DIY subwoofer cable - AVS Forum
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post #1 of 24 Old 04-01-2007, 10:24 PM - Thread Starter
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Hello All,

A little background on what I'm trying to do before I ask my question. After replacing carpeting with bamboo flooring, I decided to run surround speaker and subwoofer cables under the floor and terminate it on top of the new flooring using binding post mounting plates to keep things clean. Routing and terminating the speaker cables was easy, but I ran into a snag with the subwoofer cable. I decided to recycle an AR subwoofer cable I had by cutting off the old connectors ,routing this cable under the floor and terminating it with WBT RCA sockets. The AR cable is a dual conductor cable with foil shielding. Sounds easy enough, or so I thought. Upon stripping the insulation and exposing the conductors, I noticed another bare wire wound around the ground conductor. Should I solder this wire to ground on the socket along with the ground conductor?

Dissecting a pair of AR stereo RCA interconnects left me more confused. Only one plug out of four had this wire soldered to ground.

Thanks in advance for your help.
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post #2 of 24 Old 04-02-2007, 05:36 PM
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Quote:
Originally Posted by fourg63 View Post

Upon stripping the insulation and exposing the conductors, I noticed another bare wire wound around the ground conductor. Should I solder this wire to ground on the socket along with the ground conductor?

It's a 'drain' wire. You can solder it to ground or leave it. In some cases manufacturers will solder it to ground on one end and leave the other end unconnected. It is supposed to reduce noise YMMV.

Your best bet (and cheapest) for a DIY Sub cable is RG6 (Cable TV wire) with a solid copper core (not copper plated), but as you've already ripped the AR cable apart it will be just fine.

"One of the problems of taking things apart and seeing how they work--supposing you're trying to find out how a cat works--you take that cat apart to see how it works, what you've got in your hands is a non-working cat."
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post #3 of 24 Old 04-02-2007, 09:58 PM - Thread Starter
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Thanks. I appreciate your response. For the time being, I will use this cable and probably upgrade in the near future.
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post #4 of 24 Old 10-06-2007, 05:33 PM
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I have upgraded my system and am putting my 5 year old Definitive 10" sub in zone 2, about 75' from my receiver. I decided to run 12 gauge speaker wire to the sub since I had a 500' spool instead of purchasing a 75' sub cable.

I cut an old 10' Monster sub cable (since the people at 2 Radio Shacks were no help) and found:

2 primary conductors, roughly 20 gauge, one brown, one black.

One uninsulated silver wire wound with the two wires mentioned above.

All three of these wires were wrapped in foil.

Then, there's a braided copper wire mesh wrapped around the foil.

Lastly, there's 2 layers of rubber insulator.

I cut everything away and connected my 12 gauge speaker wire to the brown and black wires first mentioned on both ends (black to black and brown to red).

I DID NOT connect the uninsulated wire or the copper wire braid to anything else (although I was NOT careful to avoid a short between those two).

Finally, it's together! I've got signal, but when I shut down the receiver, the subwoofer continues to hum. I thought it was the air conditioner at first, but when I walked by the sub, I realize it was the sub. So, I am figuring the other 2 conductors do a job.

HELP! How should I connect the sub at both ends (receiver end and sub end) from 4 conductors in the old sub line to 2 conductor speaker wire to eliminate this hum? I have already run the speaker wire for zone 2 behind trim, so I am pretty much locked in...

Thanks for the help.
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post #5 of 24 Old 10-07-2007, 03:18 AM
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Unfortunately, you can not use speaker wire for a line-level signal, like that used with a powered subwoofer. You need to use shielded cable to prevent hum.

Here's an example:

http://www.monoprice.com/products/pr...seq=1&format=2


You could also make you own subwoofer cable with 75ohm RG6 and RCA connectors.
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post #6 of 24 Old 10-07-2007, 07:57 PM
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Quote:
Originally Posted by Rupert View Post

Unfortunately, you can not use speaker wire for a line-level signal, like that used with a powered subwoofer. You need to use shielded cable to prevent hum.

Here's an example:

http://www.monoprice.com/products/pr...seq=1&format=2


You could also make you own subwoofer cable with 75ohm RG6 and RCA connectors.

The same misinformation time after time. The only thing you need is an audio patch cord. No foil, no braid, no ground wire, no drain wire and certainly no 75 Ohm RF/video cable will render you any benifit or hum protection for your subwoofer.
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post #7 of 24 Old 10-07-2007, 10:19 PM
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Quote:
Originally Posted by NightHawk View Post

The same misinformation time after time. The only thing you need is an audio patch cord. No foil, no braid, no ground wire, no drain wire and certainly no 75 Ohm RF/video cable will render you any benifit or hum protection for your subwoofer.

