[Boatanchors] 2nd Newbie Q

[Brian Clarke]

For really good quality interconnects at audio frequency, you should run 
balanced lines, ie, two-core shielded cable. In this way the shield performs 
only one function - shielding. The two internal cables then carry the 
differential that is your audio signal. If you try to get the outer shield 
to perform two functions, ie, shielding and carrying one part of the 
differential signal, then any mains transformer, motor or electrical noise 
generator within cooee will induce AC Voltages in the outer shield. This 
Voltage on the shield will be seen at the 'receiving end' as signal; we will 
hear it amplified as hum and noise.

Coaxial cable for carrying audio signal is OK for very short lengths inside 
a chassis, or inside one metal cabinet where there are no enclosed mains 
transformers. How to use twin-core shielded? At the sensitive (receiving) 
end, connect the shield and one of the internal pair (eg, black) to the 
ground and the other of the internal pair (eg, white) to the input 'hot' 
terminal. At the other (sending) end, connect the black wire to ground and 
the white wire to the signal output. Leave the shield unconnected.

If after all this, you still have hum and noise, you may need to lift the 
mains ground lead above the chassis with say a 100 Ohm resistor. This will 
give you safety and it will significantly attenuate any ground-loop induced 
Voltage. The next step after this, is to use truly floating differential 
circuitry, such as transformers. This is the kind of thin,king applied in 
high quality audio recording studios,

At RF, different rules apply. Here, you should ground the shield at both 
ends and at somewhere between every one-twentieth and one-fiftieth of a 
wavelength along the coaxial cable. In this case, you are making the coaxial 
cable part of the ground plane.

73 de Brian, VK2GCE.