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.
- posted
16 years ago