Our great mains

[PeterSt]

Hi all,

Throughout times I have been posting about "noise" problems I experience once in a while. This is another such post ...

All is about "it can be happening to you just the same", but you wouldn't know it. I mean, I'm in the position of having an analyser at hand, so I will know instantly. Or at least I can check things a normal human being wouldn't be able to. He would just perceive lousy sound.

So, I have been away for 3 weeks (back now), and when I got home of course the first thing I did was switching on the amps and stuff. Heat it all up, and at last listening to decent sound again. But it wasn't there;
Since my amps need some 24 hours to get normal after being off for a day (let alone 3 weeks), I thought it would be that after listening the first night. But it wasn't.

At working on some "DAC ideas" the next day, I noticed great distortion from measurements, but since this was about this "new idea" I wasted a day on getting that right, while in the end the normal situation measured as bad. Hmm ... what happened ?

Well, to be brief on that, at this moment I still don't know. All I know is that some huge noise is coming from the mains (or through the air, I can't decide for that at this moment), and using protective earth (PE) makes it the worst (what about noise being there up to -40dB !!).

2 days after being back I once again noticed one of my devices being off, while I didn't recall to have shut it off. This time I looked more close, and saw it was still switched on, though not working (this is my FireFace800). Aha, so that must be something to keep in mind. But again, what happened ? Did that device die by accident, or has something happened during my being away to it was killed (lightning perhaps -> must ask the neighbours), or did that device die in a dangerous fashion and something else is destroyed without me knowing yet ?

In any case, something must have happend while being away, and now I'm up to the task of finding out what is the problem.
I have talked before - in between the lines - how much nasty stuff is going on anyway within our mains, but I never talked about it in concrete because I first need to know *all* the ins and outs about it. But you can bet there's much more going on than all of us know. I mean, I asked around amongst some other engineers I know, and nobody has a clue about what I all experience, once some base "problems" can be mimiced, and worked out further to extensions you can get crazy about. For now I give you one example, only because it is happening to me right at this moment (and this wasn't so before) :

I plug my analyser in mains without PE (otherwise it will measure noise natively because it receives noise itself);

I measure the analogue outputs of the NOS1, and it is way too noisy (something like -80dB while it should be at -113dB or so in my situation). The NOS1 is NOT connected to the means at all. Notice that this is dangerous, because in this situation all is out of (ground) control, so not connecting it to the mains may incur for MORE noise (think about that one).

I plug the NOS1 into mains again without PE; There's the On/Off switch which has double poles. So, hot and neutral are both switched off.
In this situation there's not even connected anything towards the DAC itself. So, behind the mains inlet are the transformers, and behind the transformers are the PSU's. But, the PSU's are not connected to *anything* (all is cut). And remember, since the mains switch is off too, and the PE is not connected, there shouldn't be connected anything, right ? So, the cable is in, and the only thing which connects a little, is the PE from the cable towards the all outlet. There it stops, because there's no PE in that outlet.
Well, only putting in the cable makes the noise go way up (to really unacceptable levels).

Thus, there's a PE wire in the cable (cable is 2 meters or so long), that PE wire is connected to the case, and the case connects to 100% nothing in my situation (all ground inside is floating). The interlinks out of the DAC are connected to the analyser, so what happens from there (the analyser) is unknown. In any event, plugging in that mains cable to the DAC will let rise the noise significantly.
In my view this can only be something picked up from the air. It should be a combination of the PE wire in the mains cable acting as an antenna on one side, and the Faraday construction of the case doing something on the other side (the latter in a kind of upside down behaviour ?).
And if I, in the same situation, put the cable in a PE connected outlet, the noise goes up to -40dB.

What I expect at this time, is that something very nasty is going on in my mains to begin with (some device shortcutting to PE ?), and that this by itself radiates through air as hell. The FFT of both situations (mains cable connected with and without PE) looks exactly the same, but the former at a 40dB or so higher level (now the noise being connected directly).

And about problems people may have ...
This is so bad, that turning an output level knob of an active crossover I have sitting in my chain somewhere, makes stop working the soundcard in the DAC ... (when all is normally working and connected, though without PE in this case).

For now the message is : there is no reason such a thing won't be happening to you. But how will you know without something like an analyser. You won't. You will have bad sound though.
Also, one day things may be all right, but a next day all will be wrong. That too will go unnoticed;
Or like in my situation, something happens while you're not there. Sound is suddenly bad, but you will be without clue forever, because you don't pocess discrete measurements from before. Maybe 7 years later when you replace your TV for a new one, suddenly all is right again.

