Because of the presence of mains current in mains-powered audio equipment as well as ubiquitous AC electromagnetic fields from nearby appliances and wiring, 50/60 Hz electrical noise can get into audio systems, and is heard as mains hum from their speakers.
In addition to mains frequency humming, the mains power can also contain higher frequency noise that is often referred as EMI (Electromagnetic Interference) noise. Problems range from straightforward ‘hums’ (which normally include various levels of the mains harmonics, such as 50Hz, 100Hz, 150Hz, and so on in the UK, or 60Hz and higher multiples in the US), to a wide range of scratches, ticking, buzzing and other digital gremlins.
Don’t let buzz, hum, or hiss ruin your AV experience. Check that your wiring is is right and after that if problem persists turn to look mains power quality. It’s only worth ‘cleaning up the power signal’ if it’s dirty, and a huge number of background noise problems are caused not by mucky mains, but by audio wiring that results in ground loops. This is the source of lots of unwanted nasties that sneak into your audio signals.
There are many products advertised to help power noise issues. Advertised tools include mains filters, isolation transformers. UPS devices and even special power regeneration devices. Slightly cheaper than an online UPS, but absolutely effective against all kinds of line noise is an isolation transformer.
To get idea of mains power noise take a look at Electrical noise on your power line video
One EMI meter that is commonly used to measure mains power noise is Greenwave Broadband EMI Dirty Electricity Meter sold at Amazon. Greenwave Broadband EMI Dirty Electricity Meter measures the dirty electricity (a.k.a. electrical noise, line noise, power line EMI) present on the wiring in buildings. It is designed to easily compare EMI levels before and after installing dirty electricity filters. Simply plug the meter into electrical outlets to get measurements. This instument says that it measures a broad range of dirty electricity frequencies at approximately 3 kHz — 10 MHz frequency range. The meter includes a speaker that lets you listen to the dirty electricity on a circuit and hear noise to decrease as filters are installed.
Listen To Dirty Electricity with Greenwave EMI Meter. Hear the Difference Greenwave Filters Make!
There are also other mains EMI meter brands. Ebay sells also OLED Digital AC Noise Meter EMI Tester Wideband AC Power Ripple Analyzer and similar.
There are many companies that sell mains filters that promise to make wonders in the mains power filtering. Not all of them seem to live up to their promises, and it seems you need to be careful what you buy. And you should test the product before buying.
PS Audio is selling incredibly small and advertised to be effective noise harvesting instrument converts damaging line noise into harmless light. Their video What’s a Noise Harvester? tells about the product and promises a lot:
The question is if PS Audio’s product is incredibly small and effective noise harvesting is as effective as promised. How does it work and how effective is it? It seems that based the on several tests it is not very effective.
PS Audio Noise Harvester Review (AC Filter)
In the video above Amir dropping the Hammer on the Snake Oil. According to this video this looks like to be true snake oil…that real living snakes.. wouldn’t want to go near..but some audiophiles… really like snake oil.
Does PS Audio Noise Harvester Really Work For Cleaner Electricity video shows a quick test to show whether or not it really does what the manufacturer says it does.
Video comments: Ps audio, ” it does something “. Yes it flashes a blue light. AudioScienceReview has measured this device. Thank you for this review!
Audio Science Review on PS Audio Noise Harvester AC Cleaner Review has lots of measurement results that do not seem to be promising on this filter performance. Audio Science Review proved with measurements that this product does absolutely nothing useful. In some of the tests made the noise actually went up when you plugged in the snake oil.
Collinsaudio took some measurements and wrote a report that has also some technical details what is inside the device. If anything this devise adds switching noise, because inside there is a transistor, triac, inductor (choke) and a few caps.
“If I were to speculate on the operating principles of this device, I wouldimagine that the power line is AC coupled then applied to the primary of the toriodialtransformer. The secondary is then connected to some type ofhigh-pass filt er an d fin ally t o a mo n o-stablethat flash es the LED.While it’s somewhat hard to imagine the benefits of this type of line filter, I was unable see any objective improvement with the Noise Harvester™.”
This seems be be pretty muuch purely a psychological device. This seems to be audiophoolery specifically “designed” to rip off gullible people. Brand should be more like BS Audio.
A real effective mains noise filters has power flowing through it and has several LC filter stages. Something like this:
118 Comments
Tomi Engdahl says:
DIY Hifi emi filter
https://youtu.be/yFjMZhhj9PE?si=ETq9OAht7SDA7DEg
Tomi Engdahl says:
Internal structure: Nordost QRT Qv2 AC Line Harmonizer | odear
https://m.youtube.com/watch?si=lekfUBqXqTiDc4tO&fbclid=IwZXh0bgNhZW0CMTEAAR0RgYGu2oMkreKaiL9iO44jiOoeMSofVG56SedEruhGbey4twBFwbc95Ms_aem_LpOvUPMP54TZkf8-N7270g&v=uw9xdTjZPBQ&feature=youtu.be
Tomi Engdahl says:
Ilmainen sähköisessä muodossa oleva kirja EMC-suunnittelusta https://link.springer.com/book/10.1007/978-3-031-14186-7?fbclid=IwY2xjawER_vVleHRuA2FlbQIxMAABHYi6VAyR6bWIBVuCCO3GYF-ku2Thtd56pMFLTfdByy0RpzT53ESR-OT5og_aem_rfR3Fp-8cDaLfZEDqhPd0w
Liitteeet J ja K kertovat EMC-vaatimukset. Ellei niitä ymmärrä, pitää lukea kirja siihen saakka. Kirjan lukemalla saa myös osviittaa sihen, miten vaatimukset voidaan täyttää.
