Quick Answer / Summary: In a standard 3-pin XLR audio connection: pin 1 is shield and ground, pin 2 is the in-phase signal (hot) and pin 3 is the anti-phase signal (cold). The shield connects only to pin 1. Reversing pins 2 and 3 inverts phase; leaving pin 1 open removes shielding and the ground reference, which produces hum and instability.
The XLR pinout is short, standard and frequently miswired. Three conductors, three pins, one correct assignment – and a set of symptoms that tell you which pin is wrong before you open the connector. This is the reference for anyone commissioning, repairing or specifying XLR cable.
The Standard Assignment
Pin 1 carries the shield and the reference ground, and it is the pin that must bond to the equipment chassis rather than float. Pins 2 and 3 carry the same signal in opposite polarity, which is what a differential input needs. Because the pair is twisted and its impedance balanced, noise coupled onto the cable appears equally on both pins and cancels in the input stage.
Why Pin 1 Goes to Chassis
The shield’s job is to intercept interference and conduct it away to the chassis, not to carry signal current. When pin 1 is tied into the signal ground path inside a device, shield current can share the audio return and produce hum that changes with what else is plugged in.
The Symptoms of Each Miswire
Reverse pins 2 and 3 and the channel is inverted: thin, hollow, and cancelling when summed with a correctly wired channel. Open pin 1 and the cable loses shielding, giving hum and RF sensitivity. Bridge the shield into pin 2 or 3 and the cable becomes unbalanced, losing common-mode rejection while still passing audio.
Verifying With a Meter or a Continuity Tester
With the cable disconnected at both ends, check continuity from each pin at one end to the corresponding pin at the other. Then check that no two pins show continuity with each other. Finally test pin 1 continuity through the shield; a reading that changes when the cable is flexed indicates a frayed braid inside the plug, which is the fault that passes bench testing and fails on stage.
Documenting Pinouts on a Project
Every installation should carry a wiring schedule that states the pin assignment in plain text, including any legacy device that deviates. Where a device documents pin 3 hot, the schedule should name the cable that adapts it, so a technician never has to guess which of two identical-looking cables is correct. Writing pin 1 to shield, pin 2 hot, pin 3 cold once in the drawing set removes the ambiguity for the life of the room.
XLR Pin Assignment and Fault Symptoms
| Pin | Signal | Role | Symptom If Wrong |
|---|---|---|---|
| 1 | Shield and ground reference | Interference drain to chassis | Hum, RF pickup, unstable grounding |
| 2 | In-phase signal (hot) | Positive leg of the balanced pair | Phase inverted when swapped with pin 3 |
| 3 | Anti-phase signal (cold) | Negative leg of the balanced pair | Thin or hollow channel when swapped with pin 2 |
| Shell | No connection to signal | Mechanical only | Short if shield is bonded to the shell by mistake |
| Shield drain | Pin 1 only | Never a signal path | Unbalanced behaviour, loss of noise rejection |
Real-World Application Scenarios
Commissioning a new room, checking a batch of incoming cable stock, and diagnosing a channel that sounds wrong in a rack where several vendors built the wiring. In rental inventory the same check runs before every job: continuity on three pins, no bridge between them, and a flex test on pin 1.
Recommended Internal Resources
Editorial Note: Internal linking recommendation: (1) Product cluster – https://zh-zong.com/productlist/ (anchor: XLR cable assemblies wired and continuity-tested to the standard pinout); (2) Quality documentation – https://zh-zong.com/certifications/ (anchor: inspection standards applied to every cable batch); (3) Sizing reference – https://zh-zong.com/voltage-drop-conductor-sizing-guide/ (anchor: conductor and resistance reference for long audio runs).
Frequently Asked Questions (FAQ)
Q: What are the pin numbers on an XLR connector?
A: Pin 1 is shield and ground, pin 2 is hot and pin 3 is cold in the standard balanced audio assignment. The pins are numbered on the insert of nearly every professional connector, and the numbers read the same whether you look at the male pins or the female sockets.
Q: Does it matter if pin 1 is not connected?
A: Yes. An open pin 1 means the shield has no drain path, so interference picked up along the run has nowhere to go and the ground reference is missing. The result is hum, RF sensitivity and level instability, especially on long cables.
Q: How can I tell if a cable has pins 2 and 3 reversed?
A: Listen to two microphones on the same source, or compare channels: a reversed cable inverts phase, so the combined signal loses low end and sounds hollow. Confirm with a meter by identifying which conductor reaches pin 2 at both ends.