Ground Loop Hum: Find and Fix Electrical Noise in a Home Studio

The short answer: ground loop hum is a steady drone at your mains frequency (60 Hz in North America, 50 Hz in most of the rest of the world) and its multiples, caused when two connected devices are grounded through more than one path. Fix it by putting everything on one power strip, unplugging the laptop charger to test, using balanced cables, and adding a transformer isolator or DI box on the link that closes the loop. Never cut or lift the safety earth to stop it.

A voice actor treats the room, tightens the mic technique, and a faint, steady hum is still sitting under every take. A podcaster plugs a second laptop into the mixer for a remote guest and the whole show suddenly sits on a low buzz. Both are the same family of problem: noise that is low, constant, and completely unaffected by foam, blankets or closing the window. It does not come from the air in the room. It comes from the wiring, and it has electrical fixes.

This guide is built from the manufacturer application notes the pro audio industry has relied on for decades, chiefly Rane's Note 110 on sound system interconnection and the Jensen Transformers notes by their chief engineer Bill Whitlock, along with support guidance from Shure and Focusrite and consumer electrical safety advice from ESFI, CPSC and OSHA. Every source is linked at the end. It explains what a ground loop is, how to tell hum from buzz and 50 Hz from 60 Hz, why laptops and USB interfaces are the usual suspects in a home studio, a five-test diagnosis you can run in fifteen minutes, which fixes are safe and which are never acceptable, and how to remove whatever hum is left in a file you have already recorded.

What a Ground Loop Actually Is

Every mains-powered device with a three-pin plug has its metal chassis connected to the building's safety earth. That earth exists so a fault inside the device trips the breaker instead of making the case live. Audio cables also connect grounds: the shield of an XLR, TRS, RCA or USB cable joins the ground of one device to the ground of the next. Connect two earthed devices with an audio or USB cable and there are now two paths between their grounds: one through the cable shield, and one back through the power cords and the building wiring. Those two paths form a closed circle, the loop.

The earth pins of two outlets are never at exactly the same voltage, especially when the outlets are on different circuits. Whitlock notes that a loop typically forms when connected equipment is powered from different branch circuits, because their chassis sit at slightly different potentials. That small difference pushes a current around the loop, at the mains frequency and its harmonics. The loop itself is not the hum. The hum appears because part of that current flows through the cable shield, and the shield has a little resistance.

Rane's Note 110 puts it precisely: current alone does not make hum, but current flowing through a unit's audio signal ground does. In an unbalanced connection the shield is also the signal's return conductor, so the tiny voltage the loop current creates across the shield is added straight onto the audio. Whitlock calls this common-impedance coupling and identifies it as the root cause of most hum in unbalanced systems. He also makes a point that saves people money: hum is rarely caused by bad cable shielding or bad AC wiring. It is usually caused by how the equipment is interconnected.

One distinction matters before you change anything. A loop that runs through your speakers often puts hum only in what you hear, not in what you record. If the hum is in the speakers but the recorded file is clean on headphones, your recording is fine and only the monitoring path needs fixing. If the hum is in the file, it is in the input path, and every take is affected. That single check, covered in the diagnosis below, decides where to spend your time.

Hum, buzz or hiss: name the noise first

• Hum is a smooth, low drone with a definite pitch near the bottom of the piano. It is the mains fundamental and a few low harmonics, and it is the classic ground loop and magnetic-field symptom.
• Buzz is harsher and brighter, like an electric razor in the next room. It has the same root pitch but many more harmonics climbing up into the speech band. It is common with laptop chargers, cheap power supplies, dimmers and unbalanced links, and it is harder to remove in software because so many frequencies are involved.
• Hiss is broadband and pitchless, like air escaping a tyre. That is not a ground loop at all; it is usually gain set too high on a quiet mic. See our guide to gain staging and recording levels.
• Whine or chirp that changes as you work (scrolling, clicking, a progress bar moving) is digital or USB noise from the computer, which travels the same ground paths as hum and has the same fixes.