I think I said it before ... part of the base equipment of any self-respecting audiophool should at least be a scope ...

Peter

[Nick]

Peter hi,

Very interesting, I certainly have system "bad days" for sound....

One question, can you use your analyzer to do an FFT on the mains 60hz supply ? Perhaps setup a resistor potential divider to pick up the single level you need for the analyzer.  It would be very interesting to see a post of what the spectrum looks like. Bet its not a nice sharp spike at 60hz.

I'm in the car now but I'm going to have a careful read through your notes later.

Nick,

Ps change number 4 on the NOS1 is running now, will pm you with the details

[PeterSt]

Hi Nick - somehow I expected you to respond here ... 

I'm not sure whether you're talking about the 60Hz "possible" output of my 230-110V transformer ...
"Possible" because I expect it to output just 50Hz (same like the input).

Actually I never did an FFT of the 230-50Hz ... maybe because I don't want to know and sleep better.
But somehow I expect the sine not to be a sine at all, and merely have it mixed with the other phase (attenuated/leaked through / shifted a little perhaps).

Peter


PS: Tweaks 1.5, 2 and 6 have been applied, but no measurement results yet because the picked up noise has the overhead at this moment.

[Nick]

Hi Nick - somehow I expected you to respond here ... 

I'm not sure whether you're talking about the 60Hz "possible" output of my 230-110V transformer ...
"Possible" because I expect it to output just 50Hz (same like the input)

Peter


PS: Tweaks 1.5, 2 and 6 have been applied, but no measurement results yet because the picked up noise has the overhead at this moment.

60 Hz doh! 

Tweeks 1.5 and 6 sound interesting as I have only 1 to 4. hope all is sounding good even if they cannot be measured 

Nick.

[Gerard]

For now the message is : there is no reason such a thing won't be happening to you. But how will you know without something like an analyser. You won't. You will have bad sound though.

Hi Peter back in holland!

That is not what i expect to hear from you after your holiday  especially after Nick his findings. Is it you who is static loaded 

How was your holiday? Some inside info before you are totaly in the NOS1 stuff You survived a tornado 

 

[manisandher]

Hi Peter, thanks for sharing.

Since completing my dedicated mains line, I've become convinced that the mains is the single most important element contributing towards the sound. However, as you said, it's complicated and simply buying a mains filter, isolator and/or regenerator may not be the answer. Like you, my experience is that there seems to be a substantial air-borne component also.

I think I said it before ... part of the base equipment of any self-respecting audiophool should at least be a scope ...

Agree 100%. I'd like to buy one - do you have any recommendations?

Mani.

[PeterSt]

Well, today I spent on finding the culprit, and I found ... nothing.

The below is a nice 50Hz picked up through air;

Analyser is not connected to PE, analysing PC is on batteries, DAC is switched off but PE-connected mains cable is in (see earlier description about the latter). Interlinks are XLR connected to the DAC, but in-DAC there's no connection at this time (to make this as clean as possible so nothing else can pick up (but since the interlinks are "open" at the DAC side, this is of course not the best idea for "no noise", but it really doesn't matter because of this overwhelming noise)). SE connected is even much worse.
The Audio PC is not connected and Off.
This 2mV compares to the normally 6 uV.

Without PE connection there's no problem at all.

I wish I had measured my mains before, for reference. Now I can only tell that this morning it was 217V (seems way low to me !!), and that the same voltage is in between phase and PE. Between neutral and PE there's a varying 200mV and 650mV or so (it is one or the other !).
The FFT (mind you, all picked up through air) shows a very regular pattern of peaks at certain (higher level) frequencies, that dropping, and right after that another higher frequency peak briefly arising and dropping back again. This seems to be happening at a simlar interval I see the beforementioned 200mV and 650mV alternating (each ~15 seconds).
Also there's a steady DC of 474mV between neutral and PE, 20-24mV between neutral and phase, and 26-30mV between phase and PE (both vary at the same time with a distance of 6mV amongst them).

I have no normal means anymore to get rid of the normal DAC-output DC offset values (which always have been 4-7mV), and *if* I succeed, it may go way up by itself later (but "by itself" seems to be switching off the TV or something).

