How can I measure the noise on my AC mains with a 2ch oscilloscope?

I’m not an EE so although I have some nice test equipment I need help. Here is the problem, the AC Mains are 120 vac but I’m trying to measure the noise on the AC signal , millivolts. I’d like to answer two questions how bad is my power and second  does my isolation transformer make a difference.


thanks for the help 


Hey OP!

I’ve done this before and I have some thoughts. DIYaudio is a better place for this type of hackery. You should have a 10:1 or 100:1 probe for your scope.

The thing I learned though is that an oscilloscope by itself is not actually very useful except in seeing really gross issues. For instance, I had an LED lamp which caused audible buzzing and you could see the "bite" the power supply took out of every positive going AC cycle. The bad news is that unless things are gross you don’t really get any sense of how good or bad the AC waveform is. For this you need to know things like harmonic distortion and/or a spectrum analyzer. First will give you a basic readout of how close to ideal the wave shape is, second will tell you more or less where in the frequency spectrum your problems are.

You might want to consider a PC based software solution, which uses the audio inputs, and maybe a custom, isolated interface to prevent the risk of injecting raw AC into your PC’s audio input. There are some good online resources for how to make one of these.  The limiting factor here is maximum frequency response will be limited to about 20 kHz. You won't see any RFI noise this way.

Your components' power supplies are designed to putput clean DC to activate the circuitry. No need to worry neurotically about noise on the AC line. Competent engineering takes care of that.

Thanks you sharing your insights and perspective. I never thought of this as being a Hack… hmm 

yes the AC into the OScope is an issue even with my 10x or 100x probs.

Can you share some leads to these “on line resources” ?

Hey OP? 



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OP.  While that is a nice experiment, it isn't really necessary.  Nobdy has good quality power coming into their home today.  Too many non-synchronous loads on the line that mangle the sine wave.  My power generally runs 4% thd or so.  

I don't have a stand along o-scope.  My PSA P10 has a built in scope with fixed settings to measure it.  Here is a random graph:


Carlsbad2 - thanks for the graph. I’ve looked at my waveform and it’s clipped at the peaks and some junk on the wave too .  I’m really looking to see how well my power conditioner works. If we can agree that street power and house loads make it worse, but well designed gear will handle it, what was the reason you’re regenerating your power? Your PSA was an expensive investment?




OP stands for "Original Poster" or the person who started this thread.

It's a good idea to tag those you reply to so they are aware. 

Shame on everyone discouraging you from experimenting.  Shame!  We need more hobbyist and tinkerers, not fewer, and we need more of them to have first hand knowledge of home power noise.  It also doesn't matter WHY the OP wants to tinker.  I remember many years ago I was thinking about making my own power regenerator.  It was a fun project and I would have learned a lot.  I went into an EE forum asking for help and one member immediately pulled out some thread I had typed in an audio forum to basically say no one should help me. 

Here we are now in an audiphile forum and I can't believe the number of opinions discouraging the OP.  I'm really disappointed in those of you doing so. Maybe this is just a learning experiment for the OP, and that's all it has to be.

Here is an article on doing so with a scope and an interesting thread from Stack Exchange and another from DIYaudio.

Like Jason said.  Power doesn't matter.  Hook it up to a honda generator.  It'll be fine.  🤣

It’s surprising that Honda didn’t have a booth at Axpona.

Eric… I learned what OP means and that’s relevant! Perfect.

It’s amazing how much can be learned by reaching out to folks, and with a little bit of logic and some engineering, and like my Dad thought me … he would say well it’s already broke so we might as well try to fix it.

ok so I now have a fully active 3-way and subs open baffle system that I have turned . Convolution filters running in Roon with 132k taps to exploit digital processing!! Measurements with Earthworks Mic.

the amps I designed from Class D Pascal Audio modules … it’s amazing how critical it is the have amps that deliver Slew rates to follow the instruments!

Eric I try not to let “knuckleheads” get in the way of a hobbie !! Thank you for these links 


I just installed a PS Audio Power Plant 15 yesterday and has made a HUGE improvement in my listening enjoyment. I was using a APC H15 power conditioner previously with good results but as soon as I lit up the PP15...bruh. Clarity, detail and dimensionality were significantly improved. Slam dunk purchase for me. PS Audio is selling them a 20% discount r/n on their website. 


@badbruno If you are asking me: The PP15 max output is 1500W. If you need more, they have the PP20. I'm running everything off my PP15 and only at 40% load.

Also: The reason I upgraded is b/c I noticed the system sounded better at different times of the day. 

@badbruno I think power is very important to all audio equipment.  Indeed, there are some amps that can handle bad power but all amps appreciate good power.  The advice to "plug your amp directly into the wall" comes from dealing with bad power conditioners, which is most of the less expensive ones, including some name brands. 

I am a big fan of PA Audio but I know there are others that are ok.  Bottom line it, filtering won't do it.  PS audio, and other effective models, rectify the power and then invert it (let me know if you don't know those 2 terms) back into very high quality power.  

So you got your scope working and got a trace?  excellent.  


