- Jul 26, 2021
here comes how main voltage looks in London city UK , outlet in home , all of these harmonics IS in your audio system .
Left/rights grounds all are on the same 0V/earth level. I won't see anything so.
I'll check, knowing how to sync on them. And see if my low noise 5V PSU (for the DAC) gets disturbed or not whilst the pulses pass through.
I have been fooled by scope shots quite a few times when looking at noise levels near the noise floor of scopes (which isn't great) and things were shown on the screen that later on did not come from the gear.
The way I probe (on two cables ends, cable having ferrites on both ends), and depending of the burst, I'm getting up to 3V peak-peak. This is away from my scope noise floor...
The only thing I do not know, is what exactly that cable impedance is. I see the voltage at ends, but I won't be able to tell what the current exactly is.
for voltage yes , but not for current
So there ARE power cables that do actually reduce noise
Did the CAL Sigma just suddenly fail or did it go back to normal on regular power?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:
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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:
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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:
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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:
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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):
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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:
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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:
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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:
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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:
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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:
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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:
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We definitely have a distortion factory here. Let's feed this one clean power to see if it makes a difference:
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Nothing. Everything from distortion to mains hum and general noise remains the same. But maybe the torture test of dimmer simulation does it:
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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.
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.
As always, questions, comments, recommendations, etc. are welcome.
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