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weird goings on with Audio and Video


Bill Beach:
we have a relatively small church (around 300 seating capacity) and we run a mostly contemporary worship service.  we have video projection for lyrics and sermon notes (on 4 projectors).  we record video to DVD for our Sunday morning service (sermon portion is available at  we recently upgraded from a Mackie to a Allen & Heath GL2400-32.
our difficulty may have been there before, but masked by other things going on that now are gone.  our Mackie was at least 10 years old, so the A&H is definitely an upgrade!
after installing the A&H, and having turned off the entire sound system, as I was heading to the front of the auditorium to leave, I notice a slight buzz that seems to be coming from the stage.  in checking it out, I find that the left monitor speaker on the stage is indeed emitting a low level sound like a ground loop hum.  this is with ALL the power OFF to any portion of the sound system!!  on further examination, we have found that if the mic line for our red mic (color immaterial, really) is disconnected, the buzz goes away, power on or off.  this mic line and the speaker quarter inch share the same floor box (and 3 other mic lines -- other side of the stage has 4 mic lines, a monitor speaker line and right and left balanced line level lines for those monitor speakers generated from the Aviom personal mixer the Worship Leader uses, but no noise problems).  there is an XLR connection from the stage box to the lines that run to the FOH position at the back of the auditorium.  if the mic line is disconnected from the A&H console, the buzz goes away.  I switched to a different front to back mic line and had the same problem.  on the 'new; mic line, I disconnected the shield where the line connects to the snake going into the sound both (via barrier strip).  that took away the buzz.
there is also another thing.  the video projection is affected by this situation as well.  we get differing 'sound lines' jumping around in the projected image.  the only 'connection' between the video projection and the sound is quite remote.  we send a VGA signal converted to composite video through a scan converter to the video mixer upstairs for the camera operator to be able to mix in the sermon notes as needed.  the video mixer is also connected to the (2) DVD recorders and the sound meets the video signal at the DVD recorders.  (the video mixer does not have any audio run through it, only video.)  also, with the shield disconnected on the offending mic line, the interference in the video projection goes away.
now this evening (we also have Saturday night services), I noticed we were getting signal on the red mic channel.  that seemed odd, being as I had taken the mic off the line because it no longer was working (tried it on other channels known to work and it didn't work -- perhaps related?).  I soloed the red mic channel with no signal being sent to mains and heard what seemed to be the full mix sounding rather weak and tinny.  forgetting that the channel was still connected to the Aviom, I cranked it up to better hear what was on this channel (that should not have had anything on it).  this caused a bit of feedback in the Aviom, heard by the drummer that uses headphones, but not loudly.  (also very short term as i also noticed and turned it down right away.)
why would this input mic channel be carrying the output signal?
the whole scenario seems really weird to us!!
please help!!
any further info gladly supplied, just ask!
I am volunteer and work a full time job, so may not get back to check this as often as I would like, but will try to stay on top of it and will post any more anomalies we find.[/SIZE][/SIZE]

Don Sneed:
you said the 1/4" monitor spk is in the same floor box as the mics, does the 1/4" jack has an isolated washer to isolate the ground from the box? If not known I would remove the 1/4" from the box to see if the noise goes away, I had this problem before & traced it to an un-isolated 1/4" mpnitor spk jack. Is the guitars & electric piano going through direct boxes to the mixer, could be one of is the amplifiers & mixer on the same electrical phase? a few things to check to start...good luck

Bill Beach:
I was using the 1/4" isolation jacks, but they are plastic and were getting torn up real fast.  didn't realize there was such a thing as an isolation washer.  I will need to check into getting some.
right now we don't have an electric piano (have one but not used at the time) and the electric acoustic guitar does go through the same floor box after the DI.
amps & mixer are on the same phase.
thanks for the input!


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