[Nick]

Peter hi,

I plug the NOS1 into mains again without PE; There's the On/Off switch which has double poles. So, hot and neutral are both switched off.
In this situation there's not even connected anything towards the DAC itself. So, behind the mains inlet are the transformers, and behind the transformers are the PSU's. But, the PSU's are not connected to *anything* (all is cut). And remember, since the mains switch is off too, and the PE is not connected, there shouldn't be connected anything, right ? So, the cable is in, and the only thing which connects a little, is the PE from the cable towards the all outlet. There it stops, because there's no PE in that outlet.
Well, only putting in the cable makes the noise go way up (to really unacceptable levels).

Thus, there's a PE wire in the cable (cable is 2 meters or so long), that PE wire is connected to the case, and the case connects to 100% nothing in my situation (all ground inside is floating).

One idea but mainly questions.

Reading you first post the only connection from inside the DAC to the outside is the PE which goes from the DAC power socket into the mains cable but is not connected to the household PE at the wall socket.

Could the 2 meters of PE wire within the mains lead running parallel with the live and neutral leads be coupling the noise. Reading a few papers recently (as you know) there may be capacitive noise coupling between the live / neutral and PE wires within the mains cable. There could be more coupling here to mains noise than say from air borne RF. Could this be injecting the noise ?

• Does the noise still happen with no mains cable connected to the DAC ?
• What happens to the DAC noise if you connect a cable in the same way but with no PE inside the cable (ie just a two core live and neutral cable)?
• If the 3 core mains cable length is varied (eg the PE to live parallel run is varied in length) does this inject more / less measured noise ?
  

217v does sound like a lot of sag its more than 10%, could there be something on your distribution spur (external to your house) that is dragging down the supply voltage and injecting supply noise ?

Nick.

ps Is the plot your mains wave form or an air bourn noise trace I am not clear ?

[PeterSt]

Hi there Nick,
 

Reading you first post the only connection from inside the DAC to the outside is the PE which goes from the DAC power socket into the mains cable but is not connected to the household PE at the wall socket.

In all cases the PE wire is in the cable, and connected to the DAC case (I never tried detaching it from the DAC case because I'm fairly sure no problem would be there then). So, the problem emerges when the PE wire is connected to the mains PE in the wall outlet.
 

Reading a few papers recently (as you know) there may be capacitive noise coupling between the live / neutral and PE wires within the mains cable. There could be more coupling here to mains noise than say from air borne RF. Could this be injecting the noise ?

Theoretically, yes. So, the phase and neutral in the cable (formed in a triangle fashion in my cace) could project something on the PE wire (remember, the neutral and phase are not connected in the DAC (double pole switch is off) but both are connected to the wall outlet).

Does the noise still happen with no mains cable connected to the DAC ?

No.
(the answer really is a little "yes", but too long winded for here, and not relevant; anyway what then happens is just normal).

What happens to the DAC noise if you connect a cable in the same way but with no PE inside the cable (ie just a two core live and neutral cable)?

I did not try this, but I can't imagine anything happening. Remember, this is to be compared with the not PE connected cable and switch Off. Then nothing bad happens either. So, you may want to compare it with switch On and see whether something which is OK (because it is in that case) then suddenly is not ?

If the 3 core mains cable length is varied (eg the PE to live parallel run is varied in length) does this inject more / less measured noise ?

Didn't try that either, but I can imagine so. On the other hand, this will be similar to being able to measure DC with all off (neutral and phase cut by the switch) and cable connected to PE (which actually is another story from longer ago). At first I always thought this was capacitance in the (PE) cable or something (and the length indeed may vary the capacitance), but today I think this is about a ground being able to create (DC) voltage from a Faraday cage which latter acts like a (magnetic) collector or something. Put your hand in it (touch nothing) and the voltage goes away.

217v does sound like a lot of sag its more than 10%, could there be something on your distribution spur (external to your house) that is dragging down the supply voltage and injecting supply noise ?

All I know is that I have been measuring in the past and for sure saw more than 230V, if not in the 240's range ...
Whether there's something dragging on "my" mains is to investigate, but it is not what I expect (btw, I live in the country side). What I merely expect is out of phase stuff. So, very simplified (because the calculation goes different) : when we'd have 3 phases (and I do have three coming into my house) two of them creating the 240, but they are not properly alined, then all individual phases are still 120 (hence no exceptional drag) but my received voltage is less (but the two 180 degrees out of phase and 0 will remain).
In the mean time this will create "noise" as such, because no sine will result from it.
How this relates to the PE is something else, and depending on how the mains is setup in my situation (outside of the house and yet to find out), PE *can* be a reference for the phases, so it can be influenced as well. But this is not necessarily true at all, because it can work without PE reference as a floating system. But but but ... there you have it again ... if the floating system is not in balance (degrees deviation) the presented PE is not 0V opposed to ... my earth pin which I use for audio, but not for the PC.

