Sensitivity Loss Over Time

Jim Keller · July 8, 2008

OK, this doesn't actually qualify as "...for Film and Video" but I'm hoping someone here may have some clue what's causing this.

We have a sound system installed in our church, with drops at various key locations (pulpit, lectern, etc.) all running (under floors, in walls, etc.) to a sound board at the back of the church, similar to what you'd have in a live theatre.

Well, one of the lines is giving us a lot of trouble. We've used two different microphones and run it to two different channels on the board, so the problem is clearly in the line. The problem has only occurred with phantom-powered mics (we don't own any that don't require phantom).

Basically, when we first plug the mic in, it works fine. Then, slowly, and at an unpredictable rate, it loses sensitivity, until it picks up nothing at all. If we yank the line out of the board and plug it back in, the sensitivity always comes right back (with requisite loud pop if we've forgotten to kill it from the mix). We're needing to do this 1-4 times per service on average (though occasionally we make it through a lengthy service without it occurring at all).

Since it's a gradual signal loss and always returns reliably by unplugging and re-plugging the line, it doesn't seem to my admittedly inexpert sensibilities like a short in the line. There does not seem to be a correlation to the signal loss and the weather. There isn't a rodent problem (though there is a termite issue currently, but not near the run). There is more than one plug associated with each line, but we have not been able to test whether it occurs in all of them or only the one we use every week (because it won't do it at all when we're testing it, only during services). We have considered the possibility that God hates us, but would prefer to find a technological cause for the problem. :)

Has anyone ever seen anything like this? Anyone have any ideas what might be causing it or what else we should test? Obviously, if we can fix it without ripping up walls and floors to run a new line, that would make us much, much happier.

Thanks!

Adrian Sierkowski · July 8, 2008

Can't say I've heard anything like this before. Granted, I'm not techno expert. Sounds like a voltage drop in the phantom power, though. I would get a voltmeter and test out the line, seeing if it's giving you the 48v you should have. Though as it's over-time, it might not be readily apparent.

To prove its not god; turn on a radio, put it up near the mic, let it play for a bit while recording with a tape on a camera ;) then you can of course see how it drops down

Edited July 8, 2008 by Adrian Sierkowski

Hal Smith · July 8, 2008

Adrian's on to a good checkout. To me it sounds like the cable is question got really wet at some time the past and now is presenting a low resistance value to the phantom power supply. In effect the phantom supply is getting overloaded and slowly losing voltage.

Try turning off the phantom supply and testing with a dynamic mike that doesn't require a phantom supply like a Shure SM-57 or 58. You could also try measuring cable resistance from the cable's XLR connector from pin 2 to pin 1 and then from pin 3 to pin 1. Those are the lead combinations that the phantom supply powers. You should see an extremely high resistance, ideally infinity. If you see a dead short from pin 2 or 3 to pin 1, you could have a bad solder job inside one of the cable's connectors rather than a wet cable.

All audio problems relate to film and video; figuring out what's wrong with your church's system is good practice for troubleshooting when you're twenty miles out in the boonies shooting something and your sound goes bad. Plus it's good stewardship. And God will bless you for helping Him out (Maybe I've watched "Evan Almighty" too many times? ;) ).

Jim Keller · July 8, 2008

And God will bless you for helping Him out...

You're assuming that this isn't all because He hates us... :)

That sounds like a great troubleshooting program. I'll bring my voltmeter to church on Sunday. Thanks!

Hal Smith · July 8, 2008

You're assuming that this isn't all because He hates us... :)

That sounds like a great troubleshooting program. I'll bring my voltmeter to church on Sunday. Thanks!

Naw, He doesn't.

PS: Unplug the cable from the mixer before any resistance test, your meter will not take kindly to 48 volts DC when on its resistance position, unless it's a Fluke. All one of my Flukes do when I do something real stupid is start beeping at me.

Paul Bruening · July 9, 2008

Can you physically trace the whole line? Did someone coil a lot of excess cable to remove slack in the installation? My pop always taught me to stretch the loops out. If there are a lot of loops, they can act like an induction coil and give weird results. I've never heard of total signal loss from coils. But, it might be worth adding to the checklist.

Jim Keller · July 9, 2008

Can you physically trace the whole line? Did someone coil a lot of excess cable to remove slack in the installation? My pop always taught me to stretch the loops out. If there are a lot of loops, they can act like an induction coil and give weird results. I've never heard of total signal loss from coils. But, it might be worth adding to the checklist.

Sadly, the line was enclosed into the walls and floors before I joined the church...

Jim Keller · July 28, 2008

OK, the mystery deepens.

We tested the lines with a volt meter, and sure enough, the resistance was lower than it should be. So yesterday I ran a test with the microphone instead plugged directly into a phantom power generator, and no phantom power coming down the line from the board. We still lost all sensitivity from the mic (which came back when we unplugged and re-plugged the mic from the phantom power supply, but not when we unplugged and re-plugged the mic line).

I did manage to confirm, however, that the cable snake runs under the air conditioning ducts, and a few years back there was some sort of a condenser reversal that flooded everything under the ducts, so the lines were, in fact, saturated with water in the past.

Any other thoughts before I tell them we need to pull a new snake?