The hook
The keyboard has a hum. Not loud, but it's there under everything, a low mosquito drone the whole room can hear in the quiet moments of communion. Someone says "bad cable." Someone else says "it's the dimmer lights." A third person wiggles a plug and the hum gets louder.
Every audio system on earth fights this battle, and the weapons were invented decades ago: they're hanging in your cable cabinet right now. This lesson is about knowing which weapon is which, and why one type of cable can run 300 feet in silence while another picks up radio stations at 15.
The lesson
First, the trick that makes professional audio possible.
A cable is an antenna. It doesn't want to be, but it is. Every foot of wire picks up the electrical noise around it: power cables, lighting dimmers, transformers, phone chargers, actual radio. The signal from a microphone is a few thousandths of a volt (lesson 2.1); the electrical smog it swims through would drown it on any long run. So how does a mic cable cross 300 feet of stage and arrive clean?
It carries the signal twice.
A balanced cable has three conductors: two signal wires and a shield. The sending device puts the signal on both wires, but flips one copy upside down. Both copies travel the cable together, and both pick up the same noise along the way. The interference doesn't know or care which wire is which, so it lands identically on both.
At the far end, the receiving device does one elegant move: it flips the inverted copy right side up and adds the two together. The signal, flipped twice, comes back to full strength. Doubled, even. The noise, identical on both wires, gets flipped on exactly one of them, so when the wires are summed the noise meets its own mirror image and cancels itself out. The interference erases itself, and the signal walks out clean.
An unbalanced cable has just one signal wire and a shield. No mirror copy, no cancellation: whatever noise gets in, stays in. Fine for short runs; a liability past about 10–15 feet, and worse the noisier the environment.
Now put faces to the names. Five connectors cover nearly everything on a stage:
XLR. The three-pin locking connector on every mic cable. Balanced, rugged, and it latches so it can't be pulled out by a tripping vocalist. Also the pipe that carries phantom power (next lesson). The professional standard for mic and line signals.
Quarter-inch TRS. Looks like a big headphone plug with two rings on the tip. Tip, Ring, Sleeve: three contacts, so it can carry a balanced signal (common for line-level runs between gear, and for studio monitor hookups). Confusingly, the same connector also carries stereo headphone audio: same three contacts, different job. Read the context, not just the plug.
Quarter-inch TS. One ring only. Tip and Sleeve, two contacts: unbalanced. This is the guitar/instrument cable. It's the reason instruments go through a DI (lesson 2.1) before any long run.
RCA. The little push-on plugs from home stereos, usually in red/white pairs. Unbalanced, consumer line level (the quieter −10 standard from lesson 2.1's deep end). You'll meet them on playback devices and cheap media players. Keep the runs short.
speakON. The twist-locking connector on speaker cables. Carries speaker-level power (tens of volts), so it's deliberately unlike everything else: you cannot accidentally plug an amplifier's output into a mic input if the connectors won't mate. That's by design, and it's protecting your gear.
So what was that hum in the hook? The classic culprit is a ground loop. When two pieces of gear are plugged into different power outlets, and connected by an audio cable, electricity can find two paths to ground. That's a loop. That loop acts like a big antenna tuned to exactly one frequency: the power line's 60 Hz (50 in much of the world). The result is that unmistakable low drone.
The fixes, in the order to try them:
- Power everything from one source. Console, stage gear, powered speakers fed from the same outlet/circuit where practical. One path to ground, no loop. This prevents most hum before it starts.
- Hit the ground lift. That little switch on the DI box labeled GND or LIFT breaks the loop's path through the audio shield. Flip it and listen. (This is most of what DI ground-lift switches are for.)
- Isolate. For stubborn cases, usually consumer gear like laptops feeding the system, an isolation transformer (or a passive DI, which contains one) breaks the electrical connection entirely while passing the audio through magnetically.
And a habit that prevents a quieter kind of noise: keep audio and power cables apart. Where they must meet, cross at 90 degrees instead of running side by side for twenty feet. A parallel run is a long, efficient antenna coupling; a perpendicular crossing is almost nothing.
