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Old 6th January 2011, 09:54 PM   #1
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Default DIY line-level interconnects

I want to make a couple of line-level cables with male RCA plugs on both ends. They are for powered subwoofers, and need to be about 15 feet long. I do not want to bother with baluns if I can make do with a simple unbalanced line.

All the interconnects that I have ever cut open use a cable that has a stranded center wire and a woven shield. I cannot find that cable anywhere. I do find lots and lots of "RG-6", which has a solid wire in the center, and is a whopping 1/4" in diameter. I also found some with two center strands, also 1/4" in diameter. http://www.parts-express.com/pe/show...number=100-282

So... The question is, how best to make the cables? What wire to use?

I tried searching. Everything I found got into the business of baluns or active balanced/unbalanced converters.
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Last edited by Dave Jones; 6th January 2011 at 10:07 PM.
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Old 6th January 2011, 09:59 PM   #2
Geosto is offline Geosto  Netherlands
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RG59
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Old 6th January 2011, 10:15 PM   #3
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Originally Posted by Geosto View Post
RG59
I found this: RG-59/U Copper Braid Shield 100 ft. | Parts-Express.com

It's got a solid center and it's 1/4" in diameter.
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Old 6th January 2011, 10:59 PM   #4
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URM70 or if you want a little thinner RG58. Both are stranded centre, which I prefer as it's more flexible.
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Old 6th January 2011, 11:03 PM   #5
Geosto is offline Geosto  Netherlands
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Yep thats the one.
Bud why not using the microphone cable?
Use one conductor to the center and the other one to outside of the RCA plug.
Connect the shield also to the outside of the RCA only on the side where the equipment is connected to the ground.
Thats mostly the amplifier side.
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Old 6th January 2011, 11:34 PM   #6
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Originally Posted by Geosto View Post
Yep thats the one.
Bud why not using the microphone cable?
Use one conductor to the center and the other one to outside of the RCA plug.
Connect the shield also to the outside of the RCA only on the side where the equipment is connected to the ground.
Thats mostly the amplifier side.
I am going to display my ignorance now. Ready for it? Here it is.

I don't understand that last part about connecting the shield on one end. Neither the pre-amp nor the sub's plate amp is connected to ground except via a two-pronged wall plug. Both are on the same circuit, and the wall plugs have big and little sides. I am guessing you will recommend connecting the shield to the negative conductor on the end that plugs into the plate amp. But I haven't the foggiest idea why.
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Old 7th January 2011, 12:59 AM   #7
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Any shielded cable will work. It is even less critical since this is for a sub where any capacitance of the cable/HF roll-off is a complete non-issue.

I think the idea behind connecting the shield at one end is to act as a sort of "drain" for any EMI picked up by the shield. Connecting it at one end would prevent currents from flowing on the shield between the components, but since you would have a wire connected at both ends anyways (the signal ground), currents will flow - ground loops - between the components if a potential exists.

See more info here: Sound System Interconnection
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Old 7th January 2011, 01:05 AM   #8
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Dunno about benefits of RF cables at audio or their sevicability when home assembled but Radio Shack currently have an on-line special of 16ft "Monster Cable" standard duty subwoofwer interconnects on sale in the US. $17.99 ea.

As you find, not many parts stores retail quality audio screened cable any more but the major distributors all stock it by the reel. You might not save all that much with DIY on that price, either.
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Old 7th January 2011, 01:18 AM   #9
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Originally Posted by Ian Finch View Post
Dunno about benefits of RF cables at audio or their sevicability when home assembled but Radio Shack currently have an on-line special of 16ft "Monster Cable" standard duty subwoofwer interconnects on sale in the US. $17.99 ea.
That might be very interesting indeed, depending on what Monster call "standard duty subwoofer interconnect". AFAIK, at least their higher-grade interconnects are quite awesome.

I would rate the quality of interconnect as follows:

cheap OEM cable < slightly higher-grade application-specific interconnect cable < lower-grade microphone cable < RF cable < higher-grade microphone cable < top-of-the-line application-specific interconnect cable

Monster counts as application-specific interconnect; good microphone cable would, e.g., be (pro-grade) stuff like Cordial.

If this particular flavor of Monster is not that good, it might still make sense to go with RF cable.

I'm not going to go through the pain of explaining the outer shield connection; I will only state that I agree with Geosto's statement.

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Old 7th January 2011, 01:33 AM   #10
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Quote:
Originally Posted by Ian Finch View Post
Dunno about benefits of RF cables at audio or their sevicability when home assembled but Radio Shack currently have an on-line special of 16ft "Monster Cable" standard duty subwoofwer interconnects on sale in the US. $17.99 ea.
I found some on amazon for $4.29 plus shipping. That comes to $13.89 for the pair, total.

But I still want to know how to make my own, and I want to understand the mysteries of ground, neutral, and shield. I think I am starting to catch on regarding that last bit.

I did find coax with a single stranded center wire, but the smallest roll is 500 ft. But as Geosto points out, the mic cable with two stranded center wires is at least as good - if I can reconcile myself with 1/4" cables.

Question: Is 50 Ohm cable adequate, or must it be 75? I found some 50 Ohm with a single stranded center wire that is not quite so thick. RG58, I think it was called. But again, only in humongous rolls.
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