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.”

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

Full story: https://www.headphonesty.com/2025/11/industrial-power-filters-beat-audiophile-power-conditioners/

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.