Go deeperoptional
The mirror trick is differential signaling. Call the two conductors hot and cold: the driver sends +s(t) and −s(t); interference adds a common term n(t) to both. The differential receiver computes (s + n) − (−s + n) = 2s. Signal doubles (+6 dB), common-mode noise vanishes. In theory, completely. In practice, cancellation depends on the two conductors being perfectly matched in impedance to ground; any imbalance converts a bit of common-mode noise into differential signal. The figure of merit is CMRR (common-mode rejection ratio): good transformer or well-matched electronic inputs achieve 80–100+ dB of rejection. This is also why phantom power's blocking resistors must match to ±0.4% (a lesson 2.4 detail): mismatch there degrades CMRR for everything.
A subtlety worth owning: what makes the line "balanced" is the matched impedance of the two conductors, not the presence of an inverted copy. Impedance-balanced outputs (one driven leg, one grounded through a matched impedance) reject noise just as well, and are common in modern gear. The twisting of the pair matters too: each twist swaps which conductor is nearer the noise source, so exposure averages out. Star-quad cable extends the idea with four conductors in two cross-connected pairs, cutting induced hum roughly 10–20 dB further for high-interference environments (video walls, dimmer-heavy rigs).
Pinout for the record: XLR pin 1 = shield/ground, pin 2 = hot, pin 3 = cold (AES14 / "pin 2 hot"). TRS maps tip = hot, ring = cold, sleeve = shield.
Why unbalanced runs degrade with length even in quiet rooms: cable capacitance. A coax-style instrument cable runs 20–50 pF per foot; with a passive guitar pickup's high source impedance (lesson 2.1's deep end), that capacitance forms a low-pass filter whose corner frequency drops as the cable grows. Thirty feet of ordinary cable on a passive pickup can pull the treble corner into the audible range. The "long cable dullness" guitarists know is RC filtering, not superstition.
Ground loops, precisely: two chassis connected both through the mains safety ground and through the audio cable shield form a closed conductive loop; the mains' magnetic fields induce a circulating current around it (and any voltage difference between the two ground points drives current directly); that current flowing through the shield's resistance appears as a 50/60 Hz voltage in series with the signal, plus harmonics (120/180 Hz...) that give the buzz its edge. The ground lift breaks the loop by disconnecting shield-to-chassis at one end (always lift at one end only, and never lift a mains safety ground; the switch belongs on the audio shield, not the power plug). Transformers solve it categorically: magnetic coupling passes the audio, galvanic isolation denies the loop a conductive path.
Speaker cable's design follows from load impedance: an 8 Ω (or 4 Ω) speaker driven with tens of volts draws amps of current, so conductor resistance is the enemy: heavy gauge, no shield needed (the signal is thousands of times bigger than any induced noise), and damping factor (the amp's grip on the cone) degrades as cable resistance rises. The 10-gauge/short-run habit isn't audiophile ritual; it's Ohm's law.
Remember
- Balanced = two mirrored copies + shield; the receiver flips and sums, noise cancels itself. That's why mic lines run hundreds of feet clean.
- XLR: balanced, locking, the mic/line standard. TRS (two rings): balanced line or stereo headphones. TS (one ring): unbalanced instrument. RCA: unbalanced consumer. speakON: speaker power, deliberately incompatible with the rest.
- Instrument cables and speaker cables look alike and are opposites inside. Never swap them.
- The 60 Hz drone is usually a ground loop: fix with common power, DI ground lift, or transformer isolation, in that order.
- Keep audio away from power cables; where they must meet, cross at 90 degrees.
Your turn
- Sort your cable stock by connector type and label anything ambiguous, especially quarter-inch instrument vs speaker cables.
- Find every ground-lift switch in your rig (DIs, some amps and processors) before you need one on a Sunday.
- Trace one long parallel run of audio-next-to-power in your building and re-route or cross it.
- Explain the mirror trick to another volunteer in under a minute, with your hands.