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adt-audio
5 1
SRC
we use a good quality cable with a total capacity (between cores and core
to screen) of only 100 pF, we have a total load capacity of 10000 pF. This
capacity has an impedance of a little less than 800 ohms, which results in
a drop of level at 20 kHz of approximately 0.75 dB and an additional phase
shift in the range of 20 degrees. In critical situations, this will be audible.
However, if the cable is shorter, or if a low source resistance line output is
connected instead of the microphone, the situation is not that problematic.
Anyway, we are supposed to check for low capacity values of the cables.
The lower the capacity, the less chance there is of any possible influence
on the audio signal. With bad output transformers, which can be found in a
couple of so-called vintage devices, cables with high capacity can shift the
resonance frequency of the transformer into the audio band.
While the inductance of commonly used cables is not important in the
audio band, let’s check if the resistance can have another influence. We
use the 100 m microphone cable for this check again. If we use a cable
with a cross section of 0.2 mm
2
, the total resistance per core and per
meter is approximately 0.085 ohms. The total length of both cores of the
100 m cable is of course 200 m, which means that the total resistance
of such cable is 17 ohms. This resistance is added to the 200 ohms
source resistance of the microphone, which means, that as far as the
cable resistance is concerned, the input impedance of the amplifier is the
important factor. A commonly used professional microphone preamplifier
will have an impedance that is higher than 1 kOhm in any case. The
difference in the attenuation of the applied load of a 1 kiloohms input to
a 200 ohm microphone can be easily calculated. Without cable, there is a
voltage drop of 16 %, which is equivalent to approximately 1.3 dB. With
the additional resistance of the cable the voltage drop is 16.4 % instead
of 16 %. The difference is less than 0.1 dB, so we can forget about this,
unless we use cables with a cross section that is much lower than 0.2
mm
2
. This calculation also reveals, that the influence of silver plated
cables is not existing. Even though the electrical conductance of silver
is little better than the conductance of copper, the difference is not very
high. Kappa, the electrical conductance of copper is 56.2 m/ohms*mm2,
while the conductance of silver is 62.5 m/ohms*mm2. This makes a
difference of 11.2 %. Since silver plated cables have only a very thin silver
surface of less than 10 micrometers, the silver plating does not alter the
total resistance of the cable at all. As we have already seen, also the skin
effect, that would be able to make the silver-plating on the surface of the
cable more important, will not have any effect in the audio band.
What is much more important than the material is, that the cable is stable
enough to withstand the mechanical treatment and that the isolation
between the screen is sufficient to make sure, that there is no chance the
screens of the separate cable in a multicore are shorted. This is a big risk
with multicore cables that use anodized aluminum foil without additional
isolation for the single lines. It is very difficult to assemble these cables
in a way that there is no risk for shorting the screens. If the multicore
lines must be split to several single connectors, this type of cable is not
a good solution at all. If the single lines are not carefully isolated with
shrink tubes or other appropriate material, a little scratch in the aluminum
foil can create a very confusing problem when a screen makes electrical
contact with the housing of a device. Don’t use this kind of cable.
2.4 Planning of the installation
A good design and preparation of the installation will save a lot of time
and trouble. Make sure to list all the equipment that is installed in the
studio and make a list of all necessary connections. In most cases, the
biggest part of the work is not the console. All connections from and to
the console are actually predetermined by the connectors. The major part
of this work is the external processing gear in the control room and the
connections from and to the studio. You must take care that you have
enough connector panels in the studio and that these panels are placed
in a way that can handle any kind of recording or broadcasting with the
existing lines. In addition, studio headphones and playback speakers must
be installed. Another part of the work is the necessary control installation,
red lights, speakers’ tables and anything else that is necessary for the
particular studio.
External processing gear in the control room is most problematic with any
installation. All devices are different, you have to deal with balanced and
unbalanced lines, different connectors types and problems with screening
and grounding. In addition, the units that have internal power supply
sections can inject hum by the magnetic field of the power transformer
into the devices that are mounted above or below in the rack. There are
