Sharing a common for left and right speaker - need answer fast - AVS Forum
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post #1 of 10 Old 11-15-2012, 12:07 PM - Thread Starter
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I'm not sure where to put this thread so, I threw it in here.

My Uncle is having a lot of landscape work done including landscape lighting and outdoor speakers. We've been talking about it since I'm doing similar projects.

Anyways, he has a contractor running his cables and such. I was talking to him earlier and he told me how his contractor is using three-wire direct burial cable for his speakers. He is going to use two of the wires as 'positives" on a left and right and use the third wire for the common ground for both. The argument is that he only has to run a single cable.

I argued with him that he's not really saving much money by not buying two separate speaker wires, since the ditch is being dug anyways and you're not talking that much more money for supplies. The run to one speaker is about 80' and the other is 100' with 20' separating them. He said it's being done tomorrow and that the only speaker wire he can get today would be pricey and that it would also require the cost of conduit. Plus, his contractor is giving him the direct burial wire they are using.

I told him that you shouldn't use the same common for both speakers. He asked me why, and then I realized that I didn't know. It's just something that I've been told. I don't know if it puts more of a load on the amp or if it's an ohm issue or what.

I'm worried that if he goes this route, he'll either ruin his speakers and/or his amp.

Can somebody fill me in on this before lunch time tomorrow?

Am I wrong about this? If not, can you clue me into the problem so I can inform him with the reason and not just what I'm guessing?

I've been doing a Google search without luck.
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post #2 of 10 Old 11-15-2012, 06:11 PM
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I'm with you - intuitively I wouldn't imagine this would be good practice, but I'm not sure why! Having said that though, using what's known as a 'common ground' has been very commonplace in car audio circles for some time - individual feeds to the +ve terminals of a pair of speakers and the -ve sides of both speakers connected directly to a metal body/chassis grounding point [try Googling "wiring speakers common ground" or similar] - the amp's negative side obviously is also grounded - this is the electrical equivalent of what you are contemplating.

Admittedly, not a briiliant response, but in the absence of anything else, maybe a place to start.

What could possibly go wrong?! biggrin.gif
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post #3 of 10 Old 11-16-2012, 08:35 AM - Thread Starter
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I appreciate the response.

I told him to run to the hardware store and by another direct burial cable that was long enough and have the guy throw that into the ground with it.

I was successful in arguing that for $30, it's better to be safe than sorry.

Although he wont' have actual speaker wire, at least he won't be sharing the ground.

Additionally, I found something I thought was interesting when doing a search on using solid core wiring (which is what this direct burial cable is).

http://www.hps4000.com/pages/spksamps/speaker_wire.pdf
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post #4 of 10 Old 11-16-2012, 09:10 AM
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Interesting article.

Glad you got a second cable into the trench - there's an old Chinese proverb: You can never have too much cable! [Never know when the urge for volume controllers, cameras, more speakers, more lights, etc. might hit!]
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post #5 of 10 Old 11-17-2012, 09:21 AM
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Good chance it’s okay for the speakers to share a common. If you connect a VOM meter to the speaker negative (black) posts on a receiver, you’ll probably find that they have continuity. At least this has been the case with the ones I’ve checked in the past. Keep in mind that headphones are basically just speakers, and they share a common negative.

Still, like you said, better safe than sorry. It doesn’t matter if it’s “actually speaker wire.” Any wire the proper gauge that's rated for direct burial will work fine.

Regards,
Wayne A. Pflughaupt



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post #6 of 10 Old 11-19-2012, 07:07 AM
 
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Quote:
I told him that you shouldn't use the same common for both speakers. He asked me why, and then I realized that I didn't know.

It's not a problem. Power is distributed all over the world using a 'common ground'.
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post #7 of 10 Old 11-19-2012, 09:38 AM
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Quote:
Originally Posted by SAM64 View Post

It's not a problem. Power is distributed all over the world using a 'common ground'.

Agreed, but the issue is deciding if the negative side of two independent amp channels was truly common, not if 'common ground' is electrically viable.
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post #8 of 10 Old 11-19-2012, 11:03 AM
 
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but the issue is deciding if the negative side of two independent amp channels was truly common

Easily solved using a multimeter.
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post #9 of 10 Old 11-19-2012, 11:07 AM
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Quote:
Originally Posted by SAM64 View Post

Easily solved using a multimeter.

Wayne suggested this a couple of posts ago
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post #10 of 10 Old 11-19-2012, 11:09 AM
 
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Yes, I read the whole thread as well.
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