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back RF noise. Even though the input filters of the console's power supply
units and an additional, totally separated screen winding of the mains
transformer will block these disturbances as far as the console's DC supply
is concerned, the problem remains, since the only possible way to shunt
these disturbances is the ground. Therefore, the ground that is reference
for all audio devices in the entire studio becomes polluted with clicks,
hum, and RF-frequencies interferences.
In addition, the separate line is the best way to control the audio
grounding of the system. Usually, the mains line is carried out as a
4-conductor line with 3 phases and the neutral conductor. Even though
there are different rules and uses from country to country, this is a
commonly used principle that works fine as far as safety is concerned, but
not for audio systems. Since the neutral conductor is not totally separated
from the protection conductor that is used to maintain electrical safety
in different ways in most countries, an unbalanced load of the three-
phase conductor causes compensation current in the neutral conductor.
Of course, this current causes a voltage drop on the line that shifts the
entire ground potential away from that potential that can be considered
as ground node. In principle, the only 'real' ground node is the core of
the planet; however, a real-world ground node is an electrical potential
that is not different from the electrical potential of another place,
independent of the distance between the two points. These are theoretical
considerations, of course. Let's get back to 'ground'. The point in a building
that comes closest to the ideal ground node, is the mains power entry,
where the neutral connector is bridged to an earth plate, the concrete of
the building's foundations, the main water pipes and other systems that
are supposed to have a very good 'earth' connection. The closer (in the
electrical meaning of this word) you can come to this potential, the less
are your grounding problems. It is therefore crucial to have this electrical
potential transported to the main ground node of the entire studio as
'clean' as possible.
If a conventional 4-line cable is used, the ground potential depends on
the current in the neutral connector. There is no way to avoid this current,
since almost no audio equipment uses a three-phase power supply. It is
simply not possible to share all equipment between the three phases in a
way that the resulting current in the neutral conductor is zero. The result
of these considerations is that a separate, current free conductor must
be used for ground. If the mains supply line is implemented as a five-line
cable that uses an additional conductor only for the ground, you don't
need to take care about this; it's already done. However; with a 4 line
cable, it is very important to install an additional line that is current free
and transports the ground node potential from the 'low impedance point',
which is usually the mains distribution in the basement of the building.
This line should have the highest possible cross section that can be
installed. 10 mm is a minimum, 16, 20 or even 32 mm
but improve the stability of the system. We will see later, that this line is
actually not current free but has to shunt all interferences that are injected
into the ground by the connected devices.
Let's get this clear. If the ground is connected thru a separate line from
the basement that has a cross section of 10 mm
line is approx. 1.7 milli-ohms per meter. With a cable length of 50 m, there
is a total resistance of 85 milli-ohms. If we use one phase of the three
phase system for all the audio equipment and the second and third phases
for 'dirty' electric stuff, like lamps, heaters and whatever else is installed,
we can assume that we will have a final current in the neutral contactor of
something in the range of several amperes. Let's start with the assumption
that the compensation current is 5 amperes, which is a realistic value.
Simply using 'Ohm's Law', it is obvious that this current causes a voltage
drop of 5 amps * 0.085 ohms, which is approximately 0.4 Volts. As long
as the complete studio system is completely isolated from the rest of
the world, this is not a problem. However, if there is only ONE ground
connection to another room or studio that has a different ground potential,
a compensating current will flow thru this additional connection and cause
problems. A grounded computer network cable, an antenna or something
like that will always result in such an additional ground connection. The
voltage drop on the ground line is of course an alternating voltage with the
frequency of the local mains voltage, 50 or 60 Hz. It therefore injects an
audible hum into the system that changes as soon as the ground potential
of one of the systems changes. This problem is well known as a 'ground
loop'.
RF problems
Most of the modern devices use a switch mode power supply section that
SRC
5 1
2
will do no harm
2
, the resistance of this
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