Power doesn’t make a difference crowd is counter to my experience. Two caveats here, my experience is limited to my local power and secondly, I haven’t tried a Honda generator. The greatest improvement to my system was the addition of the Shunyata Everest 8000 and Shunyata power cables. A second improvement came when I moved to battery for my full system. With a Synergistic Power Enhancer plugged into the inverter, Everest off inverter and full system, except turntable plugged into the Everest. I no longer care about local power, nor worry about lightening. My battery charger connects through both a home surge protector and a heavy duty, noisy local surge protector. That said, still no guarantee that lightening won’t fry my system, but less likely.

I went through a similar exercise a few years ago, and after getting a zillion contradictory opinions from well-meaning audiophile buddies, finally brought in the big guns -- the electrical contractor who managed the replacement of Michael Fremer’s house wiring (see Mikey's YouTubes & Stereophile articles for the blow-by-blow) and Audioquest guru Garth Powell himself. I also wrangled sessions with engineers from our local power utility and from Tesla, which installed my rooftop-PV system. I figured that, with my engineering & electronics background, all I would need would be a little direction in order to analyze and address any noise issues by myself.

However, they all agreed that what you & I want to do would require tens of thousands of bucks worth of specialized analyzers. To my amazement, even my utility’s engineers didn’t have access to such devices. A simple scope and 10X probes, or even one of these PC/phone-based spectrum analyzers, doesn’t provide all the information you need to fully understand power-line anomalies. There are many types of noise (including 2d-order FX produced by the interaction of noise components with each other & with an audio signal) that are relevant in this type of use case that even a top-notch amplfier’s internal power supply isn’t designed to address.

For example, as Garth has demonstrated so effectively, even high frequency noise -- hundreds of KHz, I presume -- can create low-amplitude sidebands that aren’t filtered out by a conventional component PS, and that leak into the audio path to compromise the resolving power and transient response of even a modest audiophile-class stereo. Analyzing & mitigating the FX of line noise is not as simple as measuring an amplfier’s THD or filtering 60-cycle hum.

Despite all this, I still didn’t give up & picked up a prosumer "line-noise analyzer" that gave me a ballpark idea of aggregate noise levels within different frequency bands. I was then able to at least qualitatively understand how the levels of certain bands of noise fluctuated as a function of time-of-day. Surprisingly regular. But again, not real useful, beyond confirming that the relatively inexpensive inverters in my PV system produced more noise than did my power utility.

So I finally, finally! gave up and sunk $7-10K into a Garth-designed Audioquest Niagara conditioner and complementary power cords. A lower-end version of what Fremer himself was using with his Dartzeels & Chronosonics.

Happy ending. The Niagara produced one of the most dramatic improvements in SQ -- imaging, soundstage, detail, physicality, air, you name it -- I’ve ever experienced in a system I’ve owned. Beyond anything I’d expected. And I’ve been building systems from Stereophile-class components for almost 50 years.

When it comes to power, as with cables or anything else audiophile, the devil is of course in the details. What worked for me might be ineffective for you, especially if power quality is not your weakest link. But I can say this with the authority of personal experience: Once I started interviewing people who really knew what they were talking about, it quickly became obvious that trying to fully quantize a typical home’s line noise before taking corrective action is neither necessary nor sufficient -- and would require pricey, specialized gear that cost more than a full-blown, top-of-the-line conditioner.

My 2c.

Gents your experiences are extremely helpful. I’m using a Core Power Technologies DEEP=CORE 1800. And with my AMT tweeters it’s quieter. But not sophisticated as your solution. I do believe the Honda inverter is cleaner power ! 

I’ve heard the same comment from a friend that switched from PS Audio P20 to a Shunyata he never looked back .

i have a High freq current probe for the o scope that I might look at up stream and dwn stream of the power conditioner? 

thank you all for your contributions




Are you really interested in answering the questions posed in your starter post, or are you interested in your system sounding more like live music?

I'm definitely sold on the Audioquest Niagara solution at this point, but Shunyata, despite a very different design philosophy, also makes superb gear.  Just be sure that you deploy comparable power cables on your amplification components.  The conditioner is only part of the story.

Good luck.  Hope I was able to help even a little.



yes your involvement was helpful and has given me new factors to consider while on this path. And good luck while enjoying your superb system !

it’s been a pleasure have the conversation 



If only Edison had won the power war. We would have DC direct from the wall outlet. 😊

"Live Music" is full of phase anomalies, microphone differences, crowd murmurs and smells, bow rosin dust, insane sound technicians, a lack of sound technicians, balding head reflections, unseasoned wood flooring, drapery...all of which you can try to measure or simply sit there and listen to it. What a mess... 

No need to worry neurotically about noise on the AC line. Competent engineering takes care of that.

I wish this was the case, and it is obviously helpful for many components, but as many people remark about hearing system differences depending on time of day or what else in the house is running concurrently, then it is not a blanket "fact"

Like Jason said.  Power doesn't matter.  Hook it up to a honda generator.  It'll be fine.  🤣

The emogee gives me hope that this was said in jest. Even the quality of the inverter when running a battery power system makes a world of difference. Unfortunately, there is no one solution that works for everyone, hence all the companies doing a brisk business these days.