Impact of AC Distortion & Noise on Audio Equipment

[voodooless]

As I wrote recently, there is some case to be made that there is a class of devices that are sensitive. It isn't just a matter of being poor quality. It is is being intrinsically sensitive, so much so that they measure well with good power and degrade with bad.

They might not be of poor quality, but they are then poorly designed. These products should just not have this property. Stating the fact that there might be examples that exhibit this problem, doesn't justify them, nor the devices that pretend to fix these issues.

 

[solderdude]

What's tested here is the voltage between L and N.
As all audio equipment has a rectifier in there followed by reservoir caps the only thing that matters for the incoming AC is the tops (both negative and positive) of the incoming AC as only during short moments (at the peaks) the reservoir caps are replenished.
The actual waveform is not of much importance for the vast majority of audio equipment.

It's a good thing this is verified/tested because a lot of nonsense is written about power supply. It's mostly about 'audiophile special qualities' though which cannot be measured anyway.

The biggest culprit in audio gremlins, however, are common mode currents (and thus also voltages).
Common mode crap is equal on L and N (so there is hardly anything measured between L and N) but exists between ground.
This can be safety ground but also a floating ground (signal ground) that always leaks a bit to actual ground.
Those grounds are NEVER clean because of EMC filters used and or capacitive leakage.

Those gremlins, the common mode stuff, is what folks are usually bothered by.
Each device, even when on battery power, has a different leakage to ground. These different leakage currents want to find the easiest (think lowest resistance) path. That path is the interlink ground connection.

In case of a balanced connection that ground (or shield) is not directly in the audio path so folks using balanced audio connection usually are not plagued by this.
Most folks use regular RCA cables or 3.5mm TRS connections.
In those connections the audio signal is between the shield and signal.
What happens in this case is that the common mode currents (containing all kinds of crap between audible and cell phone frequencies) travel through the shield and as it always has some resistance a small voltage drops across the shield.
This 'crap' voltage thus is added to the actual signal resulting in signal + crap being heard.

This kind of thing is highly dependent on:
• used interlink cables i.e. screening and resistance of the screen incl. connector.
• wiring and PCB layouts in the used equipment.
• leakage current of the used power supply in the audio component
• mains voltage
• filtering in the devices
• safety ground or not and if there is safety ground how 'dirty' it is.
• signal voltages
• balanced or SE audio signal

As all audio equipment (consider a connected PC, TV, monitor, streamer as well) differs and the cables people use differ as well we can quickly conclude that the combination the owner has is a one off. For everyone, even when using the same equipment, the situation differs.

For this reason it may be problem free for person A but someone else may be plagued by this.

Here comes the difficult part. This can ONLY be properly measured in each particular situation. One could measure leakage currents of individual devices but that is only part of the puzzle.

This is where power conditioners (in whatever form) MIGHT do something. It is pointless to use something like this if it is below audible levels.
In fact.. use the wrong 'conditioning' and you may even make it worse.

This all means... it requires different solutions in different cases and not everything (may) work. It also cannot be tested properly on a test bench as it also depends on the used cables and power supply in your wall.

 

[wwenze]

Excuse me but 3 out of 3 use PWM power supply units; why not to check some with "analogue" linear PSUs?

I feel there is some merit in this question. Because SMPS have the rectifier before the transformer i.e. you can actually feed it pure DC or pure garbage and it will probably not care. (Not sure how APFC would react tho.) While normal 60Hz supplies the transformer can be particular about the frequency component. "Oh you are going to half or double the frequency? I'm going to whine and then burn myself to death."

However, the typical transformer also adds distortion, which is cleaned up by the capacitor anyway. And if the AC mains has even more nasty frequency components, these are unlikely to make it to the secondary side due to poor bandwidth of the transformer.

 

[pma]

What's tested here is the voltage between L and N.
As all audio equipment has a rectifier in there followed by reservoir caps the only thing that matters for the incoming AC is the tops (both negative and positive) of the incoming AC as only during short moments (at the peaks) the reservoir caps are replenished.
The actual waveform is not of much importance for the vast majority of audio equipment.

Correct.

And I think the only equipment that would show any difference in @amirm measurements would be Pass ACA and tube amplifiers.

BTW, I made quite a lot of measurements on home AC power and the results were similar to the one posted in #1, including THD.

 

[DACs_Lover]

How many Pink Panthers Amir has?