Which part is misinformation? The part where I linked to a shielded audio patch cord (as you suggest) or the part where I correctly comment that you shouldn't use speaker wire for line-level connections.
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post #8 of 24 Old 10-08-2007, 06:29 AM
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A shield on a subwoofer cable is useless. While RG6 or any coaxial cable would work as a sub cable it provides nothing an unshielded two-wire audio patch cord can't. Any hum in a subwoofer is due to a ground loop and not because of any inadaquate shielding.
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post #9 of 24 Old 10-08-2007, 07:05 AM
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Quote:
Originally Posted by NightHawk View Post

A shield on a subwoofer cable is useless. While RG6 or any coaxial cable would work as a sub cable it provides nothing an unshielded two-wire audio patch cord can't. Any hum in a subwoofer is due to a ground loop and not because of any inadaquate shielding.

It's not useless. A shielded cable will provide protection from RFI "radio signals causing rf demodulation interference (I can hear the radio on my stereo)" and EMI "ever time that light is switched on I hear a pop". A 20 foot unshielded cable make a very good am antenna.

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post #10 of 24 Old 10-08-2007, 07:47 AM
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Quote:
Originally Posted by NightHawk View Post

A shield on a subwoofer cable is useless. While RG6 or any coaxial cable would work as a sub cable it provides nothing an unshielded two-wire audio patch cord can't. Any hum in a subwoofer is due to a ground loop and not because of any inadaquate shielding.

I think you may be confusing terms. Your typical Radio Shack RCA cable is made from "shielded cable".:

Radio Shack 6-Ft. Shielded Cable, RCA Plug to RCA Plug


Here's a definition from Wikipedia:

A shielded cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer.



The poster (cctmbb) took a piece of SPEAKER WIRE and put some RCA connectors on the ends. Bad idea....because the cable is not shielded, you will likely pick up a hum.
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post #11 of 24 Old 10-08-2007, 09:09 AM
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Quote:
Originally Posted by Rupert View Post

I think you may be confusing terms. Your typical Radio Shack RCA cable is made from "shielded cable".:

Radio Shack 6-Ft. Shielded Cable, RCA Plug to RCA Plug


Here's a definition from Wikipedia:

A shielded cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer.



The poster (cctmbb) took a piece of SPEAKER WIRE and put some RCA connectors on the ends. Bad idea....because the cable is not shielded, you will likely pick up a hum.

At 60hz that braid and foil provides almost no protection from a 60hz magnetic field. luckly it's very easy to route the cable away from most sources because the field is very small around most household appliances (not motors or unshielded transformers).

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post #12 of 24 Old 10-08-2007, 12:18 PM
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Quote:
Originally Posted by mntmst View Post

It's not useless. A shielded cable will provide protection from RFI "radio signals causing rf demodulation interference (I can hear the radio on my stereo)" and EMI "ever time that light is switched on I hear a pop". A 20 foot unshielded cable make a very good am antenna.


Unlikely since there is a low-pass filter at the subwoofer input (crossover). Even at line level there's a faily low impedance at both ends and interference from an AM radio station would be difficult unless it was next door.
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post #13 of 24 Old 10-08-2007, 12:20 PM
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Quote:
Originally Posted by mntmst View Post

At 60hz that braid and foil provides almost no protection from a 60hz magnetic field. luckly it's very easy to route the cable away from most sources because the field is very small around most household appliances (not motors or unshielded transformers).

My original point. You can't shield audio frequencies.
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post #14 of 24 Old 10-08-2007, 12:26 PM
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Quote:
Originally Posted by Rupert View Post

The poster (cctmbb) took a piece of SPEAKER WIRE and put some RCA connectors on the ends. Bad idea....because the cable is not shielded, you will likely pick up a hum.

I disagree. It would work fine in most circumstances.
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post #15 of 24 Old 10-08-2007, 12:52 PM
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Quote:
Originally Posted by NightHawk View Post

Unlikely since there is a low-pass filter at the subwoofer input (crossover).


If it's a active filter high levels of RF can drive the amp into nonlinear amplification that can demodulate the rf signal. Even if it has a passive low-pass audio filter at rf frequencies it will look like a straight wire into the amp unless somebody also added a rf filter circuit on the input.

