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Impact of AC Distortion & Noise on Audio Equipment

amirm

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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:
Impact of AC Distortion and Noise on Audio Equipment Test and Review.jpg


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.

Warning: any tests involving high voltage mains can be very dangerous resulting in deadly electric shock. Please do not try any of these tests unless you are fully qualified and know the risks.


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:

AC Mains Distortion Measurements.png


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:

PS Audio P300 Power Plant Measurements.png


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:

BK Precision 9801 AC Mains Measurements.png


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):

BK Precision 9801 AC Mains Dimmer Simulation Measurements.png


Let's agree this is the mother of all dirty AC feeds! :eek: 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:

Topping A90 Measurements Generic AC Power.png


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:

Topping A90 Measurements BK Precision 9801 Lab AC regenerator Power.png


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:

Topping A90 Measurements BK Precision 9801 Lab Dimmer AC regenerator Power.png


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:

California Audio Labs Sigma DAC Measurements Generic AC Mains.png


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:

California Audio Labs Sigma DAC Measurements BK Precision 9801 AC Mains.png


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:

Loxjie P20 Tube Amplifier Measurements Generic AC.png


We definitely have a distortion factory here. Let's feed this one clean power to see if it makes a difference:
Loxjie P20 Tube Amplifier Measurements BK Precision 9801 AC.png


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

Loxjie P20 Tube Amplifier Measurements BK Precision 9801 Dimmer Simulation AC.png


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/
 

Nango

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Brilliant!!

So far ~ 300$ burned (1x Allo Shakti PSU, 1x Aqvox PSU) in that kind of "enhancements", who can beat this figure?
 
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Francis Vaughan

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I might suggest that choosing a couple of ****** little tube amps that under the best of circumstances are going to be distortion generators is not the right exemplar of a device to test power supply issues. They are crud no matter what happens, and the bad power is simply unable to make them worse than they already are.

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. These tube amps don't meet this description.

First cab off the rank would be a traditional class-A amplifier with a conventional transformer and capacitor PS.
Then amplifiers that tout their low or no-feedback design.

In both cases I would expect them to perform reasonably (at least within the bounds of the limitations of their design) with good power, but as a result of poor power supply rejection intrinsic to their design, bad power may show up in the results.
 

Raindog123

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| No Change! |
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…and it would be really sweet if the manufacturers of those pricey audiophile power conditioners use this thread as an opportunity for presenting their data — the data substantiating their claims of massive (or any) improvement… But they would not. Because they dont have it.
 
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Doodski

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First cab off the rank would be a traditional class-A amplifier with a conventional transformer and capacitor PS.
Are you mentioning something like a class A with linear output and a massive power supply to match or just a middle line class A. I ask because there is so much power supply filtering in a linear output class A amp that it might not have issue with cruddy dirty power.
 

Grotti

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The expected result: seems like another audio myth is busted.

It is a mystery to me, that for literally decades these devices are being sold on the basis of sheer belief.....

Thank you very much for your efforts and cary on. Please! :)
 

H-713

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It's not busted, at least not completely.

I have always seen power conditioning as being something to do on an "as needed" basis. It's only needed on equipment that is sensitive to incoming AC power, which is really uncommon these days. Where I have seen it is on older mic preamps (and a couple power amps). The only audio equipment that I've seen have issues with dirty AC power is more than 20 years old. Even modern tube equipment seems to be pretty good.

Where power conditioning is most likely to be worthwhile is on some old vacuum tube mic preamp that got pulled from a 1960s mixing console, with zero considerations made for EMI filtering. Lots of gain, little to no feedback, questionable power supply filtering. Such equipment was often built at a time when there were far fewer switching power supplies in use. I can imagine that an old tube microphone might have similar issues.

Where stuff like AC regenerators originates is in exotic / unusual situations, where it actually does solve a problem, and then spreads to an area where it isn't needed. There's enough garbage on the AC line these days that most new equipment (even if it's not particularly low distortion) is designed to deal with it.
 

Doodski

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Where stuff like AC regenerators originates is in exotic / unusual situations, where it actually does solve a problem
Metrology uses AC waveform generators for providing the cleanest possible waveform when measuring and calibrating down to parts per million and such. Think like calibrating the equipment that is used to calibrate metering equipment. The last one I saw came on a small pallet and was about 1/2 the size of full size kitchen home refrigerator. I suspect that is where the legend and lore comes from.
 

Blumlein 88

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You failed to improperly implement a null test to show an erroneous difference.

Still, this ought to about do it for AC conditioning as a fidelity enhancer.
 

Rja4000

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Just for the sake of it:
Does it make a difference if nominal is 240V 50Hz ?
And what if voltage fluctuates at very low frequency and drops -25% below nominal ?
Could that have an impact on an amp dynamic range or max power ?
 
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H-713

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Metrology uses AC waveform generators for providing the cleanest possible waveform when measuring and calibrating down to parts per million and such. Think like calibrating the equipment that is used to calibrate metering equipment. The last one I saw came on a small pallet and was about 1/2 the size of full size kitchen home refrigerator. I suspect that is where the legend and lore comes from.

Yeah, I've used regenerators on really sensitive lab equipment before. It's pretty rare, but now and then it makes sense for really critical measurements.
 

Doodski

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Just for the sake of it:
Does it make a difference if nominal is 240V 50Hz ?
And what if voltage fluctuates at very low frequency and drops -25% below nominal ?
Coukd that have an impact on an amp dynamic range or max power ?
With a conventional linear power supply yes the power supply output voltage will be reduced. With a PWM/switched mode power supply the drive IC will compensate and drive a bit harder to make up for the reduction in AC mains voltage or even a reduced DC supply if there is a external DC supply.
 
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amirm

amirm

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Just for the sake of it:
Does it make a difference if nominal is 240V 50Hz ?
Everything should carry over just the same to 240 volt/50 Hz.
 
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amirm

amirm

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In both cases I would expect them to perform reasonably (at least within the bounds of the limitations of their design) with good power, but as a result of poor power supply rejection intrinsic to their design, bad power may show up in the results.
You have seen any measurements to indicate such?

You mention amplifier. Most people won't run their amplifiers through such boxes. They don't have the power capability usually so they are reserved for source devices, and preamps.
 
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amirm

amirm

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The B&K, 0-12 Amp, 9805 Series AC Power Source comes at the expense of ~$5590.00 USD.
Cost is one thing. The weight at 115 pounds/52 Kg is another!
 

Doodski

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Cost is one thing. The weight at 115 pounds/52 Kg is another!
That caliber of test gear requires a strong metal workbench. Tack on another $2000.00+ for the workbench/table. Actually how large is this thing...lol. Might need a pallet.
 
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