You see ? I must stop (or shouldn't start) because there are tons to investigate first.

ps Is the plot your mains wave form or an air bourn noise trace I am not clear ?

As per the reasoning of my previous post, it must be through air. Or in either case it is not a direct connection to the mains (then you would have seen that 217V, while this is 2mV).
Let's also keep in mind that what we call "noise" here, is totally inaudible through my system as noise as such (it's completely quiet). But, the system sounds like sh*t, because everything rides on it. For the DAC all is fine when not using PE (remember, at this time it's not even used and only connected to the case !), but for the amps it still will. So, tonight I will connect those without PE, but it's a little "dangerous" to just try it without measuring *those*, with again no reference for me, because I never measured them.

Peter

[Per]

Hi Peter, thanks for sharing.

I think I said it before ... part of the base equipment of any self-respecting audiophool should at least be a scope ...

Agree 100%. I'd like to buy one - do you have any recommendations?

Mani.

Hi Mani and others, If you consider an AC regenerator / monitor, maybe this would pass the bill as it is an improved version of the older powerplants AND - from what I understand from Paul McGowans newsletters - a true scope built in... Don't know anything about the price, though...

http://www.psaudio.com/ps/products/detail/perfectwave-power-plant-5?cat=power

Per

[manisandher]

Hi Per, I think I'd prefer the flexibility of a separate scope. Also IIRC, the new PS Audio P5 and P10 units are quite expensive, >$4000. I've never bought a scope before, but I suspect you could get quite a nice one for this sort of price.

Of course, a scope wouldn't clean the mains up for you... But having had a PS Audio P300 (still have) and P600 (sold a few years ago) and also a PurePower 2000 (still have), I'm not convinced that a pure sinewave is necessary to get the best SQ. Indeed, on their older products, PS Audio offered 'multisine' functions which deliberately flattened the top of the sine wave, in order to improve the SQ.

I haven't given Peter's first post in this thread enough time to analyse and fully understand, but it seems to me that the earthing/grounding system is key. My understanding is that no filters, isolators or regenerators are permitted to manipulate the earthing/grounding system, for safety reasons. My hypothesis is that it's not just earth/ground loops that we have to worry about, but also 'noise' getting transmitted from one component to another via the earth/ground connection. This situation is exacerbated by using a PE which is shared with the rest of your house... and street! I was first alerted to this by the work of Arthur S. Kelm at 'Ground One' (http://www.ground1.com) who seems to be pretty much the expert in this field. If you read his site, you'll notice that I've adopted much of his approach and am very happy with the result. But I think air-borne 'noise' still needs to be addressed, ehich he doesn't seem to cover.

I'm still interested in hearing recommendations for a good scope...

Mani.

Mani.

[Per]

I think you are absolutely right, Mani. Ground is very important. Some deliberately make ground wires thicker than signal / hot wires - both in a powercord and inside cabinets.

Regarding seperate scopes I have heard praise for the Tektronix scopes over at the Naktalk (Nakamichi tapedeck) forum. If I am right you won't need one in the multichannel / gigahertz range - a two channel megahertz scope will be enough for audio - even to monitor for instance a S/PDIF squarewave signal in the low megahertz range.

I bought a used 2-channel 20 mhz HP scope back in 2006 for 1000 DKK (around 120 pounds) but never got to (learn how to) use it due to a worsening in my muscle disease.

Wish you a good hunt for a cheap used scope or, maybe a handheld one or a PCI card that uses an older PC as screen and recorder.

Best Regards,

Per

PS: Please take VERY much care measuring the mains - at 110 / 230 volts ac it takes only mA to kill you if goes left arm - right leg...

[PeterSt]

Mani,

I didn't reply to your scope question yet, because it is not so easy for me to judge this. All I know is this :

My second hand analogue scope is 20MHz, and it is just sufficient for even 40MHz range measurements, if only the 10x "magnifier" is used and you'd know in advance what the results should be. I'm talking audio clock levels here, but which you normally wouldn't need.
So, 20MHz is sufficient for sure ?

