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[baking and welding audio]

We have a system that is experiencing sudden volume spikes intermittently. it will get really loud then go back to normal.

signal chain is as follows.

shure mx condensor gooseneck mic
shure scm800 mixer
rane dual 15 grapic eq
sabine fbx1020 feedback eliminator
crown com tech 810 amplifier
bose pan array system

also a split signal is going to a bose 70v system

we have bypassed the feedback eliminator, and changed out the mic. and things have remained the same. it doesnt seem to record to tape the same that it appears in the house system (amp?.

can anyone help out on this one?????????

thanks
jc

[Tim Padrick]

From what part of the signal chain are you taking your recording feed?  You need to keep moving it down the chain until you get to a point where:

You are recording the problem - the last unit added is the culprit.  (Make sure that you take a feed from the input and then the output od each device in the chain, so you don't falsely accuse a device that is being fed by a faulty cable.)

You have eliminated everything but the amp (and to do so would mean taking the feed at the amp inputs, so you are after the cables feeding it.

[Rodney Connelly]

Sometimes cables or connectors can cause these types of problems.  Type wiggling the cables one by one in the system and see if you can duplicate the problem.

[Bennett Prescott]

Could be the loose cable nrgdjs mentioned... if you've got a balanced line that is having connectivity problems with pin 2 or pin 3 you'll experience a 3 (or 6, I can never remember. Tom?) decibel drop in signal when the pin isn't connected. This is because it's only getting half the balanced signal, and without the other half to add back in, you lose signal.

[Rob Timmerman]

'Tis 6db.  

If the signals sum coherently (which they will if they are in phase), you get double the amplitude of the signal, which is 4 times the power, and thus 6db.  If the signals don't sum coherently, you only get double the power, or 3db.  In a balanced signal, both sides of the balanced line are in phase, but the polarity of one side is reversed, so that the signal is inverted.  (This is why we must remember that a polarity reversal and a phase shift of 180 degrees are NOT the same.)  When the signal is inverted again at the receiving end, any noise on the line is canceled out because it was received (in theory) equally by both sides of the line.

[Too Tall (Curtis H. List)]

baking and welding audio wrote on Wed, 22 December 2004 16:41

We have a system that is experiencing sudden volume spikes intermittently. it will get really loud then go back to normal. signal chain is as follows. shure mx condensor gooseneck mic shure scm800 mixer rane dual 15 grapic eq sabine fbx1020 feedback eliminator crown com tech 810 amplifier bose pan array system also a split signal is going to a bose 70v system we have bypassed the feedback eliminator, and changed out the mic. and things have remained the same. it doesnt seem to record to tape the same that it appears in the house system (amp?. can anyone help out on this one????????? thanks jc

TimmyP had the correct advice for troubleshooting the problem.

   I had something similar with an old White EQ. It has a set of jumpers that look like big staples. They patch across if you don't use an outboard transformer or xover.
   I was working along and the monitor system went into massive feedback. The jumpers were oxidized and found a clear path.
Oops!

The tip off should have been that I had to drive the front of the system (in this case a Ramsa 24 channel) way too hard to get the speakers to a working volume.

   So in addition to using TimmyP's advice on troubleshooting steps you should figure out if-

the system is running at a normal level and you are getting much more line level gain sometimes.

or

The system is losing gain somewhere, you are turning things up to make up for it and every once and a while it goes back to the level it should be at.

So take a look at your levels.

You could set the system to a nominal level (Rane EQ and Sabine set to flat) and note the speaker level and then feed the amps directly from the board see if the volume changes drastically.

I would be running noise through this and adding one piece or cable at a time. The only trouble with TimmyP's method is it depends on something changing. I would want to know if everything was running at the correct level.
   Looks like a great time to check your gain structure.


PS- do you have an analyzer or even a DMM?