Behringer only hums in church

[drewl]

But it only hums in their church!
Its fine elsewhere.
I could swap out IC1 on all three as a precaution.

[Enzo]

or you could go find out the real problem. Why swap out a million parts when the thing sits there working? You plug signals in and it works, they plug THEIR signal in and it doesn't. Maybe their signal isn't right?


I have a plain old FM tuner in the warehouse as a signal source. I run that through one of those mic transformer things to make a balanced signal into the powered speakers. I also have some 1/4 female to XLR adaptors. Two wired pin 2 hot, the others wired pin 3 hot. WHichever pin 2 or 3 is hot, the other is grounded. That lets me check either hot pin used on its own unbalanced. So with my three adaptors I can take a few seconds and check that both balanced and unbalanced inputs work.


In teh church, you have so many possibilities. A ground loop may have resulted. The output from the mix station may not be fully balanced anymore or maybe a ground trace burnt open on the mixer (or whatever the last piece of gear is in the signal path at the mix station) Maybe the output of the mixer is OK, but the long cables out to the speakers are not. Bring a mic cord with you so you can connect the mix board itself directly to a speaker.

So remember that this is a sound SYSTEM, not some powered speaker working all alone on a deserted island.

[drewl]

I agree and have tested it as you suggested, but we have an unsatisfied customer here and as usual it is probably a user error.
I was trying to eliminate any possibility of there actually heijgva problem with these units before I go and check out their set up with my own cables.

As I mentioned they have hooked up a couple other of the sane units in the same place without the hum and noise.

[g1]

They probably mean "in the room" rather than "in the exact position with all the same connections as the one that hums". At least that has been my experience with these type of installs.

[drewl]

My friend the shop owner took the units to the church and verified the noise.
They then took two other units and hooked them up in the same place and they were fine.
He's no rocket scientist but he knows how to hook up pa stuff.

I think they are friends of his.
Anyway, I tore apart all the filtering circuits for the hell of it and everything checks out okay.

Looks like I'm going to church.

[The Dude]

This may or may not apply in this particular instance. We don't know if a ground loop is the problem. For that matter, a ground loop may have caused the problem. At any rate, this link should be read by anyone who installs or uses sound systems. Note particularly that the ground is not necessarily connected at both ends of a balanced signal line to prevent ground loops. It is only a shield and therefore does not need connection at both ends. I've fixed lots of hum problems by simply opening a connector and snipping a wire. This can happen particularly when (for instance) front of house gear is on a different power leg than stage gear.

[drewl]

Well I know they dimmers on the line and the cables are run to an XLR panel with cables run in the walls like a studio.
Thanks for the, it may help me debug their problem.

[g1]

For all I know their "good" units may have an open ground preventing hum.
We'll see.

This is a very valid point and worth looking into.
A shop I worked at had a stereo power amp do the rounds of all the techs for way too long. The complaint was that one side was weak. In the end it turned out that the other side was faulty and actually had too much gain. I try to always remember that one when making assumptions due to the customers description of the symptoms.

[Chuck H]

Nah, Behringer just forgot the words. All it can do is hum along.

The Behringers know all the NON secular stuff. It must be church related

[Enzo]

OK, so your repair units hum when connected to their system. But if they unplug the input cable from your unit, and plug it directly into one of theirs, it doesn't? Same exact cord, only change is which cab it connects to? It may be nothing, but plugged them in "in the same place" just leaves some doubt for me.


If indeed one cab works without hum at the end of the cord, but the other cab hums. Assuming the exact same feed cord and power cord are swapped onto it. then the speaker cab itself must have the issue. And if that cab works fine in your shop, then we can eliminate filter caps and all that sort of stuff, because they would cause hum in your shop as well as at the venue.

Besides, this amp has a SMPS, so filter caps are not filtering 60Hz anyways.

The problem would more likely be at the input. or a ground loop. These were lightning damage? I might look for a damaged or open ground trace on the boards. In your shop an open ground might not cause hum becuase everything is otherwise on the same page electrically.

But speaking of pages, look at page 2 of the schematic, should be the power supply. AC power comes in upper left, earth goes to chassis with the "rake" symbol (my names here). There are a lot of different grounds here. Note upper right the +/-15v supplies are grounded by what I will call a tree, since it looks like an upside down Xmas tree...sorta. Note the circuits on the left ground to an open triangle symbol. I'll call it a triangle, they call it ACGND. I don;t like that because it gives the impression it is grounded, when that is actually -170vDC there. Now lower right. See the triangle and tree are linked by a couple caps C7 C45. And a bunch of caps C126-131 link the Earth rake and the tree grounds. I'll ignore the obvious drawing error there for now. And bottom center, between the two 40v rails is 10 ohm R86 joining tree to earth. Look just left of the drawing of a transistor.

Thing is we can lose some of those connections and the thing will still work.

So I propose looking at one of the fixed by hummy amps, look close at the circuit board to see if somewhere a ground trace has been burnt open. Is R86 open? Where the power cord ground pin enters, is that trace burnt open anywhere? Traces tend to burn open at narrow spots. Likewise the input preamp board, the shell of the XLR is grounded, but that isn;t necessary for a balanced input, but can still result in hum.

I don;t recall, did we try not only balanced inputs, but did we also try unbalanced? Both grounding pin 2 and feeding pin 3 and grounding pin 3 and feeding pin 2?

One question, when it is conected to their circuit, it hums, if we pill the input cord out of it but leave it powered, does it still hum? Of so, then that is weird, but if that stops the hum try this: Touch the metal shell of the input cable to the metal body of the input jack or to some exposed piece of metal on the speaker amp chassis. In other words, we want to ground the input cable to the thing without plugging it into the input jack. If that makes it hum, we have a ground loop.