50 Hz vs 60 Hz Hum, and the Harmonics

Mains hum sits at the frequency of your electricity grid. The USA, Canada, Mexico and much of South America run at 60 Hz; the UK, Europe, Australia, New Zealand, India, China and most of Africa run at 50 Hz. The IEC's World Plugs reference lists the frequency for every country. Japan is the famous exception: according to Kansai Electric Power, eastern Japan runs at 50 Hz and western Japan at 60 Hz, with the boundary at the Fuji River and Itoigawa.

Hum is rarely a pure tone. Power supplies draw current in short pulses rather than a smooth sine wave, and anything nonlinear in the path adds harmonics, so the noise contains the fundamental plus whole-number multiples of it: 120, 180 and 240 Hz on a 60 Hz grid, or 100, 150 and 200 Hz on a 50 Hz grid. This matters in two practical ways. First, the harmonics are what you actually hear on small speakers and earbuds, which barely reproduce 50 or 60 Hz at all, so a high-pass filter at 80 Hz removes the fundamental but leaves the audible part of the hum behind. Second, a buzz with a long ladder of harmonics reaching into the low kilohertz overlaps the voice directly, which is why fixing it at the source is far better than trying to filter it out later.

Knowing your grid frequency also helps diagnosis. If you are in a 60 Hz country and your spectrum analyser shows a peak at 60 Hz with neat peaks at 120, 180 and 240 Hz, the noise is mains related, so it is a loop, a field, or a power supply. If the peaks are at unrelated frequencies, look instead for a fan, a fridge compressor, a hard drive or a computer whine.

USB Bus Power, Laptop Chargers and the Home Studio

In a home studio the commonest loop is not between rack units, as in the classic pro audio literature. It is between a laptop, its charger, a USB audio interface, and a pair of powered speakers. The USB cable has a grounded shield, so it connects the computer's ground to the interface's ground, and from there through the speaker cables to the speakers' earthed power cords.

Bus-powered interfaces

A bus-powered interface takes its power from the computer over USB, so it inherits whatever noise rides on the computer's supply and ground. If the computer is a laptop on battery and nothing else is plugged into the interface except a mic and headphones, there is no second path to ground and no loop is possible. That is why a lot of people never hear this problem until they add speakers, a charger, a second screen or a mixer. If your interface has its own power input, try it both ways: a supply approved by the interface maker sometimes cleans up a noisy bus, and sometimes adds a loop that was not there before.

Laptop chargers

Focusrite's support guidance puts the laptop charger at the top of the suspect list: if the hum stops when you unplug the laptop's power cable, the power supply or the building wiring is the likely source, and it recommends using only a supply made or approved by the computer's manufacturer. Hugh Robjohns, technical editor of Sound On Sound, describes the mechanism: a mains-powered laptop and a mains-powered converter can form a loop through the grounded shield of the USB cable, and running the laptop on battery breaks it.

A charger with a two-pin plug has no earth connection, but it can still cause trouble. Switching power supplies contain filtering components that pass a small leakage current from the mains side to the low-voltage side, and Jensen's notes describe that kind of leakage current, flowing through signal cable shields, as a leading cause of hum and buzz in unbalanced systems. Cheap replacement chargers are frequent offenders, which is one more reason to use the manufacturer's own.

Recording on battery is a legitimate fix for a session, not a hack, but it is not a permanent answer for a four-hour audiobook day or a long podcast recording. For that, plug the laptop charger into the same power strip as the interface and speakers, and if the hum persists, isolate the link to the speakers as described under DI boxes and isolators below.

USB isolators: useful, with a catch

A USB isolator sits between the computer and the device and breaks the ground connection while passing data. The catch is speed. Many isolators are built on chips such as Analog Devices' ADuM4160, which supports USB low speed and full speed (1.5 and 12 Mbps) but not USB 2.0 high speed (480 Mbps). Robjohns notes that full-speed isolators are fine for a MIDI keyboard but often inadequate for a multichannel audio interface. Check whether your interface works at full speed before buying one, and treat the isolator as a later step rather than the first fix. Do not tape over the power or shield pins on a USB cable either: Robjohns notes that breaking the ground in the cable often stops the computer recognising the device at all.

