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Author Topic: Odd problem  (Read 3261 times)

Bob Henley

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Odd problem
« on: May 26, 2009, 04:13:05 pm »

I have an odd problem I can't figure out:

We have monitor outputs scattered around our stage. They are fed from AUXs on our Soundcraft MH3, through microphone cable in conduit to an amp rack, then speaker cable through conduit to the output. There are six outputs total, and each conduit holds four single-conductor cables (two outputs).

Here's the issue: I hook up a speaker to one of the outputs, I play a source through the AUX. Sounds fine. I turn that one off Then, I turn up it's partner AUX with no speaker attached. I hear soft, distorted sound out of the first output. The second output won't work at all.

I checked the board by using a pair of powered speakers direct out of the AUXs and that doesn't appear to be the problem. I also swapped out the amp for a brand new RMX850, and that didn't solve anything either. In the process I re-connectorized the speaker cables.

I assume it's some kind of cabling issue, but I can't find any physical damage on the parts of the cable I can see.

Any suggestions?
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Mark Mattocks

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Re: Odd problem
« Reply #1 on: May 26, 2009, 04:29:15 pm »

Bob,

Have you:

1) Plugged a source directly into the amplifier - channels a and b - to check if the issue is cabling from the console to the amp rack?

2) Metered the resistance of the speaker cable from amplifier to monitor output with and without speaker connected?

These two steps should tell you where your issue is.

Mark
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Bob Henley

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Re: Odd problem
« Reply #2 on: May 26, 2009, 04:38:18 pm »

No, I have not - I'll do that tommrow.

I hadn't thought of running a different cable to the amp (durr Embarassed) - it's maybe 45 feet in the conduit, but 150 outside due to walls. I should have enough 50-footers to do that...

What should I be looking for with the resistance- just a value vs 0 (cable cut) or something special?
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Charlie Zureki

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Re: Odd problem
« Reply #3 on: May 26, 2009, 04:48:34 pm »

Bob  Healey wrote on Tue, 26 May 2009 15:13

I have an odd problem I can't figure out:

Quote:

We have monitor outputs scattered around our stage. They are fed from AUXs on our Soundcraft MH3, through microphone cable in conduit to an amp rack, then speaker cable through conduit to the output. There are six outputs total, and each conduit holds four single-conductor cables (two outputs).



Quote:

Here's the issue: I hook up a speaker to one of the outputs, I play a source through the AUX. Sounds fine. I turn that one off Then, I turn up it's partner AUX with no speaker attached. I hear soft, distorted sound out of the first output. The second output won't work at all
.

 What?...  You turn up the Aux that does not have a speaker connected to it and expect to get sound? Or do you mean it doesn't produce with that speaker plugged in? But does give you sound from the other Aux's speaker?

 I think, Although... you're not coming across clearly.... there's something wrong with the Speaker pair's termination... conductors are swapped on the pairs. (If the other Amp that was swapped is doing the same thing)

And, are your speaker cables in "twisted pairs"? or just some thnn or tnn pulled through the conduit?

If your cabling and terminations are proper, you should get sound from either speaker by flipping the Aux output at the board.

Quote:

I checked the board by using a pair of powered speakers direct out of the AUXs and that doesn't appear to be the problem. I also swapped out the amp for a brand new RMX850, and that didn't solve anything either. In the process I re-connectorized the speaker cables.



Quote:

I assume it's some kind of cabling issue, but I can't find any physical damage on the parts of the cable I can see.

Any suggestions?



Good Luck,
Hammer
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Mark Mattocks

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Re: Odd problem
« Reply #4 on: May 26, 2009, 04:51:16 pm »

Bob,

By a source at the amplifier, I meant plug an I-Pod or CD player or other line level device directly into the input on the amplifier - through proper adapters as needed - bypassing the console and all wiring between the console and the amplifier.

A speaker run with no speaker should measure open - as in infinite resistance.  If you get a reading of 0 ohms from a speaker wire run with no speaker attached, then there is a dead short connecting the two conductors at some point.  You should also measure impedance between each speaker conductor and the conduit to determine if one of your wires got chewed up as it was pulled.  With a monitor connected, you should see the same impedance as the monitor by itself - say 5-6.5 ohms for an 8-ohm wedge.

