Ground Loop Hum Ground Loop Hum and Buzz in Home Theater Systems

What ground loop hum is

A ground loop forms when two points in a circuit are meant to share one ground reference but sit at different potentials. Wikipedia describes the loop as acting like a single-turn secondary of a transformer, so stray magnetic fields drive a current through it. Loop resistance is often below one ohm, so a small induced current produces a voltage drop that rides along with the audio. In speakers it is heard as a 50 or 60 Hz hum. Acoustic Frontiers, a calibration practitioner, defines hum as the 60 Hz mains frequency superimposed on the audio or video signal and buzz as its harmonics, such as 120 Hz and 180 Hz. Video shows it too: Jensen refers to 60 cycle hum bars in video.

Not every hum is a ground loop. Acoustic Frontiers lists three origins: interchassis currents from stray capacitance in power supplies (with or without a safety ground), true ground loops caused by safety-ground voltage differences between interconnected gear, and electromagnetic coupling between AC and signal cables. A ProSoundWeb author notes that hum can sit at 60, 120, or 240 Hz and suggests a real-time analyzer to identify the frequency. So 120 Hz buzz does not prove a ground loop, and 60 Hz does not rule out a poorly shielded cable.

How the loop gets into a home theater

The textbook case is the cable TV line. Wikipedia describes a coaxial cable entering a TV that is grounded by the cable company at a different point than the house ground, which causes mains hum in the speakers. The loop runs from the cable's outer shield, through the set-top box or TV, through the AVR's interconnects, and back to the house ground at the outlet. Disconnecting the coax is the quick test: if the hum disappears, the loop includes the cable line.

The second case is the subwoofer interconnect. A powered sub is plugged into a different outlet than the AVR, the RCA shield ties the two chassis together, and a ground-potential difference drives current through it. Blue Jeans Cable says directly that hum can come from a ground loop, that cable cannot fix it, and that the remedy is to correct the grounding or insert an isolation transformer between source and subwoofer. A better shield still helps a different problem, induced hum, as covered in subwoofer cable vs RCA cable.

A worked example, with illustrative numbers: 100 mA of leakage current through the 20 ft run of a 1.8 ohm per 1,000 ft shield (0.036 ohm, from Blue Jeans' figure for the Canare LV-77S) creates 3.6 mV on the signal return. That is about 49 dB below 1 V, and a subwoofer amplifier with gain will present it alongside the bass.

Fixes, in the order to try them

First, plug the system into one outlet strip on one circuit. Acoustic Frontiers names equipment on different circuits, or in different outlets on the same circuit, as the common root cause, and recommends a single grounding point with short, heavy power leads. Second, isolate the offending link. For cable TV, Jensen's VRD-1FF is a passive isolator for 75 ohm CATV lines that its spec lists at 2 to 1300 MHz, with insertion loss of 0.19 dB at 5 MHz, designed to reduce 60 Hz ground currents. Markertek shows it as no longer available, and it is not suitable for satellite or RG11, so a current CATV isolator needs the same checks. For a sub, use a line-level isolation transformer. For HDMI, Acoustic Frontiers lists fiber optic HDMI cable, and for digital audio, TOSLINK is galvanically isolated by design, as Wikipedia notes (see also coaxial S/PDIF). Third, balanced connections reject ground-loop noise as common-mode interference, per Wikipedia, so an XLR sub input beats RCA (see balanced XLR vs unbalanced RCA).

Wikipedia calls an isolation transformer the quickest, quietest, and most foolproof way to resolve hum. It sits in the signal path, so choose one made for the signal type: a video or CATV isolator for coax, an audio transformer for line level.

What not to do

Do not float the safety ground. Acoustic Frontiers says plainly that the ground is there to protect you from electrocution, and Wikipedia states that removing the safety ground to cure hum creates a shock hazard and, if the chassis then picks up leakage, can produce a very loud hum that may damage speakers. Rane's interconnection note repeats the rule that safety regulations require the original grounding means to stay intact. A cheater plug that breaks the third prong is the classic mistake, and it trades a hum problem for a safety problem.

Do power conditioners fix it?

Sometimes, but not because of filtering. An isolation transformer inside a conditioner can break a loop, as the power conditioner entry explains, but a filter bank or surge strip does nothing for a ground-potential difference between two chassis. The ASR forum position quoted there is that filters change the mains but usually not the audio output. Treat the conditioner as a possible fix only when it has real galvanic isolation.

Common confusions

Hum and buzz are different tones with overlapping causes, so a 120 Hz buzz does not by itself prove a ground loop. A notch filter is not an audio fix either: Wikipedia notes that notch filters cannot be used in audio because of objectionable audible effects on the wanted signal. Start by identifying which connection carries the loop, then fix that link. The wiring context is in the home theater wiring guide.