No, sadly for what we're talking about here, it is not.

With 20MHz, unveiling normal noise levels already is quite difficult. Normal for NOS1 output stuff would be in the us range, while this is something like mv range. With my 20MHz you may see a somewhat thicker signal line, but nothing like "moving" and certainly not for analysing the wave shape.

If we'd take the picture in this post : http://www.phasure.com/index.php?topic=1559.msg15810#msg15810
as an example of something you'd want to know, you'll see that this needs a resolution on the X-axis of around 0.2ms and something like 10us or better on the Y-axis. So actually that are your requirements ...

Peter

[Scroobius]

Hi Peter

Not sure I can help much on this but it seems difficult to believe this could be a problem caused by anything within your home. Mains droop down to 217V would be of big concern here in the UK and I am sure it must be in NL. I would be on the phone to the electrical supply company right away. I have come across peculiar ongoing supply problems in the past when I worked for an IT hosting company and when we took it up with the electricity supply company they found problems with partial transformer failure in the local sub station. In my experience Electrical companies take reported supply problems very seriously.

Just out of interest is your house earth bonded to the incoming supply cable earth or (if you are remote with an old supply system) do you have an earth "spike" sunk into the ground (this is sometimes the case in the UK with older properties). Also do you any friendly neighbours where you could make some similar measurements in their house?.

Hope you get it sorted soon as days without NOS1 are not as good as days with NOS1 as I have come to learn.

All the best

Paul C

[PeterSt]

Ok, a couple of days further I seem to have tackled my problem. The story is too long (and maybe too unsure at this moment) to lay it all out, and FYI I just reported what happened in 3 or for pages to an electrical engineer helping me with this.
But what came from it, is a little easy to apply trick to find out whether your SQ can be changed for the better. Or at least this is something which *can* happen to everybody, if not that it *will* happen, depending on your particular situation. I only use an anlyser to check for te truth of this little trick; what you need is a volt meter (230V) ...

Let me first simply "state" that having more mains groups around the house is prone to prolems.
This, while we thought doing good with that. So, very generally, and a bit speculative for now, is that more of these mains groups - meaning each with their own Protective Earth (PE) pin or otherwise with different PE's, may or *will* create a potential difference between those two PE's (potential for our dutch readers in this case means "pontential to ground" (and not potential as in "possibly" ).
This does not seem to be an inherent thing (meaning : without anything connected to your means the difference will just be 0V - so no problem), but it is created by the devices you connect. Example :

Put the PC in one mains group, and put the audio devices in the other, thinking that the PC will spit its noise into its own group only. Aren't we all thinking just *that* ?? I think we do.
But for simplicity, let's think that whatever it spits out, exhibits as the voltage difference between the two groups. And *that* is noise, because the noise now can't go anywhere (please leave this latter be for now, and let it come across as something logical to you).

So, before I attempt too many explanations for phenomena I really can't at this moment, let's go and measure our differences in potential between *all* the groups related to audio. So, look at really *all* the devices you use, even if it's a liniar PSU for your USB connected device. Think about how all interconnects (via-via-via), and of course explore the different mains groups you use for it. Remember, groups with different PE (pin, etc.).
Hook up your multimeter (ready for 230V) to the earth pins of both groups (just find two outlets, each of the other group), and look what you've got. Remember, have *all* devices and interlinks connected as should !!

If you see more than 0V, you can improve.
If you see more than 0V, an analyser will show something like you saw in the earlier picture in this topic ...

If you have the problem, immediately stop trying to solve it by whatever commonly known means, because you probably will make it worse (and you can't see that without an analyser), nevermind you end up with 0V in the end.
Instead, move all your noise spitting sh*t to this one group of which you think it is made for it anyway. Remember, 100% everything.

It also works the other way around :
You see 0V, but this may well be because the problem has been solved the wrong way. Example : there is a USB connection between the PC and your audio chain, and the "solution" to it now will travel over this connection. Mind you, I am *not* saying this is really bad, but it isn't solved the way it should. This noise should travel over PE, and now it doesn't.
So what does this mean ?

If you use different (PE) groups but see 0V, work the other way around. Start DISconnecting stuff like USB and actually everything like interlinks, and wait for the voltage to go up. If it does at disconnecting something, you may think about what the connection actually will do for you, knowing that it actually carries away noise. Mind you, this is not the proper expression, but let's all agree that PE is for this, and you just prooved to yourself that this something else is used.
So ... if you see the voltage go up, again think like I explained before, and better move all to this same group. And yes, the first thing prone is the PC ...


