dumb question- Shield?

[setboy]

so, this is kind of a dumb question, but i got to ask it. what is the Shield? keep reading about it. but don't know what it is

[Roving Sign]

Or an FX cop show!!!

The idea is the the shield will catch any RF and shunt it off to ground...(sorry - EE terms I have NO business using...)

Becomes the (-) in a unbalanced config...

[setboy]

Thanks Guys

i think i get it now

Raphael

[Roving Sign]

Actually,.... In an unbalanced config, the shield is left open on the downstream terminal, and tied to ground on the upstream terminal, so as to shunt/route the rf back to the upstream source ground (the ground coming from the the shielded inner core group of wires).
Unbal is much more susceptible to the effects of rf, so proper shielding is more critical in the unbal config.

Relative to a balanced arrangement - yes...but my unbalanced mic cables have 2 conductors...hot and ground(the "shield")

[Roving Sign]

I like to pound a 6 foot copper rod into the ground and attach that to the shield - no RF gets by that baby!

Just Kidding...!

[whatboutbob]

So, err, if this is the thread for dumb questions, could someone give me the lowdown on balanced v unbalanced? 

[Lil Kim Jong-Il]

sometimes a shield is made of foil and sometimes both foil and braid.

there are more in depth explanations but basicly the balanced cable uses two signal paths to carry a differential signal relative to reference ground.  This is good because noise introduced on the cable is introduced on both lines, so it can be negated at the other end by adding one signal and the inverse of the other.  It's called Common Mode Rejection and you can find lots of references on google.  It's especially good for small signal wires, like microphone cables.

Unbalanced cable has one signal path carrying a signal relative to reference ground so there is no way to cancel induced noise.

[Lil Kim Jong-Il]

This comment about "leaving the downstream (or upstream) sheild unconnected puzzles me.  I've never read that this is something to consider.  It must only come into play with a 3 conductor + shield cable architecture... or xlr to 1/4"   I just peeked at some star quad xlrs that I have and the sheild is soldered to the 3 pin at each end.  Im confused.  (There's a shock.)

With the starquad balanced cables you have no choice but to use the shield for the ground.   I've come across a few unbalanced IC construction recipes that put the signal and return on a twisted pair, then shielded the twisted pair with the shield tied to ground at one end just like described above.  Usually the shield ground is connected to the source component, as mentioned earlier, so that the collected noise is not drained into the subsequent gain stage.

If you wanted to try this with balanced cables, you need an additional path inside the shield for common or run an additional braided sleeve around the outside of something like starquad.  I never tried this but I have read several reports that shielded cable sounds worse than unshielded twisted pair when no interference is present.  So I wonder if the additional braid would be bad or good.   Also, I don't imagine that this is much use for mic cables since the source is a small signal and there is not place for the noise to drain at the source end.