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Apparent issue with subwoofer cable

post #1 of 22
5/16/12 at 4:33am
Thread Starter 
When I was wiring up my room before drywall, I ran lengths of 75ohm RG6 cable - thru ceiling and wall cavities - to various places in the room for subwoofer drops.

A few weeks ago, I got my system set up and used one of these drops to add my subwoofer. The placement is completely across the room (22ft) from where my AVR and components reside in a cabinet. I added RCA plugs (RG6 connects via 2 screws) to the ends of the RG6. Everything worked fine. Until now.

I had a couple of instances where I would turn on the woofer and it would hum. I would simply toggle the main on/off switch and it would fix the problem. Then, a few days ago, the hum came back - and wouldn't go away.

On advice from a Velodyne rep on these forums, I did some troubleshooting last night, and I believe I've determined that the issue has to do with this RG6 cable.

When I connect the sub directly to the AVR (after dragging it across the room) with a boughten cable, it works fine. When I turn on the woofer when attached via the RG6 drop, it hums - even when the AVR is not on, which I think means it isn't receiving any signal.

This fact leads me to believe that somewhere in the ceiling, my RG6 cable is in too-close proximity to some Romex carrying power. Isn't this the only reason that there would be a hum when there isn't any signal coming from the AVR? Or have I somehow not correctly installed the RCA ends?

Short of pulling the cable out of the wall/ceiling altogether, I don't know how to troubleshoot this issue. And, even if I pull it out and run the same or a different run of RG6 cable back through - if the path is still the same, then theoretically I would still have the same issue?

Would running a boughten subwoofer cable make any difference here?

Really grasping at straws here to try to fix this issue.
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post #2 of 22
5/16/12 at 5:35am
Quote:
Originally Posted by AC2011 View Post

When I was wiring up my room before drywall, I ran lengths of 75ohm RG6 cable - thru ceiling and wall cavities - to various places in the room for subwoofer drops.

A few weeks ago, I got my system set up and used one of these drops to add my subwoofer. The placement is completely across the room (22ft) from where my AVR and components reside in a cabinet. I added RCA plugs (RG6 connects via 2 screws) to the ends of the RG6. Everything worked fine. Until now.

I had a couple of instances where I would turn on the woofer and it would hum. I would simply toggle the main on/off switch and it would fix the problem. Then, a few days ago, the hum came back - and wouldn't go away.

On advice from a Velodyne rep on these forums, I did some troubleshooting last night, and I believe I've determined that the issue has to do with this RG6 cable.

When I connect the sub directly to the AVR (after dragging it across the room) with a boughten cable, it works fine. When I turn on the woofer when attached via the RG6 drop, it hums - even when the AVR is not on, which I think means it isn't receiving any signal.

This fact leads me to believe that somewhere in the ceiling, my RG6 cable is in too-close proximity to some Romex carrying power. Isn't this the only reason that there would be a hum when there isn't any signal coming from the AVR? Or have I somehow not correctly installed the RCA ends?

Short of pulling the cable out of the wall/ceiling altogether, I don't know how to troubleshoot this issue. And, even if I pull it out and run the same or a different run of RG6 cable back through - if the path is still the same, then theoretically I would still have the same issue?

Would running a boughten subwoofer cable make any difference here?

Really grasping at straws here to try to fix this issue.

The most likely source of a hum that won't go away is a ground loop.

Ground loops come from a system that has too many grounds which may seem counter-intuitive. A common source of ground loops in home AV system is equipment that is connected to cable systems - as they are grounded per electrical code.

It is possible that a cable in a wall is inadvertently grounded by a nail or screw that hit the shield of the cable but did not break the circuit path.

One way to break a ground loop due to a signal cable is to install a ground isolator. These can be purchased online or from Radio Shack stores for under $20.
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post #3 of 22
5/16/12 at 5:40am
Quote:


boughten cable

Sounds painful.


Quote:


Isn't this the only reason that there would be a hum when there isn't any signal coming from the AVR? Or have I somehow not correctly installed the RCA ends?

No. Most likely a ground loop. What kind of connectors did you use?
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post #4 of 22
5/16/12 at 6:06am
Thread Starter 
Quote:
Originally Posted by SAM64 View Post

Sounds painful.




No. Most likely a ground loop. What kind of connectors did you use?




These are the connectors I used. Exposed the inner copper, inserted and screwed down. Rear screw on outer jacket.



