Audio cables pick up noise.
They sit next to power cables, fluorescent lights, computers, phones. Every long run of copper is also an antenna picking up whatever electromagnetic field happens to be passing through. The longer the cable, the more it hears.
You'd think the fix would be better shielding. It isn't, not really. The fix is stranger and more elegant: send the signal twice, with one copy upside down, and do some arithmetic at the other end.
That's a balanced cable. Here's why it works.
The trick: phase inversion
A balanced cable carries two versions of the same signal. The first one, call it A, is the original. The second, B, is the same wave with its phase flipped 180°. Where A goes up, B goes down. They look like mirror images.
Drag the slider below. Signal A stays put. Signal B rotates. Watch the sum.
At 180° the sum is a flat line. Identical waves with opposite phase add up to zero everywhere. This is the same trick that lets noise-cancelling headphones work. It's also what makes balanced cables the default in pro audio.
Why noise cancels
The clever part isn't the inversion itself. It's what happens when noise gets added to both signals along the way.
The two conductors run side by side, twisted around each other. Any external interference (hum from a power line, RF from a phone) gets induced onto both wires almost equally. Same noise, same direction, on A and on B.
At the receiver, B is flipped back to its original phase. That flip also flips the noise on B. Now A's noise points one way and B's noise points the other. Sum them: signal doubles, noise cancels.
Try it. Inject a noise pulse and watch the OUT line at the bottom.
This is why long cable runs in studios, on stage, and in any pro audio context use balanced lines. A 50-foot unbalanced TS cable will pick up audible hum. A 50-foot balanced XLR will not.
Inside the cable
The geometry matters. For noise to cancel cleanly, the noise on A and the noise on B has to be as identical as possible. Twisting the two conductors together makes sure they sit, on average, in the same position relative to any external field. The shield around them grounds out whatever it can before it ever reaches the conductors.
Common confusions
Balanced is not stereo
A balanced cable carries one signal in mono. A stereo cable carries two signals, left and right. The connectors look the same (TRS jack, for example), but inside they're built for different purposes. Plugging a stereo cable where a balanced cable belongs can work, but you lose the noise rejection. There's no twisted pair, no inverted copy. You just have left and right going down two independent shielded paths.
The cable doesn't flip the phase
The phase inversion happens at the source (a balanced output stage) and the receiver (a differential input). The cable is just the highway. If you connect a balanced cable to gear that doesn't have balanced I/O, you don't get the benefit. The gear has to do the work; the cable just preserves the symmetry.
Balanced doesn't mean better-sounding
In a quiet, short-run setup, a good unbalanced cable sounds the same as a good balanced one. Balanced earns its reputation in long runs and noisy environments. On a 6-foot patch into the same rack, it's overkill. Just not harmful.
Why I find this beautiful
Most engineering problems get solved by adding more material: thicker insulation, heavier shielding, denser construction. This one gets solved by adding information. You send a redundant copy, agree on a convention with the receiver, and let the math do the cleaning. The signal doesn't have to fight the noise; the noise just doesn't survive the comparison.
I think about this every time I run a cable.
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- *CMRR (Common Mode Rejection Ratio) is the technical name for how well a balanced input rejects identical noise on both wires. Higher number, cleaner signal. ↩
- †A "balanced stereo" cable does exist for niche cases (some headphone setups), with two balanced pairs in one jacket. Most XLRs and TRS cables you encounter day to day are mono balanced. ↩
- ‡Originally written for minimastering.com (2020). Re-edited and made interactive for this site. ↩