Good luck,
Mark
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Bob Henley

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Re: Odd problem
« Reply #5 on: May 26, 2009, 04:58:27 pm »

No, I turn up the AUX with no speaker connected and GET sound out of the other AUX with a speaker attached. Distorted sound, but enough it's audible. With the exact same set-up, turning up the first AUX works as normal.

The pairs are separate, not twisted. Therefore, there are four conductors in the conduit up to the first output, then two continue to the second.

Maybe I should draw a diagram, this is a bit tricky to explain.

EDIT: Drew the diagram  Smile

index.php/fa/22924/0/
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Charlie Zureki

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Re: Odd problem
« Reply #6 on: May 26, 2009, 05:11:11 pm »

Bob  Healey wrote on Tue, 26 May 2009 15:58

No, I turn up the AUX with no speaker connected and GET sound out of the other AUX with a speaker attached. Distorted sound, but enough it's audible. With the exact same set-up, turning up the first AUX works as normal.

The pairs are seperate, not twisted. Therefore, there are four conductors in the conduit up to the first output, then two continue to the second.

Maybe I should draw a diagram, this is a bit tricky to explain.



 No... I think I'm getting the picture... Now try the reverse.

Now disconnect the speaker from one and connect the speaker to it's other connection and turn off it's Aux, while turning the other Aux up ... and see if it does the same thing.

Hammer
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Bob Henley

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Re: Odd problem
« Reply #7 on: May 26, 2009, 05:28:08 pm »

Mark Mattocks wrote on Tue, 26 May 2009 15:51

Bob,

By a source at the amplifier, I meant plug an I-Pod or CD player or other line level device directly into the input on the amplifier - through proper adapters as needed - bypassing the console and all wiring between the console and the amplifier.



Ah. I should have thought of that earlier.

Quote:


A speaker run with no speaker should measure open - as in infinite resistance.  If you get a reading of 0 ohms from a speaker wire run with no speaker attached, then there is a dead short connecting the two conductors at some point.  



Thanks!

Quote:


You should also measure impedance between each speaker conductor and the conduit to determine if one of your wires got chewed up as it was pulled.  With a monitor connected, you should see the same impedance as the monitor by itself - say 5-6.5 ohms for an 8-ohm wedge.



The conduit is PVC - is there an alternative method?
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Mac Kerr

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Re: Odd problem
« Reply #8 on: May 26, 2009, 05:34:53 pm »

Swap the speaker cables at the output of the amp. Now aux 1 will be on the old aux 2 floor pocket, and vice versa. Do the test again and see if it is the speaker cable, or something before that.

If the conduit is metallic, and the hot conductor of aux 2 was damaged in the wire pull so that it shorts to the conduit the signal could be getting back into your amp on the ground. You can check this by disconnecting everything and measuring for something other than an open circuit between each conductor and the conduit.

Mac
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Bob Henley

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Re: Odd problem
« Reply #9 on: May 26, 2009, 05:43:45 pm »

Mac Kerr wrote on Tue, 26 May 2009 16:34

Swap the speaker cables at the output of the amp. Now aux 1 will be on the old aux 2 floor pocket, and vice versa. Do the test again and see if it is the speaker cable, or something before that.

If the conduit is metallic, and the hot conductor of aux 2 was damaged in the wire pull so that it shorts to the conduit the signal could be getting back into your amp on the ground. You can check this by disconnecting everything and measuring for something other than an open circuit between each conductor and the conduit.

Mac


I'll make the swap when I get in tomorrow, but all the conduit is definitely PVC.
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Duane Massey

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Re: Odd problem
« Reply #10 on: May 26, 2009, 08:55:17 pm »

If it worked before, and nothing has been altered, I would suspect the amp, but you've already tried that.
If the problem has always been there, I'd suspect a wiring error at the speak-on connector(s).

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Robert Sims

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Re: Odd problem
« Reply #11 on: May 27, 2009, 02:33:47 am »

Do you have the amp in the bridge mono mode? This will disable the second input. If you have the input paralleled you would hear aux one feed in both monitors.  


Do you have anything inserted in the Aux feeds? EQ,s ? I've had inserts cause crosstalk in the past. More so on a group inserts, but it might be a mis-wired or half connected insert if you're using them.

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Bob Henley

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Re: Odd problem
« Reply #12 on: May 27, 2009, 07:32:16 pm »

OK, new developments today:

1) I have ruled out the amp and everything before it: I connected a pair of speakers directly to the amp and could not reproduce the problem.

