SoundCraft Five Monitor Series manual Shielding, Points to Remember

lConnect 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:

lExtraneous electrostatic or electromagnetic fields.

lNoise and interference on the earth line.

lCapacitive 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 cou- pled to the screen. This current will ultimately be returned to the source of the sig- nal, 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 indi- rect 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 0.01μF 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.

Points to Remember

lIn all cases, use good quality twin screened audio cable. Check for instability at the output.

lAlways connect both conductors at both ends, and ensure that the screen is only connected at one end.

lDo not disconnect the mains earth from each piece of equipment. This is needed to provide both safety and screen returns to the system star point.

lEquipment which has balanced inputs and outputs may need to be electrically isolated from the equipment rack and/or other equipment, to avoid earth loops.

It is important to remember that all equipment which is connected to the mains is a potential source of hum and interference and may radiate both electrostatic or elec- tromagnetic radiation. In addition, the mains will also act as a carrier for many forms of RF interference generated by electric motors, air–conditioning units, thyristor light dimmers etc. Unless the earth system is clean, all attempts to improve hum noise lev- els will be futile. In extreme cases there will be no alternative but to provide a com- pletely separate and independant ‘technical earth’ to replace the incoming ‘noisy earth’. However, always consult your local electricity supply authority to ensure that safety regulations are not being infringed.

FIVE Monitor Installation 2.5