Analog ground separation (“ground lift”)
Anyone who connects external amplifiers to the PC in analog rather than digital form may also be familiar with the slight humming that is difficult to eliminate at first glance because all cables are actually of high enough quality and, above all, are plugged in correctly. Incidentally, this effect also occurs with normal music systems if not all components are connected to the same socket or power strip. However, the causes are usually the same and if you don’t know them, you’ll quickly end up searching to death. This is because it is not only due to the cables and their shielding, but also to the so-called potential differences in the earth line (earthing, protective conductor).
This ground hum (also known as ground loop), which is unfortunately clearly audible as interference voltage, occurs when there is a so-called potential difference between the respective protective conductors (“earthing”) of the connected devices. This is caused by the internal resistance of the respective supply line up to the common earthing or grounding point. The further away this point is, the higher the probability of this potential difference leading to an interference current. DIN VDE 0100-410:2007-06 regulates the connection of all conductive enclosures to an earthed protective conductor and to the main earthing busbar of the respective building / main connection. The ideal case would also be local equipotential bonding, which leads to the main earthing busbar via a protective equipotential bonding conductor.
However, if you are not building your own house and have set great store by hi-fi-compatible cabling, you can remedy this by plugging all the relevant devices into the same power strip! If this is not possible, you should still look for the shortest route to a common potential. Sockets that are far apart are highly problematic here. If you wire symmetrically, you are in the clear, as the interference current flows exclusively via the shielding, which does not carry an audio signal. This means that no hum is generated by the flowing interference current. But what do all those who have not been able to change (and reconnect) anything up to this point do?
There is a simple but extremely effective solution for these users: galvanic isolation in the analog branch using a ground isolator (“ground lift”). Such a ground isolator (picture above) is simply used as an audio filter in the signal line. The whole thing also has a nice side effect in the living room if, for example, a receiver is still connected to a TV cable for the satellite system, a cable connection or an aerial. The very distant earthing points of such signal sources can also result in different earth potentials. I don’t want to know how many unnecessary and expensive installation hours users have had to spend on such problems, even though an inexpensive filter would have done the trick.
The impedances of such filters with galvanic isolation are usually in the range between 500 Ohm and 1 kOhm, which is completely unproblematic as long as the connection between the individual components is concerned. However, low-impedance inputs and outputs should not be used, as low frequencies may be lost. That should be enough for now, because if you follow the advice, the hum should be history.
Asymmetrical cables
As a general rule, asymmetrical cables (as in our case) always have the disadvantage that the signal is transmitted via the actual audio line and via the shielding in the cable, which also serves as an earth conductor. With balanced cables, which will certainly be the subject of the second part, the signal is transmitted via two different audio lines in the cable and no longer via the shielding, which also reduces the ground hum described in a moment. The biggest advantage, however, is that these balanced cables virtually “cancel out” external interference signals. This works so well because the signals are polarized in opposite directions and are therefore phase inverted. The signals subtract each other and the interspersed interference signals are elegantly canceled out.
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