2) I was able to reproduce the problem with an iPod connected to the amp.

So, it looks like that speaker cable.

Should I replace both pairs, or just the one that's giving me problems? A quick search for speaker cable comes up with Belden 8477 for a 2-conductor 12 gauge - is that OK, or should I look elsewhere?

Here's the new table:
index.php/fa/22957/0/
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Charlie Zureki

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Re: Odd problem
« Reply #13 on: May 27, 2009, 10:13:44 pm »

Bob  Healey wrote on Wed, 27 May 2009 18:32

OK, new developments today:

1) I have ruled out the amp and everything before it: I connected a pair of speakers directly to the amp and could not reproduce the problem.

2) I was able to reproduce the problem with an iPod connected to the amp.

So, it looks like that speaker cable.

Should I replace both pairs, or just the one that's giving me problems? A quick search for speaker cable comes up with Belden 8477 for a 2-conductor 12 gauge - is that OK, or should I look elsewhere?

Here's the new table:
index.php/fa/22957/0/



Bob,

 Before you replace any cabling/wire, check the  solder connections in the panel (wall or stage pocket?) to the  (NL4) Speaker connectors.

Are they terminated correctly? No solder or wire bridges between these two Monitor connectors inside the floor pocket or Wall panel?

 If you have to replace speaker cable in the conduit... I'd suggest replacing both pair. (maybe leave the good cable in the conduit for spares)  And, pull BOTH new pair at the same time.


Measure the distance of all the conduit length , extra for inside the rack, and add extra for a safety margin. DOUBLE that LENGTH and cut ENTIRE LENGTH from the spool ( example: two 100ft runs = 200ft and add extra, cutting 200+ ft from spool)  ..... fold the ENTIRE length in half and tie middle of cable on to pull string/fish tape and.....make your pulls.

MEASURE CAREFULLY, MEASURE TWICE ....

Any Belden 12 ga should be fine.... use twisted pair.

Good Luck,
Hammer
 
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Charlie Zureki

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Re: Odd problem
« Reply #14 on: May 30, 2009, 12:24:21 pm »

Bob,

  Did you find the answer to the problem?

 Keep us advised.

Thanks,
Hammer
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Bob Henley

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Re: Odd problem
« Reply #15 on: May 30, 2009, 01:56:02 pm »

Charlie Zureki wrote on Sat, 30 May 2009 11:24

Bob,

  Did you find the answer to the problem?

 Keep us advised.

Thanks,
Hammer


No, I found other problems...

Somebody had unplugged the control wire leading to the sequencer from a power strip in the back of the amp rack, then thought the power strip was broken.

They proceeded to plug everything into the other power strip. Meaning we went from four twenty-amp circuits to two twenty-amp circuits.

It's been like that for at least a year. I really don't understand what people think sometimes.

I did take a look at the connectors and didn't see anything amiss, but I'll get back in on Monday and replace them. We'll probably order new speaker cable on Monday if the connector change doesn't fix the problem.
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Mark Mattocks

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Re: Odd problem
« Reply #16 on: May 30, 2009, 10:19:19 pm »

Bob -

I've read through the rest of this thread and I have trouble believing that four runs of single conductor 12 ga in conduit are now bad.  Did it work before?

I would now check to be sure that the pair of speaker wires connected to output 1 of the amp are indeed the same pair that is connected to the output in the floor pocket.  Are the runs to monitor output 2 - in the farther box - continuous or are they junctioned in the first box?  The more that I think about this, the more it seems to be a mis-wiring issue or a short in the junction box to a steel plate possibly.

I would also meter all conductors to all other conductors with no speakers or amplifier attached.  Any reading other than infinity between the speaker wires would indicate a short somewhere along the way.

Best,
Mark

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Bob Henley

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Re: Odd problem
« Reply #17 on: June 01, 2009, 02:57:53 pm »

OK, I finally got back in.

I inspected all the connectors again and didn't find anything suspicious.

Then, I metered the conductors. One pair returned infinity, the other a value.

It looks like two of the cables were damaged and are now shorted. The cost of cable isn't a problem and we've got lots of downtime this summer, so we'll replace the cables next (as soon as we order new stuff)

Thanks for the help, everybody. This forum is awesome!
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