[Coco] [off-topic] cabling problems

[gene heskett]

On Friday, April 20, 2012 03:54:16 PM Mike Pepe did opine:

> Gene - on this topic, I was under the impression that twisted pair cable
> only really worked right if the signal is balanced. If it's a
> single-ended signal, the twisting does nothing, since one side of the
> pair is grounded. Is that right?
> 
> -Mike

That is pretty much true for the 'short' runs we would be involved in.  The 
signal coupling between the wires tending to force a balance at the far end 
wouldn't be of much help until the load end was several tens of wavelengths 
away, and by that distance the cable would also be functioning as a low 
pass filter anyway unless being driven with a source impedance that matched 
the cables surge impedance.  For the usual smallish 2 conductor audio cable 
used by the megamile in broadcast facilities, this is approximately 60 
ohms.  This sort of power in the distribution amplifiers that for decent 
headroom, need to be able to output 20+ volts peak to peak into both sides 
of the cable even though the average level peaks at 2 volts or less, is a 
major heat handling problem that I partially solved using the CMOS tlo-84 
family of op-amps.  Unforch, they did not tolerate the EMP's induced in the 
long runs by nearby lightning strikes, and we had a 255' lightning rod 30 
feet out the back door, so I had to stock those in "by the tube" 
quantities.  It got "interesting" at times. ;-)

Cheers, Gene
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