So, with a ground loop issue, is it possible that the ground issue is actually from my satelite receiver? It has 2 RG6 inputs (with F connectors) on the back, it is then connected to my AVR via HDMI. Possibly these connectors are not tight and I may have shifted the cables when I was doing cable management in the rear of the cabinet, which caused the loop?
LL
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post #5 of 22
5/16/12 at 6:26am
Thread Starter 
On second thought, if there is no hum when I connect the sub to the AVR with a different subwoofer cable, that MUST mean that any potential ground loop problem must be in this piece of RG6 cable.

Should I have exposed the silver shielding to the screw at the back of the connector? Currently, that screw is screwed down on the outer jacket. Previously, I think I had one end where the shielding was exposed to the screw, and one end that wasn't. In my re-jigging last night, I re-did the ends and I didn't expose the shielding on either end. Maybe the issue is I should have exposed the shielding on the one end that wasn't - now 2
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post #6 of 22
5/16/12 at 7:09am
Quote:
Originally Posted by AC2011 View Post

On second thought, if there is no hum when I connect the sub to the AVR with a different subwoofer cable, that MUST mean that any potential ground loop problem must be in this piece of RG6 cable.

Should I have exposed the silver shielding to the screw at the back of the connector? Currently, that screw is screwed down on the outer jacket. Previously, I think I had one end where the shielding was exposed to the screw, and one end that wasn't. In my re-jigging last night, I re-did the ends and I didn't expose the shielding on either end. Maybe the issue is I should have exposed the shielding on the one end that wasn't - now 2

If I understand what you are saying, there is a possibility that the ground portion of the connector does not make a metal-to-metal connection with the cable's shield.

It should. That wouldn't be a ground loop, it would the absence of grounding something that needs to be grounded.
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post #7 of 22
5/16/12 at 8:25am
Just to clarify arnyk's statement:
The outer braid should be folded back over the outer insulation so the set screw provides the proper ground on the outer shell of the RCA connector. This should be done at both ends.
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post #8 of 22
5/16/12 at 9:11am
Quote:


Should I have exposed the silver shielding to the screw at the back of the connector? Currently, that screw is screwed down on the outer jacket.

yes, the insulation cannot carry any current.
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post #9 of 22
5/16/12 at 11:06am
Yeah... good info.
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post #10 of 22
5/16/12 at 7:12pm
Thread Starter 
So, still no luck finding the problem.

I disconnected both coax inputs on my satelite receiver - this didn't stop the hum. I even unplugged the box - still humming.

Attempted various other disconnects, with no luck. The only thing that stops the hum is removing the RG6 cable from the receiver. Funny thing, though - I removed this cable while the woofer was humming; it continued to hum. But then I turned the woofer off from its remote (or rather put it in standby, as there is a main on-off toggle switch on the woofer itself) and turned it back on and the woofer was gone. So it seems like it takes the energy from whatever is causing the hum a little while to dissipate even after I completely disconnect it from the AVR.

So, I'm at a loss/impasse - I guess I need to try an isolator between the AVR and the woofer?
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post #11 of 22
5/17/12 at 8:06am
Quote:


In my re-jigging last night, I re-did the ends and I didn't expose the shielding on either end. Maybe the issue is I should have exposed the shielding on the one end that wasn't - now 2

Is this still the case? Both conductors at both ends of the cable must be connected.

Quote:


So it seems like it takes the energy from whatever is causing the hum a little while to dissipate even after I completely disconnect it from the AVR.

No, nothing is storing AC.
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post #12 of 22
5/17/12 at 8:59am
try another subwoofer
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post #13 of 22
5/17/12 at 10:59am
Thread Starter 
Quote:
Originally Posted by chikoo View Post

try another subwoofer


^^Thanks, but this is not really an option right now.


As noted, I can plug another cable (one that I bought from a store) into the sub, and into the AVR (not using this problematic RG6 connection) and all appears to be well, so it definitely has to do with this particular connection.


This piece of RG6 that I am using is an older piece that I had - it is 75ohm RG6/U - and the "inside" of it is a little different.

I bought some new RG6 for the other runs, and when you take the outer black casing off, there is the usual silver sheild that looks like many threads. You can bend them back over the black casing.

On this particular piece, when you take the outer casing off, the inner white casing is covered with a silver foil. And if I am not perfectly delicate with it, this foil has a tendency to come off with the black casing.

Should I take away enough of the outer casing so that the rear screw on these connectors screws down on this foil sheild? Or should I be trying to peel this back over the black outer casing and have the screw make contact with the inner white casing?


In the end, this location might not be optimal for flatest room response anyway, so I might end up using a different location. But, I would like to fix the issue with this particular line as it might be the optimal location, either now, or perhaps later if/when I add more subs.
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post #14 of 22
5/17/12 at 11:21am
I just want to reiterate what was said above. If the ends are put on incorrectly it will cause a hum.

