Updated July 2026 · Links checked July 2026.
A subwoofer that hums with nothing playing is running one of audio’s oldest cons: the ground loop. Two pieces of gear, two slightly different ground potentials, one RCA cable between them — and the difference leaks into your sub as a steady 60 Hz drone. It is the cause in most cases, it costs little to fix, and one thirty-second test confirms it before you spend anything.
Short answer: pull the RCA/LFE cable out of the sub while it is powered. Hum stops → ground loop — plug sub and receiver into the same outlet or strip, and if that fails, add an inline ground-loop isolator on the cable. Hum continues with no cable attached → the noise is inside the sub or its power path — steps 4–6 below.
The fix sequence
- The unplug test. Sub powered, cable out. This one move splits the universe: cable-borne noise (ground loop, interference) versus sub-internal noise (power supply, amp plate). Everything else depends on this answer.
- Same outlet for everything. Ground loops form when sub and receiver use outlets on different circuits. One quality power strip feeding both kills the loop at its source — the free fix that works remarkably often. (Never “lift” the ground with a 3-to-2 cheater plug — that trades a hum for a shock hazard.)
- Add a ground-loop isolator. If same-outlet is impractical or insufficient, an inline RCA isolation transformer breaks the loop electrically for modest money. Cable routing matters too: keep the sub cable away from power cords, and cross them at right angles where they must meet.
- Hunt the buzz character. A pure smooth hum is ground-loop territory; a harsher buzz (120 Hz with edge) points at power-supply trouble or a nearby dimmer switch, LED driver or phone charger on the same circuit. Flip suspect dimmers off and unplug wall-warts one at a time — cheap electronics pollute mains audibly.
- Check the sub’s own settings. Auto-on modes can idle noisily on some amps — try switching auto/standby to always-on (or vice versa). Turn the sub’s gain down and see whether the noise scales: noise that ignores the gain knob lives after the volume stage, deeper in the amp.
- Accept the amp-plate verdict. Hum with no input cable, on a clean circuit, that ignores the gain knob is a failing plate amplifier — aging capacitors are the classic cause. Plate amps are replaceable on many subs (a reasonable intermediate DIY job with the sub unplugged), but on budget subs the math usually favors replacement.
If the sub is dying anyway
When the repair quote meets the replacement price, these are dependable-value picks: the Klipsch R-100SW 10″ subwoofer for the honest step up, the Acoustic Audio PSW-12 on a tight budget, and the BESTISAN 6.5″ powered sub for small rooms and apartments. For a serious upgrade, our SVS SB-1000 vs SB-2000 comparison covers the enthusiast tier.
Subwoofer hum FAQ
Why does my subwoofer hum when nothing is playing?
Most often a ground loop – sub and receiver on different circuits with slightly different ground potentials, leaking a steady 60 Hz hum through the signal cable. The confirm test: unplug the RCA cable at the sub; if the hum stops, it is the loop, not the sub.
How do I fix a ground loop on a subwoofer?
Plug the subwoofer and the receiver into the same outlet or power strip first – that alone often ends it. If not, add an inline RCA ground-loop isolator. Never use a 3-to-2 cheater plug to lift the safety ground; it works by creating a shock hazard.
My sub hums with no cable connected – what is wrong?
The noise source is internal: typically the plate amplifier’s aging power-supply capacitors, or occasionally mains pollution from dimmers and cheap chargers on the same circuit. Test on a different circuit with dimmers off; if the hum persists, the plate amp is the patient.
Is a humming subwoofer dangerous?
The hum itself is harmless, though at high gain it can heat the voice coil over long periods. The real risk is the tempting wrong fix – defeating the mains ground – which converts an annoyance into a safety hazard. Fix the loop, keep the ground.