Balanced vs Unbalanced Cables

A balanced cable (XLR, or TRS on a quarter-inch jack) carries the signal on two conductors with the shield as a separate third conductor. The receiving input listens to the difference between the two signal wires, so noise that appears equally on both, including most of the voltage a loop current creates, cancels out. An unbalanced cable (RCA, TS guitar cable, 3.5 mm minijack) uses the shield as the signal return, which is exactly where the loop current flows. Rane's Note 110 says balanced lines are excellent at rejecting ground loop noise, but only when the whole connection is properly balanced, and that balanced and unbalanced interconnects are not compatible: mixing them is where much of the trouble starts.

• Speakers: if your interface has balanced quarter-inch outputs and your speakers have balanced inputs, use TRS to TRS or TRS to XLR cables. Focusrite notes that the line outputs on its interfaces are balanced and recommends balanced TRS cables to reduce hum in the monitors. A TS guitar cable fits the same socket and throws the balancing away.
• Keep unbalanced runs short. Rane recommends keeping unbalanced cables under about 3 metres (10 feet).
• Phones, tablets and second laptops usually connect through an unbalanced 3.5 mm jack. A podcaster feeding a remote guest from a laptop on its charger into a mixer has built the textbook unbalanced loop. That link is the first one to isolate.
• Microphones are already balanced. An XLR mic cable rarely causes a ground loop by itself, which is why a loop usually lives in the speaker, computer or headphone-amp links rather than in the mic line.

Rane also documents what it calls the pin 1 problem: equipment that connects the XLR shield (pin 1) to the internal signal ground instead of directly to the metal chassis lets shield current wander through the audio circuitry. The AES48 standard, which Rane cites, specifies tying the shield to the chassis right where the cable enters. You cannot fix this inside your own gear, but it explains why one particular interface or mixer can hum in a setup where another is silent, and why swapping a single box is sometimes the whole fix.

Diagnose It: Five Tests in Fifteen Minutes

Ground loop hunting is methodical, not mysterious. Change one thing at a time, record ten seconds of silence with the mic open after every change, and compare the clips by ear on headphones and by eye on a meter. Write down what you changed. Do it with your normal gain setting, because hum that is inaudible at low gain can be very audible at the gain you actually record at.

Before you start, record a baseline: the whole setup running as normal, mic open, nobody speaking. If you have a meter that reads the noise floor in dB, write the number down. Every later test is measured against it, and a change of a few dB is a real result even when your ears are not sure.

1. Speakers or file? Play the baseline back on headphones with the speakers turned off. If the file is clean and the hum only comes out of the speakers, it is a monitoring loop. Your recordings are fine; fix the interface-to-speaker connection with balanced cables, a shared power strip, or a line isolator.
2. The charger. Unplug the laptop charger from the laptop and record again. If the hum stops, the loop runs through the charger and the USB cable. Use the manufacturer's charger, plug it into the same strip as the interface, or record on battery.
3. Everything else. Unplug every cable from the interface except the mic and the USB cable to the computer: speakers, second monitor, mixer, phone, headphone amp, MIDI keyboard. If the hum stops, plug them back one at a time, recording after each, until it returns. The last thing you connected closes the loop, and that link is the one to isolate.
4. Fields and lighting. If the hum is still there, rotate the mic, move its cable away from power bricks and power cords, and switch off dimmers, LED strips and desk lamps one at a time. If the hum changes with the mic's position or with a light, it is magnetic or electrical interference rather than a loop.
5. A faulty part. If the hum survives with only the mic, the interface and a laptop running on battery, there is no loop left to break. Swap the mic cable, then try a different mic, then a different interface or USB port. A failing cable or a faulty power supply is the likely culprit, and the maker's support team is the next call.