Tomi Engdahl says:
Thomas Anderssen lainaus linkkaamastani kirjasta:
AC mains power filters. AC mains power filters require capacitors with a
sufficient safety rating (IEC 60384-14; USA: UL 1414 and UL 1283; Canada:
CAN/CSA C22.2 and CAN/CSA 384-14; China: GB/T 14472) because a failure
of a mains power filter capacitor could result either in fire (short circuit of an
X-capacitor) or in electric shock (short circuit of a Y-capacitor). The safety
classifications for X- and Y-capacitors according to IEC 60384-14 [7] are
presented in Table 11.1.
Tomi Engdahl says:
These small filters are popular devices that can be supplied with a wide range of X and Y values to suit most purposes.
https://lcrcapacitors.co.uk/capacitors/delta-filters/
Tomi Engdahl says:
https://www.audiosciencereview.com/forum/index.php?threads/ps-audio-noise-harvester-ac-cleaner-review.14122/
Tomi Engdahl says:
https://www.audiosciencereview.com/forum/index.php?threads/ps-audio-noise-harvester-ac-cleaner-review.14122/page-2
Tomi Engdahl says:
EMP testing, or Electromagnetic Pulse testing, involves evaluating the resilience of electronic devices and systems to electromagnetic pulses. These pulses, which can be caused by events such as nuclear explosions or solar flares, can disrupt or damage electronic equipment.
ATLAS-I better known as Trestle, was a unique electromagnetic pulse (EMP) generation and testing apparatus built between 1972 and 1980 during the Cold War
https://en.m.wikipedia.org/wiki/ATLAS-I
Tomi Engdahl says:
By the way Marx Generator was invented 100 years ago
https://en.wikipedia.org/wiki/Marx_generator
Tomi Engdahl says:
https://www.planetanalog.com/a-short-primer-on-x-and-y-capacitors-in-ac-power-supply/
Tomi Engdahl says:
https://www.facebook.com/share/p/169VN7dCYK/
Tomi Engdahl says:
There is a difference on how well different amplifiers/electronics do filtering, some do it well and some not do filtering so well. Of your incoming power is noisy and amplifier is one with not so good built in power supply, it is possible to hear external power filter to have some effect. If incoming power is clean and devices well built, no effect if power filter or not. I have has two devices and several locations where power filtering has helped. One was my DIY amplifier located on same building that has powerful HAM radio station in it. Other was home hifi amplifier as part of mobile DJ PA system, without filter there was strong annoying pops from light controls and with filter there was still some noises but at low enough that system was usable. None of the filters have been expensive audiophool products but mains power filtering components used in industrial electronics or IT systems.
Tomi Engdahl says:
Kemp Mains Noise Detector
Tool to convert mains interference (EMI/RFI) into sound
https://www.technologyfactory.eu/en/kemp-elektroniks/discontinued/kemp-mains-noise-detector/a-3640-10001564
Tomi Engdahl says:
How measure safely AC mains pollution noise
https://www.diyaudio.com/community/threads/how-measure-safely-ac-mains-pollution-noise.315953/
If you use a pair of standard oscilloscope probes at 10/1 as a differential input with series capacitors of 1 nF 1000V you can safely look at the AC line.
Even if you touch the AC line after the isolation capacitors that is less than 50 micro amps of current. Well below leathal but probably enough to be educational.
Now if you want to feed a spectrum analyzer you can use capacitors feeding a voltage divider into a wide band instrumentation amplifier.
The important issue to have a high enough input impedance that the capacitor coupling will provide a high level of isolation.
Lethal current can start at 5 mA, a theory I was taught but never have tested. Normally a safety factor of 10 would be applied, but as you can do 100 quite easily that is where I would aim. Keep in mind even with a transformer has leakage capacitance.
I would also suggest you build a box to contain all the high voltage parts to maintain safety.
Of course the other method is to use an oscilloscope that is designed to measure power line distortion like the Tektronix THS735.
Tomi Engdahl says:
https://www.diyaudio.com/community/threads/snubway-noise-defender.408583/
Tomi Engdahl says:
Engineers have the graphs audiophile brands don’t want you to ask for.
Full story: https://www.headphonesty.com/2025/11/industrial-power-filters-beat-audiophile-power-conditioners/
Tomi Engdahl says:
For years, audiophile power conditioners have been sold as essential upgrades for better sound. Yet when independent testers compared them to industrial filters used in laboratories, the results were hard to ignore.