Bottom line: Always use shielded cables.

http://pdfserv.maxim-ic.com/en/an/AN3660.pdf

http://digitalcontentproducer.com/ma...l_dealing_rfi/

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post #16 of 24 Old 10-08-2007, 02:58 PM
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Quote:
Originally Posted by mntmst View Post

If it's a active filter high levels of RF can drive the amp into nonlinear amplification that can demodulate the rf signal. Even if it has a passive low-pass audio filter at rf frequencies it will look like a straight wire into the amp unless somebody also added a rf filter circuit on the input.


Well again I disagree. It would take a very special set of unlikely circumstances for a subwoofer to be bothered by an AM radio broadcast. A low pass filter, passive or active is going to present a low impedance to ground to those frequencies. The same is true of the driver circuit at the other end making the unshielded subwoofer cable a poor receiving system. The transmitter would need to be in your neighbor's yard for starters. Not impossible but not a problem for 99.9% of systems.
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post #17 of 24 Old 10-08-2007, 05:48 PM
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Quote:
Originally Posted by NightHawk View Post

Well again I disagree. It would take a very special set of unlikely circumstances for a subwoofer to be bothered by an AM radio broadcast. A low pass filter, passive or active is going to present a low impedance to ground to those frequencies. The same is true of the driver circuit at the other end making the unshielded subwoofer cable a poor receiving system. The transmitter would need to be in your neighbor's yard for starters. Not impossible but not a problem for 99.9% of systems.

Ok, sure.

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post #18 of 24 Old 10-09-2007, 01:59 AM
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You can roll your eyes if you like but if you can develop the minimum voltage necessary to get a response from a subwoofer amplifier, which is about 100 -> 200 millivolts inadvertantely from a 20 piece of wire terminated in a line impedance (probably less) you need to move.
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post #19 of 24 Old 10-09-2007, 09:27 AM
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It one time I thought the same way as NightHawk, how could signals in the high MHz range have any effect on audio equipment? But new communications equipment makes it a whole new ball game. Some new stuff sends out packets of information at 217 packets per second. If this signal get into your equipment it can cause 217 DC offset per second! Now with new EMI/RFI sources everyday I would shield everything.

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post #20 of 24 Old 10-21-2007, 07:31 PM
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Thanks for the informed discussion. Am I correct in interpreting that there is nothing special about Monster's special subwoofer cables (other than the price)? ... or at least nothing a decent patch cable would not deliver to a non-audiophile "enthusiast"?
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post #21 of 24 Old 01-03-2008, 10:47 PM
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WOW! Off the tracks and through the woods, off the cliff and plunge into the lake!

Here's what I did to fix the problem... I ran the Left and Right channel speaker wire to the sub binding post inputs and added 12 additional feet of speaker wire from the sub high pass output binding posts to the speakers. My inwalls are good down to 80 Hz, so I put the sub cross-over frequency in that neighborhood too.

Thanks for the input all.
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post #22 of 24 Old 06-17-2008, 11:15 AM
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Unfortunately I pretty much do have an AM transmitter in my neighbors yard. There is a station less than a mile from my home and it annoys me to no end while trying to watch a movie with the subwoofer on. The only fortunate thing is that the station is so low budget that they don't broadcast past 10:00pm most nights. I've tried using snap on ferrite core filters on both ends of the subwoofer cable but it hasn't helped. I have heard that the interference could be coming through the water pipes. I'm a total newb to A/V so I have no idea, but apparently from what I heard most homes are grounded through the water pipes, and that somehow plays into the issue. I'm using I believe a 15' cable, I'll have to check, it's run to my A/V equipment in the unfinished part of my basement, and I'll have to check but I believe I may have it close to or crossing over a water line. The cable should be shielded but I would have to cut it open to verify. Any suggestions on how I can filter out these AM signals?

Thanks!
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post #23 of 24 Old 06-18-2008, 05:35 AM
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It may be the RFI is directly entering the subwoofer amplifier through the cabinet. Turn on the sub with no audio cable connected and see if you can still detect any interference. If so there is little that can be done except to try a different subwoofer. Also does the crossover setting have any effect at all on the interference?

It may also be you're using the wrong type of ferrite. Not all ferrites work well at the low frequencies of the AM broadcast band.

It may also be the interference is being detected unintentionally by some circuit inside your receiver and passed as audio to the sub, in which case there is little you can do short of building a screen room for your equipment.
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post #24 of 24 Old 06-24-2008, 10:40 AM
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Thanks for the suggestions, I'll give them a shot and see what turns up.
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