Worked example: the podcast desk

Picture a two-person podcast recorded on a laptop with a bus-powered USB interface, two dynamic mics, powered speakers on the wall outlet behind the desk, and the laptop charger on a power strip by the window. The baseline has a clear 60 Hz drone. Test 1: the hum is in the file, so it is not just monitoring. Test 2: pulling the charger cuts the hum noticeably but does not kill it. Test 3: unplugging the speaker cables kills the rest. There were two loops, both closed through the laptop: one through the charger, one through the speakers on a different outlet. Moving the charger and the speakers onto the same strip as everything else deals with the charger loop; replacing the unbalanced TS cables to the speakers with balanced TRS cables deals with the other. Total cost: two cables.

Safe Fixes, and the One Fix That Is Never Safe

Whitlock is blunter than most: defeating safety grounds is "dangerous and illegal", and he advises against ground lifters even when a manufacturer says they are acceptable. The safe principle is simple: break the loop in the audio path, where the signal is a few volts, and leave the power path alone. Every fix below does that.

• One power strip for the whole chain. Interface, computer or charger, speakers and any mixer on the same strip or the same wall outlet. Robjohns recommends plugging everything into adjacent sockets so the earths sit at the same potential. This costs nothing and solves a large share of home-studio cases.
• Battery power for critical takes. It removes the charger's path entirely while you work out the permanent fix.
• Balanced cables wherever both ends support them, and short unbalanced runs where they do not.
• A transformer isolator or DI box on the link that closes the loop: the audio crosses the transformer magnetically, so the grounds on each side are no longer joined through the cable.
• The ground-lift switch on a DI box. It disconnects the audio cable's shield at pin 1 of the XLR output. That is a signal ground, not the mains safety earth, and lifting it is exactly what the switch is designed for.
• Rerouting. Keep audio cables away from power cords, power bricks and dimmer wiring, and where they must cross, cross them at right angles.

If you live in an older building with two-slot outlets, the safe answer is a properly grounded outlet fitted by a qualified electrician, not an adapter. ESFI's adapter guidance warns that three-prong adapters are not properly grounded, and CPSC's home electrical checklist says that if an adapter is used at all, its grounding tab must be connected to the outlet's ground. Where an outlet cannot be earthed, an electrician can fit a GFCI-protected outlet (RCD protection outside North America).

A note on power conditioners: Focusrite suggests one when many devices share a setup, and a decent strip with good connections is worth having. But a conditioner does not break a ground loop on its own, because every device on it still shares the same earth and the audio cables still join their chassis. Its main value for hum is that it puts everything on one strip.

DI Boxes and Isolators

A passive DI box and a line isolator do the same core job: an audio transformer passes the signal from one winding to another through a magnetic field, with no wire connecting the two sides. Rane calls transformer isolation the quickest, quietest and most foolproof way to connect balanced and unbalanced gear, and recommends isolating all unbalanced connections. Whitlock calls it the correct way to eliminate hum, precisely because it leaves the safety grounds intact. Radial describes the transformer in its passive DIs as isolating the source from the mixer to eliminate the ground loops that cause hum, with a separate ground-lift switch on the XLR output for stubborn cases.

• Phone or laptop into a mixer or interface (remote guests, sound effects, a backing track): a stereo line isolator or a passive stereo DI on that one link.
• Interface to powered speakers: a balanced line isolator on the speaker outputs if balanced cables and a shared strip did not fix it.
• Keyboard, guitar or synth into the interface: a passive DI box, with the ground-lift switch available if the transformer alone is not enough.
• Where to put it: Whitlock advises placing input-type isolators close to the receiving device, within two or three feet, because their output is sensitive to the capacitance of a long cable.

Not all isolators are equal. Whitlock reports that input-type transformers with a Faraday shield reject noise around 30 dB better than output types, so if an inexpensive isolator reduces the hum without killing it, a better transformer can finish the job. A poorly made isolator can also thin out the low end, which is worth a careful listen on a voice with a lot of chest.

