphone input can overload the preamplifier and destroy headroom. Many interfaces solve this with switchable pads or separate inputs.
| Situation | Recommended choice | Reason |
|---|---|---|
| Dynamic microphone, 5 m | Standard twisted-pair XLR cable | Balanced line rejects hum easily |
| Condenser microphone | XLR with phantom power on | Pins 2 and 3 carry the 48 V supply |
| Long stage run, 50 m | Quality cable or multicore snake | Low capacitance keeps highs intact |
| AES3 digital link | 110-ohm digital cable | Avoids reflections and errors |
| Interface to monitors | XLR to XLR or XLR to TRS | Keeps the line balanced end to end |
Cable quality matters mainly through shield coverage, conductor twist and capacitance. A microphone cable of 20 to 30 meters is routine with balanced lines, whereas an unbalanced link of the same length would typically pick up hum and lose high frequencies.
Common XLR problems and how to fix them
Most XLR faults come from wiring, grounding or mechanical wear, not from the connector design. The symptoms are usually recognizable.
- Hum that disappears when pin 1 is lifted points to a ground loop; the correct fix is proper grounding or an isolation transformer, never removing the safety earth.
- Intermittent crackle usually means a broken conductor near the plug or a loose solder joint, often caused by pulling the cable instead of the connector body.
- Weak, thin sound on one of two microphones suggests a polarity reversal between pins 2 and 3.
- A condenser microphone that stays silent often indicates that phantom power is off or that a pin 2 or pin 3 conductor is open.
Pressing the latch release before unplugging prevents damage to the locking tab, and regular testing with a cable tester catches faults before a session rather than during it.
In plain words
An XLR cable is the sturdy, round-plugged cable that connects a microphone to a mixer or recording box. It clicks into place, so it does not fall out when someone steps on it.
Inside, it sends the sound twice, in a clever way that lets the receiving equipment throw away noise picked up along the way. Think of two people carrying the same message through a noisy room, where the listener keeps only what both said differently and ignores the shared background noise. That is why XLR cables can be long and still stay quiet.
A real-world example
A sound engineer records a choir concert in a church. The microphones stand near the altar, while the recording position is in a side room about 35 meters away. Lighting dimmers and power cables run along the same route.
The engineer uses two small-diaphragm condenser microphones with XLR cables into a stage box, turns on 48 V phantom power, and sends the signals over a multicore snake to the audio interface. Because the lines are balanced, the dimmer buzz that would ruin an unbalanced connection is rejected by the preamplifiers.
During the soundcheck, one channel sounds thinner when both microphones are combined. A quick check shows a cable with pins 2 and 3 swapped. Replacing it restores full low end, and the concert is recorded clean and free of hum.