 

[Mrgoogle]

This is an ASR research project to see what impact AC noise and distortion has on performance of audio equipment. I have been wanting to write this article some three years ago and finally getting a chance to do the testing and release it.

As you will see, the heart of this study is a special instrument: the BK Precision 9801 Lab AC programmable generator:
View attachment 145395

With this machine I can generate a wide range of AC voltages, frequency and as you will see, distorted AC waveform. It is a massive box despite its smallish front. It goes way back and has a noisy fan to keep it cool. Heat is generated because this is basically a high-voltage audio amplifier. The audio source is a computer generated waveform but the rest is not much different than amplifier other than being above to generate hundreds of volts (which causes a lot more heat generated). The 9801 is rated at 300 volt-amps. There are higher power AC generators but they get massive in size and weight.

I also have a PS Audio P300 AC regenerator. It is even larger and heavier than the 9801 because it doesn't use a fan. It is much more limited though as you can only change the frequency. I will show its performance but to keep this project manageable, I am only going to show its spectrum.

General Test Protocol
I am going to show you the performance of three different audio products. Each will be powered by generic AC coming out of the wall, then by 9801 AC generator with and without distortion. Each one of these three modes has massively different AC spectrum of noise and distortion. If there is something to AC power making a difference in audio equipment, we will see it.

There are a lot of large dashboard shots here so you may want to review this article on a computer or a tablet. Maybe hard to read them using a phone.

PS Audio P300 and BK Precision 9801 AC Quality Measurements
Let's start as usual with analyzing the spectrum of noise and distortion of AC on my workbench. The source is a 20 amp feed going to a very stout power strip in my equipment rack that powers my workstation, Audio Precision Analyzer, and audio equipment being tested. A shared supply like this helps a bit to keep ground currents lower. Here is what AC looks like from my last review a couple of nights back:

View attachment 145396

We see our mains peak at 60 Hz which is delivering the bulk of the power. With it though we see harmonics of it and noise going all the way up to 90 kHz bandwidth of the measurements. Total distortion+noise is 1.9% and that is enough to visibly distort the waveform where the tops of it no longer look like a nice sine wave. Here is what it looks like with PS Audio P300:

View attachment 145397

Much nicer now. The sine wave looks like a sine wave now. SINAD as a measure of noise+distorion jumps massively from 34 dB to 61 dB. Second harmonic is down to -65 dB which is what mostly determines that figure.

Now let's examine the spectrum of BK Precision 9801:

View attachment 145398

Our sine wave is pure as before. SINAD is improved to 65 dB due to second harmonic dropping a few dBs. There is however mid-frequency noise that is higher than PS Audio P300. Overall, this shows that the P300 is doing its job to produce very clean power.

By the way, I am using a high voltage differential probe to sample the AC power. It divides the voltage by 100. That probe itself has some distortion. I measured that it and it is at or below 0.01%. So it is not making hardly any contribution to the numbers and waveforms you see above. I did not capture its output by it only has a tiny second harmonic spike and nothing else. So our instrumentation is much better than what we are measuring as it should be.

As I noted in the introduction, the BK Precision 9801 has a special feature to create distorted waveforms. What it can do is simulate what comes out of a light dimmer which chops off parts of the AC waveform. This causes the duty cycle to lower and with, produce lower power to the light bulb. I can adjust the amount of "dimming" by degree. Here is what the AC wavform looks like when I set that to 35 degrees (just a random number I picked):

View attachment 145399

Let's agree this is the mother of all dirty AC feeds! Due to step response in the AC waveform, we create infinite harmonics which you can see as a spray in our FFT graph. SINAD has dropped to just 15 dB and THD+N has shot up to 18%. Surely if our audio equipment cares about quality of the AC power, it should produce far lower performance using this type of AC feed. Let's see if this is the case.

Audio Equipment AC Impact Measurements
Let's stay with state of the art audio device because anyone searching for AC mains tweaks is surely also investing in the best audio gear they can. And at any rate, I never see any company advertising that their AC filters/regens are for poor performing audio equipment. Let's start with generic AC coming out of the wall:

View attachment 145400

Superlative performance as we have seen. Distortion is down to -128 dB in the form of third harmonic. Now let' switch to running it on our 9801 lab AV regenerator:

View attachment 145401

No difference at all. We expected this from prior testing of AC cleaners and such. But now let's subject it to the "torture" test of dimmer simulation in 9801 with its 18% THD+N:

View attachment 145402

Not a difference. Zilch. Nada. Clearly the A90 is filtering the AC mains well enough that even a highly distorted AC waveform doesn't bother it.