Dimmers, LED Lights and Magnetic Hum Fields

Not every mains hum comes through a ground. Shure's guidance on 60 cycle hum identifies a second common studio source: a hum field radiated by AC power lines and transformers. Dynamic mics, the usual choice for home voice-over and podcasting, are vulnerable because their voice coil picks up the field directly, and Shure says nothing can be added to such a mic to stop it. Some dynamics, such as the SM7B, include a hum-bucking coil that reduces this, though not completely. Condensers do better, and transformerless condensers do best. The telltale sign is that the hum changes as you rotate the mic or move it a few centimetres, which a ground loop does not do.

Dimmers and LED lighting add another route. A phase-cut (triac) dimmer switches the mains waveform on partway through every half-cycle, and the sharp edges create electrical noise rich in harmonics, which arrives as a buzz rather than a hum. Many LED bulbs and strips contain small switching power supplies of their own, which can do the same. A dimmer on the same circuit as your interface, or an LED driver sitting near a mic cable, is a classic source of buzz that comes and goes with the lights.

• Set the dimmer to full or switch the light off, and record again. If the buzz vanishes, the dimmer or driver is the source.
• Move power bricks, phone chargers and LED drivers at least an arm's length from the mic and the interface, and keep mic cables off the power strip.
• Light the booth from a circuit without a dimmer for recording sessions.
• Do not coil spare mic cable on top of a power brick or wrap it around a power cord.

Remember the acoustic lookalikes too. An air conditioner, a fridge compressor or a computer fan can sound like hum on a casual listen, and our guide to recording a podcast in a bedroom covers those. The test is simple: acoustic noise is still there when you run on battery with nothing else connected, and it changes as you move the mic toward or away from the source.

Removing Residual Hum in Software

Fix the wiring first, always. But if a session is already recorded with hum in it, or a guest sent a file from a setup you cannot touch, software can remove most steady mains hum because it sits at known, predictable frequencies. The tool for the job is a notch filter: a very narrow cut at one frequency that leaves everything around it alone. Audacity's manual describes the method, and the same principle applies in every editor.

1. Find the frequencies. Select a few seconds with no speech and open a spectrum view (in Audacity, Analyze then Plot Spectrum). Look for a peak at 50 or 60 Hz and its multiples.
2. Notch the fundamental at exactly 50 or 60 Hz with a narrow filter.
3. Notch each audible harmonic. Audacity recommends a narrower notch (a Q of roughly 2 to 10) on the higher harmonics to avoid audible artefacts, because those frequencies overlap the voice.
4. Stop when the hum stops. Two or three notches usually clear a hum. A buzz with dozens of harmonics may need a dedicated de-hum tool that notches a whole series at once, or it may be beyond clean removal.
5. Then do the rest of the clean-up, in the order covered in our guide to removing background noise from podcast audio: noise removal, then editing, then loudness last.

For audiobook narrators the stakes are specific. ACX's audio submission requirements set a noise floor no higher than -60 dB RMS, and a steady hum is often what pushes an otherwise quiet booth over that line. Measure the noise floor of a room-tone section before and after the notches, and check the finished file with our ACX audiobook check before you upload. If the hum is still there after filtering, re-recording on a fixed setup is usually faster than fighting it chapter by chapter.

Software is the safety net, not the plan. Every notch removes a sliver of the voice along with the hum, and buzz in particular sits right where speech intelligibility lives. A setup that records clean in the first place is worth more than any amount of repair, which is why the diagnosis above comes first.

Confirm It Is Electrical Before Treating the Room

A hum that survives every acoustic fix is a strong signal to stop treating the room and start checking the wiring. Electrical hum has a fixed pitch at your mains frequency and its multiples, stays the same when you leave the room and come back with the mic still recording, and changes or disappears when you unplug a specific device. Acoustic noise moves with the mic's position relative to the source and does not care what is plugged in where. Knowing which one you have saves money on acoustic treatment that was never going to touch an electrical problem, and on a new mic or audio interface when a pair of balanced cables would have done it. If you are not sure what your room's baseline should be, our guide to room tone and the noise floor explains how to measure it.