What makes this gap even more striking is how little data high-end brands share about their products. Once you see how industrial engineers approach the same problem, the luxury price tags start to look strange.
Full story: https://www.headphonesty.com/2025/11/industrial-power-filters-beat-audiophile-power-conditioners/?utm_source=fb&utm_campaign=caption
Tomi Engdahl says:
Audiophile Power Cords Only Sound Different for One Reason That Has Nothing to Do With the Cable, Says Hi-Fi Designer
https://www.headphonesty.com/2026/10/power-cords-sound-different-chord-dac-designer/?utm_source=fb&utm_campaign=thumbnail
The difference was convincing enough to challenge his own assumptions, but the explanation pointed somewhere unexpected.
Rob Watts, Chord Electronics’ Digital Design Consultant and creator of its Watts Transient Aligned (WTA) and Pulse Array technologies, once dismissed the idea that power cords could change a stereo’s sound.
Yet in the 1980s, he plugged a solid-core mains cable (the British term for a power cord) into his rig and heard it differ from a stranded one anyway.
That unexpected result led Watts to investigate what else might be affecting his stereo’s sound. He eventually built a filter to reduce radio-frequency (RF) interference from the mains supply, and when he compared the cables again, he could no longer hear a difference.
The cable comparison wasn’t the only thing Watts couldn’t explain. His stereo also sounded worse whenever the kitchen fluorescent light was switched on, while it often sounded sweetest and smoothest around two o’clock in the morning.
Suspecting RF interference, Watts built a mains filter starting with one inductor and capacitor, then added stages until there were twelve.
As he added more stages, he heard the system become warmer and more natural. The filter had already affected the sound before he tried the two cords again.
But when he compared the solid-core and stranded cables again, the result was different.
“There was no audible difference. None,” he said.
Watts says that once he had filtered his preamps, power amps, turntable, and everything else, the kitchen light switch and the time of day stopped changing the sound, too.
How Wall Noise Gets Past a Well-Regulated Preamp
Power-supply rejection often weakens at higher frequencies. Texas Instruments notes that analog components can have only 10 to 20 decibels of rejection around 30 megahertz. But those are general figures from modern electronics, not measurements of Watts’s 1980s equipment.
RF can enter an amplifier through its inputs, outputs, or power-supply pins. In some circuits, internal junctions rectify interference far above the amplifier’s normal audio bandwidth, producing a lower-frequency disturbance even though the circuit cannot reproduce the original RF signal as sound.
Under certain conditions, the resulting interference can follow changes in the RF signal. Watts believes this may help explain what he heard in his stereo, an effect he calls noise-floor modulation.
“The noise floor itself effectively moves with the music.”
A cord’s ability to pass or reject RF interference depends partly on the electrical characteristics of the wall socket and the equipment’s power supply.
That dependence on the connected equipment helps explain why the same cable may behave differently from one system to another, as physicist Jim Lesurf’s transmission-line analysis shows. Choosing a cord specifically to reduce RF interference is therefore unpredictable.
Before You Buy Another Cord
Fluorescent lights, some dimmers, and switching power supplies can put high-frequency interference onto household wiring. Watts’s kitchen-light clue gave him a reason to investigate that possibility in his own system.
For that reason, Lesurf recommends filtering the incoming interference directly:
“If you wish to filter the mains input, then my suggestion would be to buy a mains filter unit, not a new mains cable!”
If RF interference is the problem, buying a more expensive cord is a costly way to investigate it. A Nordost Valhalla cord cost $2,500 in a 2004 review, while a nine-piece set of snap-on ferrite clips was recently listed at $11.99.
Of course, those clips aren’t an equivalent substitute for the expensive cord or a purpose-built mains filter. The comparison simply shows the wide range of prices involved in trying to control RF interference.
Furman’s M-8×2 conditioner lists at $139.95 and specifies more than 26 decibels of RF/EMI filtration from 200 kilohertz to 10 megahertz.
But that specification only covers the stated frequency range. It doesn’t establish how the conditioner performs above 10 megahertz or whether it would improve the sound of a particular stereo.
Before buying anything, it is worth checking whether the audible problem changes when likely household noise sources are switched off. If it persists, the next step is to investigate where the noise is entering the system.
The power supply itself is another place to investigate. For compatible USB-powered equipment, Watts has suggested comparing the supplied adapter with a suitable battery bank before buying a replacement linear supply. His advice comes from a secondhand forum account rather than an independently available original post.
What the Evidence Says About Power Cords
Watts wasn’t the only listener to report that cable differences disappeared after improving RF rejection. In 2001, a reviewer initially heard aftermarket power cords improve his Herron monoblocks, but the difference vanished after the amplifiers were updated to reject RF interference better.
Both accounts involved listeners who initially heard cable differences. But a 2004 blind comparison produced a different result.
Grounding presents another possible explanation for noise in audio systems. Engineer Bill Whitlock identifies ground paths and leakage currents as common causes, which would require different remedies from RF filtering.
“It taught me that if I hear something which contradicts my existing model, I shouldn’t simply dismiss the listening result. I need to find the actual mechanism.”