So people say that maybe a "low quality" audio device cares. Hard to know what this device would be. I looked around and found this old California Audio Labs Sigma tube DAC I had bought in an auction as a donation to our local audiophile society. Let's first run it with generic AC again:

View attachment 145403

Looks like we found our low performing device! It is actually not bad for a tube device but certainly miles different than what we like to see around here.

Let's switch to BK Precision 9801 and see if performance improves:

View attachment 145404

Nothing of note. The tiny bit of variation is due to vagaries of this tube product.

I was just about to switch the 9801 to dimmer mode and the DAC went crazy with its noise floor jumping way up and then down. I played with a bit but could not get it to be stable. So I had to abandon it at this point.

I looked around and found a Loxjie P20 tube headphone amplifier I had bought last year but never tested. Here is how it does with generic AC:

View attachment 145406

We definitely have a distortion factory here. Let's feed this one clean power to see if it makes a difference:
View attachment 145407

Nothing. Everything from distortion to mains hum and general noise remains the same. But maybe the torture test of dimmer simulation does it:

View attachment 145408

Nope! Tiny variations are native to the device. There is no trace of this device caring one bit about quality of the AC mains input.

Conclusions
Up until now we have tested a number of devices that reduce AC noise/distortion only to find them do nothing for the output of audio products. In this little research project, we went the other way, producing very dirty AC feed. Yet three devices from very different origins show zero, and I mean zero, dependency on AC quality. You could argue that we got unlucky with three devices not being sensitive but hard to make a case that they simply did not care.

Of course the explanation is clear: all of these devices first convert mains power to DC and then use it. DC by definition means no variation so filtering is used to remove noise and distortion. Sure, some remains but the rest of the circuit also has immunity to power supply vagaries. By the time we look at at the output of the audio product, we are so, so far away removed from AC that its "fidelity" makes no difference.

With both engineering knowledge and objective measurements backing each other, the conclusion that you don't need to worry about quality of your AC mains is exceptionally strong. This is in the context of fidelity of course where vast majority of these audio tweaks are sold.

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As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

 

[Mrgoogle]

I have this Torus Power , it is day and night , it filter both common mode noise and differential noise , watch following, it is based on old patent from Valve tube radio time . Amir measure differential mode noise , what about common mode noise ?!

 

[pma]

The biggest culprit in audio gremlins, however, are common mode currents (and thus also voltages).
Common mode crap is equal on L and N (so there is hardly anything measured between L and N) but exists between ground.

N vs. PE, typically (+HF mess, of course)

 

[tw99]

We shouldn’t forget that audiophiles will advocate the need for “clean power” to literally ANY device, including network switches… So there’s plenty of room for people to say “but but but he didn’t test my phono stage, and my wife heard the difference from the kitchen when I changed the PSU on that”

It’s still a very useful demonstration though, even in the limited scope of devices tested.

 

[restorer-john]

@amirm How about superimposing a 1050Hz ripple of several volts amplitude on the mains waveform and seeing what gets through on conventional amplifiers (not SMPS supplies)?

I use one of these (below) Without the filter (30dB at the notch), many conventional (Class A/B transformer based PSUs) amplifiers exhibit noises from widely used ripple control on the mains. Not airy fairy audiophile foo- they are used for lighting (to reduce flicker) for ceiling fans (to reduce the noises heard) and audio gear. The filter completely removes the annoying ripple control noises heard through the amplifiers.

Like @Francis Vaughan I too own a ferro-resonant transformer (SOLA) ex-server room gear I picked up for peanuts. It's absolutely fabulous except for one serious issue. It's too noisy (physical buzz) for a listening room.

 

[Francis Vaughan]

Stating the fact that there might be examples that exhibit this problem, doesn't justify them, nor the devices that pretend to fix these issues.