The braided shielding must be folded back and the end crimped/compressed/screwed over that.


Bad



Good



I had the same problem with my in wall RG6 run.
http://www.avsforum.com/avs-vb/showthread.php?t=1396105
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post #15 of 22
5/17/12 at 11:31am
Quote:
Originally Posted by AC2011 View Post

... when you take the outer casing off, the inner white casing is covered with a silver foil. And if I am not perfectly delicate with it, this foil has a tendency to come off with the black casing.

Don't worry about the foil that covers the "white" dielectric. Peel it off.

Are you using a proper coax stripper or a kitchen knife?

I really think that if you properly terminate the coax into your connectors, the problem will be solved.
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post #16 of 22
5/17/12 at 11:41am
Thread Starter 
Quote:
Originally Posted by Ratman View Post

Don't worry about the foil that covers the "white" dielectric. Peel it off.

Are you using a proper coax stripper or a kitchen knife?

I really think that if you properly terminate the coax into your connectors, the problem will be solved.



I am using a proper coax stripper tool.

As I noted, this particular length of coax doesn't seem to have any of the "shield" under the outer black casing - so I guess this is likely the issue. But, I have no idea how this is possible. This was previously a pre-terminated cable (with F connect ends) purchased from Walmart or somewhere like that a few years ago. I cut the pre-terminated ends off because I wanted to use these RCA connectors directly on the coax, so that I could just pull the end out of the wall and plug it into the woofer without additional transition connectors. Perhaps I just F'd up the cable.

I will give it another attempt tonight, but, assuming this is the problem (ie. no shield = no connection) then I guess my only other option is to pull out this piece of cable and replace with a "proper" piece?

How in the heck did it work for a few weeks, then go off the rails???
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post #17 of 22
5/17/12 at 11:51am
Quote:


As I noted, this particular length of coax doesn't seem to have any of the "shield" under the outer black casing

the foil, which someone else told you to peel off and discard, is the shield. This is an unbalanced circuit, it must have two conductors to complete the circuit, so far, you've only connected one.
The white stuff and the black stuff are insulators, they don't conduct anything.
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post #18 of 22
5/17/12 at 12:14pm
Thread Starter 
Quote:
Originally Posted by SAM64 View Post

the foil, which someone else told you to peel off and discard, is the shield. This is an unbalanced circuit, it must have two conductors to complete the circuit, so far, you've only connected one.
The white stuff and the black stuff are insulators, they don't conduct anything.



Okay, it appears I need to inspect this cable a little more closely. I have cut each end several times in the course of all this, and usually only ever see the foil after taking off the black jacket. Once in a while I will see 1 or 2 strands of braid, but nothing like the newer RG6 I purchased at Home Depot which clearly has both foil and braid.

I must be missing something.
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post #19 of 22
5/17/12 at 1:52pm
Can you try out a new cable of similar length, running along the floor?
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post #20 of 22
5/17/12 at 2:46pm
Quote:
Originally Posted by AC2011 View Post

Okay, it appears I need to inspect this cable a little more closely. I have cut each end several times in the course of all this, and usually only ever see the foil after taking off the black jacket. Once in a while I will see 1 or 2 strands of braid, but nothing like the newer RG6 I purchased at Home Depot which clearly has both foil and braid.

I must be missing something.

Have you adjusted your stripping tool or are you using it properly?

For standard RG6, there should be:
1) outer insulation (black)
2) braid (not one or two strands)
3) foil (covering white dielectric)
4) copper center conductor
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post #21 of 22
5/17/12 at 4:38pm
Thread Starter 
Okay, I think I got it this time. Turns out the foil layer around the center insulation had/has small "ribs" in it, which are the braid making up the 2nd conductor.

This time, I ever so carefully cut the outer insulation layer and gently peeled it off - and I exposed this foil layer well back. I then added my connector, screwed the screws down and, voila - no hum.

I guess when I put the original connectors on, there must have been juuuuuuuuust enough connection for it not to hum all the time, but those couple of times it did hum were a sign. Then when I was moving wires around in the back of my receiver, I must have shifted it just enough to disable this good connection. Then, I started cutting and shifting it multiple ways, obviously with no idea of the concept of the ground.


Anyway, it seems to work now.


Thank you very much, everyone who contributed to this success. Could not have done it without you - my "speaker peeps", as my wife likes to refer to these forums.
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post #22 of 22
5/17/12 at 4:51pm
Awesome. The devil is in the details. Tell your wife we peeps appreciate her acknowledgement of our existence so endearingly
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