Frequently asked questions

Is ground loop hum dangerous?

The hum itself is not dangerous: the voltages driving the loop are tiny. What is dangerous is the most common bad fix, which is cutting the earth pin or using a cheater adapter to lose it. That removes the protection against a fault making a metal case live. Leave the safety earth alone and break the loop in the audio path instead.

Why does my mic only hum when my laptop is charging?

The charger connects the laptop to the mains, and the USB cable's shield connects the laptop's ground to the interface, which may also be connected to earthed speakers or other gear. That completes a loop, or lets charger leakage current into the audio ground. Use the manufacturer's charger, plug it into the same power strip as the rest of the setup, and record on battery for critical takes.

Do ground loop isolators work, and are they safe?

Transformer-based isolators work by passing audio through a transformer, so the grounds on each side are no longer connected through the cable. They do not touch the mains earth, so they are safe. Put one on the specific link that closes the loop, which the five tests above will identify. USB isolators also exist, but many only support full-speed USB, which some audio interfaces cannot run on.

Can I just use a high-pass filter to remove 60 Hz hum?

Only partly. A high-pass filter at 80 or 100 Hz removes the 50 or 60 Hz fundamental, but the harmonics at 120, 180, 240 Hz and up sit inside the voice and pass straight through. Those harmonics are what listeners hear on earbuds. Use notch filters at each harmonic, or a dedicated de-hum tool, and fix the source if you can.

Can a USB microphone get ground loop hum?

Yes, if the computer is part of a loop: a desktop or a charging laptop that is also connected to earthed speakers or a second screen can put hum on the USB ground the mic shares. The diagnosis is the same: test on battery, then unplug other connections one at a time. Our guide to USB vs XLR microphones covers the other trade-offs.

Is the ground-lift switch on a DI box the same as lifting the earth?

No. A DI box's ground-lift switch disconnects the shield of the audio cable at pin 1 of its XLR output. That is a signal ground carrying no mains voltage, and the switch is there precisely so you can break a loop safely. Lifting the mains earth, by cutting a pin or using an adapter, is a completely different and unsafe thing.

Sources and Further Reading

• Rane Note 110: Sound System Interconnection: how loop current through audio signal ground creates hum, balanced vs unbalanced, the pin 1 problem, AES48 shield practice, transformer isolation, and keeping factory grounding intact.
• Jensen Transformers AN004: Hum and Buzz in Unbalanced Interconnect Systems, by Bill Whitlock.
• Jensen Transformers AN003: Interconnection of Balanced and Unbalanced Equipment.
• Bill Whitlock, Grounding and System Interfacing, audioXpress: common-impedance coupling, why ground lifters are dangerous, transformer isolation and placement.
• Shure: What's That Noise and How to Fix It: hum fields, dynamic mic voice coils, hum-bucking coils and transformerless condensers.
• Focusrite Support: Why is there constant noise in my monitors?: laptop power supply test, approved chargers, balanced TRS outputs, finding a second ground path.
• Sound On Sound, Hugh Robjohns: Do laptops sound worse when using an AC adapter?: USB shield loops, battery power, adjacent sockets and full-speed USB isolators.
• Analog Devices ADuM4160 full and low speed USB digital isolator: 1.5 and 12 Mbps data rates.
• Radial Engineering ProDI passive direct box: transformer isolation and the pin 1 ground-lift switch.
• ESFI: Avoid Unsafe Electrical Adapters and ESFI Indoor Electrical Safety Check: never remove the grounding prong.
• CPSC Home Electrical Safety Checklist: adapters, grounding tabs and three-prong plugs.
• OSHA Construction eTool: Electrical Incidents: do not remove ground prongs from equipment or extension cords.
• IEC World Plugs: mains voltage and frequency by country.
• Kansai Electric Power: Electricity in Japan: the 50 Hz east and 60 Hz west split.
• Audacity Manual: Notch Filter: removing mains hum and its harmonics with notch filters.
• ACX Audio Submission Requirements: the -60 dB RMS noise floor limit for audiobooks.