That will of course be your answer as this seems to be your style and class.
I am not giving remedials, your local evening college is for that.
In the below diagram A and B are the two hi-fi equipment representing two noise voltage sources. The noise voltages are coupled onto their cases. Does not matter how. Naturally these two sources are at a higher potential.
The two cases are coupled together by a wire with zero ohm resistance.
So the voltage between the two chassis always must be zero, because they are linked by a zero ohm resistance.
The mains earth wire has a resistance R and is connected between the two noise sources and earth node. Earth node is naturally at a lower potential than the two noise sources.
The earth node has a different potential, as we are dealing with AC, everything depends on phasing and relative levels.
As the current flows from higher potential to lower potential the noise currents will flow from the two sources into the ground through R.
The above, the reverse may be true, the may be current sinking into the devices from mains earth.
So, the claim of ground earth wire having no bearing on this is plain wrong.
It has no bearing on what happens on the audio path.
And a super duper mains cable with thick earth wire the size of my wrist will still not change the fact that the two equipment cases are at higher noise voltage potential.
BUT RELATIVE TO EACH OTHER THEY ARE AT EXACTLY TH SAME POTENTIAL!!!
What happens if we remove R.
There is no path for the noise currents to drain into earth, so the chassis of each device float up to a voltage that is determined by the source potential driving the noise currents and the coupling impedances - the two chassis will be at exactly the same potential but it will be somewhere between the two voltages. Again phasing etc. matter.
If (as usual) the driving source is our mains voltage both voltages will be in phase and of the same magnitude, so both chassis will actually be at mains voltage.
That is the reason why you are not permitted legally to remove "R" as you just did.
As now both equipment are coupled together through a zero ohm wire both noise currents will look for low impedance paths to flow into within both equipment. This naturally will be the signal ground.
No, there is nowhere for the current to flow. As the impedance between the two chassis is zero and each chassis is linked to signal ground, any noise current flowing would flow through the lower impedance, it does not get lower than zero.
So there is just no way for any current to flow in the signal cable, UNLESS you remove the zero ohm link between the two chassis.
That was the essence of the argument and I hope this explains my position..
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You are plain and simple completely wrong. In literally everything you wrote.
In reality we can assume an accumulated loop impedance between the two chassis that is not zero.
Let us assume a high grade IEC connector and good quality mains plug with 5mOhm each. 6.6foot of 10 AWG cable amount to 6.6mOhm.
Two fat mains cables then will produce a loop impedance of 33.2mOhm between the two chassis and 16.6mOhmper cable.
The line cable is a common type, with a ground resistance of 0.5Ohm and around 0.2 Ohm contact resistance at either end (signal cables are not designed inherently for low impedance) for a round trip of 0.9 Ohm.
Let's further presume one item is at the legal limit for earth leakage current, say a digital mixer with a SMPS, so item A at 10mA.
And Item B has 1mA from a well designed Mains transformer and is a linear power amplifier.
After the two mains cables at the duplex outlet 11mA current flow to return to earth.
Or if we disconnect the earth at the Duplex Outlet (do not try this at home kids), no current will flow into earth.
One Mains cable will drop ~0.166mV from passing ~10mA and the other will drop ~0.0166mV from passing ~1mA. In reality a tiny proportion of the ~10mA will flow through the line cable. We need to look at the whole circuits thevenin equivalent to get an accurate solution, but a close enough approximation is ~180uA.
So actually ~180uA pass through the line cable, ~8.82mA pass through mains Cable A and ~1.18ma pass through mains cable B.
Thus the potential between the two chassis will be ~183uV. For an SE connection that is directly the noise injected into the signal, degrading SNR. For a balanced connection the noise will be reduced by the CMRR which is strongly system dependendent.
Now let's replace the "fat" mains cable with standard 18 Gauge mains cables with shitty chinese IEC and Mains plug. Our contact resistance is easily up to ~20mOhm and 6.6' of 18 Gauge is ~42mOhm.
So our loop impedance per mains cable is ~80mOhm, 0.16 Ohm in total. Our line cable has not changed, 0.9 Ohm.
The higher impedance of the Earth connection changes the proportion of the current that flows in the line cable, the higher resistance of the mains cable means now ~1.78mA will flow in the line cable and ~8.22mA in Cable A and ~ 2.78mA in Cable B.
Thus the potential between the two chassis will be ~1,600uV, a nearly 20dB increase in Noise.
Now let us try to reduce the noise. Let us lift the Earth on Mains Cable A. This means the full 10mA will flow in the line cable and we will have ~9,000uV between the two chassis.
Now let's lift the second mains cable earth.
The potential difference between the two chassis becomes very close to zero, but now 220V are present on the chassis with a current capacity of 11mA. Touch that and you get a pretty painful Zap or worse, which is why electrical safety regulation disallow simple lifting of earth on Class 1 equipment.
The same incidentally holds true if we have both mains cables with earth but lift the earth at the duplex outlet. The voltage between the chassis becomes zero but the two chassis now are at a dangerous voltage.
Are we now clear what is going on and why all you write is wrong?
Thor
