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Author Topic: Odd problem  (Read 3262 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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