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Author Topic: Speaker Wire Question  (Read 2363 times)

Andrew Bob

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Speaker Wire Question
« on: May 10, 2006, 08:08:48 am »

Hello all,
I'm fixing up a sound system that is installed in a school hall. They have a very basic set up that is made up like this.
[mic]-Microphone cable-[PA Amp]-figure 8 cable-[Speakers]

At the moment there is a hum in the system, unpluging/pluging in a microphone dosen't stop the hum and nether does turning all the channels down in the PA Amp. So my throught was the that there is some electrical interferance with the speaker cable, between the PA Amp and speakers. I came to this thinking because when a bank of lights located right near the speakers in turned off the hum dies down a little bit. Slowly over time the hum has gotten worse.

I was wondering if using a shielded cable between the amp and the speakers would help reduce electrical interferance.

Any opinions/thoughts/suggestions will be greatly appreciated.
If possible I need to have this fixed in two weeks time.

Thanks
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Andrew Bob

Andrew Sharpe

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Re: Speaker Wire Question
« Reply #1 on: May 10, 2006, 09:25:56 am »

In my experience if the noise lessens when the lights are off, that is probably where your problem is.  Check to see if your speaker wire is near any of the AC wiring, and STAY FAR FAR AWAY from ballasts.  

As for shielded wire helping, it wouldn't hurt but might not be the answer depending on too many factors.  But if they noise is still there with the lights off it might be the amp that has the buzz, maybe run a direct line to the speakers (if possible) for test purposes.  

Anyway I've rambled on enough, I'm sure you know how to trouble shoot.
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Austin Parker

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Re: Speaker Wire Question
« Reply #2 on: May 10, 2006, 11:19:46 pm »

Does the cabling already have ground connected on both ends? balanced or unbalanced? If you are looking to put new cable in try to look for cable that has a ground lead, along with a full foil / paper sheild.
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Austin Parker

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Andrew Bob

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Re: Speaker Wire Question
« Reply #3 on: May 11, 2006, 07:31:50 am »

I'll have a closer look tomorrow @ where it runs. But thats hard as it goes through ceiling and walls.
At the moment it just two wires running from the amp 2 the speakers. one + and the other -. Not balanced.
If I can find a run of shielded cable long enough i'll test it with that, along with some unshielded cable and compair the results.
I'm still trying to work out how shielding on the cable helps, compaired to just running + and - on two different wires.


If i was to run shielded cable would I run 2 wires + Sheilding so have a - wire and Ground shielding connected together and the + on a another wire, eg using Mic cable, or Just 1 wire with sheilding so it would be + connected to the wire wire and - connected to the sheilding, eg Guitar lead cable.
I havn't really had much to do with sheilding cables as it's not usally used in home theare systems and car audio systems, only when it comes to microphone leads.

Thanks
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Andrew Bob

Rick Johnston

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Re: Speaker Wire Question
« Reply #4 on: May 11, 2006, 08:08:57 am »

Leave the lights on. Disconnect everything from the amp. Put a test speaker on the output. Got hum? It's either the amp, or something funky in the AC power. Try a different AC circuit.

If you don't have hum, attach the inputs one at a time. Still no hum? Attach the speaker leads one at a time.

At least that will narrow it down.

Regards,
Rick Johnston
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Hal Bissinger/COMSYSTEC

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Re: Speaker Wire Question
« Reply #5 on: May 12, 2006, 04:19:59 pm »

If these are voice coil (8 ohm) speakers I would be very surprised if the hum you hear is being induced in the cables. I rather suspect that it is being fed to the amp via the power line or a ground loop which is the usual scenario. You disconnected the inputs but did you power down the amp or disconnect the speaker lines from the amp? If it is indeed the wiring it shouldn't go away when the amp is turned off or the wiring disconnected and I think you would have noticed that!

-Hal

Tim Padrick

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Re: Speaker Wire Question
« Reply #6 on: May 24, 2006, 02:21:40 am »

Hal Bissinger/COMSYSTEC wrote on Fri, 12 May 2006 15:19

If these are voice coil (8 ohm) speakers I would be very surprised if the hum you hear is being induced in the cables. I rather suspect that it is being fed to the amp via the power line or a ground loop which is the usual scenario. You disconnected the inputs but did you power down the amp or disconnect the speaker lines from the amp? If it is indeed the wiring it shouldn't go away when the amp is turned off or the wiring disconnected and I think you would have noticed that!-Hal


Agreed.

Also note that the capacitance of shielded speaker cable can cause some amps to go into protection at very low signal levels, and should be avoided.
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