Again, the situation needs too much writing to explain in full, and is super complex at the same time. The past week I have been trying to find people who could explain what is happening, and although still someone may be able to, at this moment I brought some "elements" together from quite some different stories and ideas, knowing that at this moment no single person came up with the real idea. I only want to say, these are my own findings / conclusions / trials of indivudual elements, and it may all appear total BS in the end. But no matter what, you can have the problem without being able to know it, and you really will find exactly nobody coming up with an analyser knowing what to do. So, at least I can in my own house, and this came from it. And mind you, I will give the example of my situtation right now, and keep in mind that picture ! :

Yesterday, in a kind of random situation - as much plugged out as possible, sutting down all the groups etc., I measured 67V over the two PE's of my groups involved.
Knowing this, this morning I started experimenting a bit, and this time all was connected again, and derived from the whole subject of this topic I solved my problem (already yesterday) I measured 0.00V. I didn't know that yet, and had the idea the 67V would still be there.
So, problem solved, and 0.00V now. Right.
Thus I started "phase two" as described above, and for your real life example - at removing an USB connection which via-via is linked to the audio stuff in the other group, I now saw ... 99V. And for those who know the subject, I also immediately got crazy of my PWM regulated pump for the heating system, which square waved exhibits its peaks at -40dB in a bad case.
No audible noise from the speakers (except for that 17.8KHz tone), and if I hadn't had that pump, I wouldn't notice it at all, except for most probably bad sound. But of course the analyser would show *that* picture again ...

At moving the PC to the audio group, nothing happens. 0.00V no matter that USB (or anything else like the DVI cable from the NOS1) connected or not. 0.00V stays forever, and the analyser will show normal 7uV or so noise levels.

Without further explanation, I know that this is not pure PC related. I mean, I had my subwoofers in again another PE audio group, and there too the voltage would occur.

And thus it is my conclusion that however it goes and travels, no different devices are allowed to interconnect once connected to different PE mains groups. Each group of such devices will create their own potential, project in onto PE, and the two (or more) PE's will create a new potential opposed to eachother, and that now can't go anywhere.

It is good to undertstand that our houses are full of noise. Yes, we knew that allright. But what it really is about is how to move out that noise out of our systems. This is not about cancelling the noise, but move it out over PE (tough to explain for me ). The ever example is my PWM pump, which really won't produce less of its square wave noise onto my mains if I punt some little gnd cable somewhere in order to let go away that noise. It helps, put the pump keeps on spitting;
When I have those differences in potential over the PE's, that noise won't go to earth, but it will go into my system instead. It will start to travel over the neutral or phase (whatever), as will any induced noise. Also think about the voltage which normally will be there between phase and PE, and you can see how it can sneek into your system, if PE doesn't move it to earth via the shortest path. Now, it will go from the one PE to the other (read : via your devices) because that path is shorter (distance between your devices is a few meters, while distance to earth may be 20 meters). Something like that.

Disconnecting all your devices from PE most probably will create hum, because now *really* all is travelling over the normal wires (neutral and phase), directly. All is "floating", but each device will create its own potential and all will be in unbalance, PLUS switching Off / On at some time may create new ground paths opposed to before (hey, I did nothing, but now it all sounds sh*t !).


Please try this out (only if you have different PE groups), and I urge you to let know the results in this topic; As you may have learned from some in between the lines texts from me throughout time, I'm really heading for working this all out at last, while nobody really seems to know. This is one step of that, although one month back I didn't now about it at all. So, there is much more out of this little teasing subject, but in the mean time this may be the most important one (oh, did I ask you to look at that picture ? ).

I guess this also puts the "isolated from the PC" in another perspective;
I don't know what others (manufacturers) are doing with this and how commercial this stuff all is, but

a. as per my example you just will create the problem (but use different PE mains groups);
b. I really can't see by measurement that a PC spits noise over its gnd connection.

Ad b.
But it really should when just that connection (think USB for your conveniency) solves the problem (described above, and when the voltage goes to zero at connecting it), and :
Ad a.
Without that connection (now meaning : connected by galvanically isolated means), you *will* have the problem (but won't see the analyser picture).

Well, again long ...
Let me know your findings please !
Peter