SoundCraft GB2R User Manual page 8

Initial Wiring Considerations
For optimum performance, it is essential for the earthing system to be clean and noise free, as all signals are
referenced to this earth. A central point should be decided on for the main earth point system, and all earths
should be 'star fed' from this point. It is common electrical practice to `daisy chain' the earths to all
electrical outlets but this method is unsuitable for audio installations. The preferred method is to run an
individual earth wire from each outlet, back to the system star point to provide a safety earth screen refer-
ence for each piece of equipment.A separate earth wire should also be run from each equipment rack and
area, to the star point. This may or may not be used depending on circumstances, but it is easier to install in
the first place, than later when problems arise.The location of the star point should be a convenient, easily
accessible place, preferably at the rear of the console or in the main equipment rack.
Install separate 'clean' and 'dirty' mains outlets, wired individually back to the incoming mains distribution
box. Use the 'clean' supply for all audio equipment and the `dirty' supply for all lighting, etc. Never mix the
two systems.
If necessary, to provide sufficient isolation from mains borne interference, install an isolating transformer.
This should be provided with a Faraday Shield which must be connected with earth.
Never locate the incoming mains distribution box near audio equipment, especially tape recorders, which
are very sensitive to electro-magnetic fields.
Ensure that all equipment racks are connected to earth, via a separate wire back to the star point.
Equipment which has unbalanced inputs and outputs may need to be isolated from the rack to prevent
earth loops.
Audio Wiring
Having provided all equipment with power and earthing connections, consideration must be given to the
method of providing audio interconnection and adequate screening of those interconnections. This must be
done in a logical sequence to avoid problems and assist in the localisation of problem equipment.
Connect the FOH or Monitor system to the console and check for any hum, buzz, or RFI. Only when you are
satisfied with the quietness of the console and the PA system should you proceed with the next step.
Connect stereo or multitracktape recorders, FX and foldback sends one at a time, checking and isolating any
connection which degrades performance.
Connect all other peripheral devices.
Connect all microphone lines.
By following this sequence much time and future trouble will be saved, and the result will be a quiet, stable
system.
Shielding
Audio equipment is supplied with a variety of input and output configurations, which must be taken into
consideration when deciding where the screen connections should be made. There are three sources of
unwanted signal being impressed on the screen, which are as follows:
Extraneous electrostatic or electromagnetic fields.
Noise and interference on the earth line.
Capacitive coupling between the screen and signal wires.
To minimise the adverse affects of the unwanted coupling to the signal wires, it is important that the screen
is connected at one end only, i.e. the screen must not carry any signal) current. Any signal) on the wires
within the screen will be capacitively coupled to the screen. This current will ultimately be returned to the
source of the signal, either directly, if the screen is connected at the signal source end, or indirectly via the
earthing system, if the signal is connected at the signal destination end. The indirect connection will cause
an increase in high frequency cross-talk, and should be avoided wherever possible.
Therefore, in general, always connect the shield only at the signal source end. In high RF areas, the screen
can also be connected to earth via a 0.01 mF capacitor. This will present a short circuit at RF frequencies,
thus lowering the effective shield impedance to ground. However, at low audio frequencies the reactance of
the capacitor will be sufficiently high not to cause an earth loop problem.
8 GB2R User Guide