I never said it did. This thread is titled "Impact of AC Distortion & Noise on Audio Equipment". So one would assume that that is the goal. Not to dismiss that the existence of any such equipment is justified. It is worthwhile finding gear that has such problems, and gaining some insights about the sensitivity thereof. That seemed to be the entire point. We might gain an understanding of other gear to avoid here. Amir has OK but nothing special mains power so far as we can see. Not everyone is so blessed. If we understand how much sensitivity exists in equipment we learn, and add to thebody of knowledge.
Right now we have learned that two cruddy tube amps are rubbish with both clean and distorted power feeds. That is a data point, but hardly a conclusion that leads us to believe that all devices are immune from mains power supply noise, or in what manner they are sensitive.

 

[amirm]

BTW did we miss a chance to see how much those power conditioners can clean that up, or does it not matter anymore.

I have measured a number of those including the Audioquest Niagara 1200 a couple of days ago: https://www.audiosciencereview.com/...view-power-conditioner-surge-protector.25443/

As you see, they essentially have no ability to filter the AC harmonics that matter (up to 90 kHz). To get cleaner power you have to use these regenerators which unfortunately are hugely inefficient and act like heater for your home!

 

[JohnYang1997]

I never said it did. This thread is titled "Impact of AC Distortion & Noise on Audio Equipment". So one would assume that that is the goal. Not to dismiss that the existence of any such equipment is justified. It is worthwhile finding gear that has such problems, and gaining some insights about the sensitivity thereof. That seemed to be the entire point. We might gain an understanding of other gear to avoid here. Amir has OK but nothing special mains power so far as we can see. Not everyone is so blessed. If we understand how much sensitivity exists in equipment we learn, and add to thebody of knowledge.
Right now we have learned that two cruddy tube amps are rubbish with both clean and distorted power feeds. That is a data point, but hardly a conclusion that leads us to believe that all devices are immune from mains power supply noise, or in what manner they are sensitive.

There are designs directly feed AC voltage to the heater in the tube right? I think those can obviously show some difference. But again those designs are probably not good ideas to begin with.

 

[amirm]

Is there a chance you can measure any of the different types of ups for powering small devices like dacs and headphone amps?

Can do. I only have true sine UPS's though. They have come down in price so much that I don't see any reason to get the non-sine ones.

 

[DSJR]

No idea if many were sold in the US, but I'd love a 1980's Naim power amp to be tested here. We were conditioned to accept the often snarling mains transformers as being *so good* they showed how bad the incoming mains quality was (mine on quiet Sundays almost used to play tunes on their own, the hum becoming a snarl, then back to a hum and sometimes disappearing, but this may well be DC on the mains and not well wound transformers for some reason - they're better today but can still buzz a bit I gather). The performance of said amps didn't seem stable either but we were conditioned to believe they were 'better' because of it...

P.S. That's a thought - DC on an AC mains line? Just thinking as this is reportedly another thing to watch out for?

 

[amirm]

We shouldn’t forget that audiophiles will advocate the need for “clean power” to literally ANY device, including network switches… So there’s plenty of room for people to say “but but but he didn’t test my phono stage, and my wife heard the difference from the kitchen when I changed the PSU on that”

Ah, did not think of testing phono stages. Will see if I can do that....

 

[amirm]

Let me be clear about something. Almost everyone buys these things to add to their systems even though they have not detected any audible problems with their system. If they had hum, buzz, etc. and went this route and problem went away, that would be fine. But that is not remotely the common case. The common case is adding such a box and then claiming noise floor has gone down, dynamics are better, bass is louder, tighter, etc, etc. This is the motivation for the testing I have done, and continue to do.

 

[filo97s]

great article!
Wondering how this will impact products with a linear power supply such as the majority of speaker power amplifiers.
With switching and low power products you have strong voltage regulation either by using switching PS or IC voltage regulators... but how the things change with something linearly powered?

 

[Mrgoogle]

 

[restorer-john]

No idea if many were sold in the US, but I'd love a 1980's Naim power amp to be tested here. We were conditioned to accept the often snarling mains transformers as being *so good* they showed how bad the incoming mains quality was (mine on quiet Sundays almost used to play tunes on their own, the hum becoming a snarl, then back to a hum and sometimes disappearing, but this may well be DC on the mains and not well wound transformers for some reason - they're better today but can still buzz a bit I gather). The performance of said amps didn't seem stable either but we were conditioned to believe they were 'better' because of it...

P.S. That's a thought - DC on an AC mains line? Just thinking as this is reportedly another thing to watch out for?

Toroidals are sensitive to DC on the mains. My main feed to my lab has a sophisticated filter which includes DC blocking.