We ran 5 microphone cables about 175 feet through PVC pipe underground
between two buildings. Shortly after the job was completed we tested
the lines. They weren't as quiet as I had hoped they would be but they
were usable. Over the past two years they have gotten more noisey. We
set up a test today. All cables were open ended, that is unterminated
on the other side. We set each input individually so that the VU meter
read 0VU noise level. Then doing one input at a time we plugged a mic
in. The noise level dropped to -6 on all inputs when checked
individually. It was an improvement but certainly still not too good.
Next we started out with open ended cables again, established our 0VU
noise reference and then shorted all three terminals on the XLR
connector together with a piece of foil. I repeated this with all the
remaining cables and the results were the same. The noise completely
disappeared and the line was clean. I next disconnected the line at
both ends and checked each conductor to ground for high resistance. I
read infinity from each conductor to the sheild as well as to building
ground. Next we took one mic and boosted it to line level at the far
end. We ran this line level signal through one of the underground
cables to a line level input on the mixer. The noise level is now way
below the signal level and the signal was squeaky clean. So now we
know that we can definitely retrofit the job and get it to work this
way, but this whole thing really puzzles me. Does anyone have any
ideas as to why we are getting all this low level noise on these
cables? They seem to acting like antennas. But when grounded they are
clean. Also since addional equipment is now going to be an added
expense to my customer, and one which they will not apreciate, any
ideas as to what I might try to remedy the situation other than pre
amplifying my mics before they go through the underground cables would
be nost sincerely appreciated. Lenny Stein, Barlen Electronics.