Subwoofer issue - will new cable help?

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andrew027

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#181166 5-Oct-2015 10:09

In my last house I had my subwoofer at the front of the room, plugged directly into the receiver with a one metre RCA cable. It just worked. In my new house, the lounge is set up as a proper home theatre room. There's a power point and sockets on the ceiling for a projector, and connections for rear and side speakers coming out of the walls at ceiling height. There is also a subwoofer connection running about seven metres to the back of the room, but this is not quite what I would have expected.

I thought the best way to do this would be to have female RCA sockets on the faceplates on the front and back wall. I would use then standard RCA cables with male plugs on each end to connect the receiver to the front faceplate, and the rear faceplate to the sub. But there are male RCA plugs hanging out of the wall on about four inches of cable - not long enough to reach the receiver or the sub so I bought RCA "extension cables" with a plug on one end and a socket on the other, to connect the plugs to the receiver and sub. Also I'm used to RCA cables being shielded (a core with braided copper or aluminium foil shield wrapped around it, plus insulating layers) but the wiring in the walls is standard side-by-side/figure 8/tru-rip speaker cable.

When I turn the sub on, there is a very noticeable low frequency hum that I didn't have in the old house. I've turned the sub volume down a bit from where it was so it's not too bad when there's a bit going on in the audio soundtrack, but before/after a film or during quiet passages it's quite annoying. I could move the sub back to the font of the room like it was in the old house, but I'd prefer to have it at the back if possible - it would mean rearranging a couple of other things as well. And I've tried giving the cable a bit of a tug and it's not moving, so I can't use it to pull a shielded cable through the walls - if I had a long enough shielded cable I'd have to run it along the floor around the edge of the room.

What is most likely causing the hum? Could it be an electrical problem like a short or grounding issue? Is it likely the type of cable, or maybe the extra connections added with the "extension" cables? Or is it just the length? What is most likely to fix it? Or will I just have to move the sub?

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Awrrr
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#1400046 5-Oct-2015 10:19

Others here know a lot more than me about this but I can tell you my experience. Shielding is important for high-impedance inputs, which this is not. It shoudn't make a difference for this kind of thing. One thing I found that did make a real difference for me was grounding. Don't rely on any cable connections between the sub and your main amp. Find some wire (anything will do) and make a direct connection from the chassis of the amp to the chassis of the sub-woofer amp. This can make a real difference and it definitely did for me.

My original sub-woofer amp died and I had to replace it. After solving the ground issue I noticed I still had hum. I checked carefully and it turned out to be mechanical vibration from the transformer. I had to shock-mount the amplifier with rubber to solve it. I doubt this is the case for you, but it is worth checking that your hum is really electrical interference.

Plesse igmore amd axxept applogies in adbance fir anu typos

Sideface

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#1400049 5-Oct-2015 10:22

What is most likely causing the hum? Could it be an electrical problem like a short or grounding issue? Is it likely the type of cable, or maybe the extra connections added with the "extension" cables? Or is it just the length? What is most likely to fix it? Or will I just have to move the sub?

All of the above.
Move the sub.
Long cables are A Bad Thing.
Extension cables with multiple connections are even worse.

Sideface

richms

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#1400063 5-Oct-2015 10:39

It's because it is unshielded. and subs with rca plugs are un balanced so get more noise than XLR balanced ones on crap cables.

Richard rich.ms

eracode

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#1400070 5-Oct-2015 10:42

Rikkitic: Others here know a lot more than me about this but I can tell you my experience. Shielding is important for high-impedance inputs, which this is not. It shoudn't make a difference for this kind of thing. One thing I found that did make a real difference for me was grounding. Don't rely on any cable connections between the sub and your main amp. Find some wire (anything will do) and make a direct connection from the chassis of the amp to the chassis of the sub-woofer amp. This can make a real difference and it definitely did for me.

My original sub-woofer amp died and I had to replace it. After solving the ground issue I noticed I still had hum. I checked carefully and it turned out to be mechanical vibration from the transformer. I had to shock-mount the amplifier with rubber to solve it. I doubt this is the case for you, but it is worth checking that your hum is really electrical interference.

That rubber you used to mount the transformer sounds like handy stuff!

Sometimes I just sit and think. Other times I just sit.

jonathan18

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#1400081 5-Oct-2015 10:46

andrew027: ... I thought the best way to do this would be to have female RCA sockets on the faceplates on the front and back wall. I would use then standard RCA cables with male plugs on each end to connect the receiver to the front faceplate, and the rear faceplate to the sub. But there are male RCA plugs hanging out of the wall on about four inches of cable - not long enough to reach the receiver or the sub so I bought RCA "extension cables" with a plug on one end and a socket on the other, to connect the plugs to the receiver and sub. Also I'm used to RCA cables being shielded (a core with braided copper or aluminium foil shield wrapped around it, plus insulating layers) but the wiring in the walls is standard side-by-side/figure 8/tru-rip speaker cable...

When I put cable in for our HT I ran the subwoofer cable (a 75 ohm one I bought from the US; cheaper than here) through the wall cavity, and had each end coming out through a PDL light switch plate - I really don't see the point of creating additional and unncessary breaks in the signal by putting plug boxes in at each end, which then require additional cables to connect to the sub and amp. So, this in itself, shouldn't be an issue - but damn stupid they left only 4 inches, forcing the use of an extension cable - they should have been left long enough to plug directly into the sub (I ensured enough cabling as available to position the sub within a metre either side of where the cable comes out of the wall).

But are you saying they used standard speaker cable for the sub cable? I've never heard this be advisable; I've always been told to use a 75 ohm shielded cable, especially for longer runs. And, yes, it's preferable to keep the length down, but sometimes practicality makes this not possible (though weird they positioned the sub at the opposite end of the room?).

Can you try running such a shielded 75 ohm cable to see if that still produces the hum?

Jaxson

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#1400087 5-Oct-2015 10:54

as an aside, I'm quite against the faceplace phenomenon in this situation.

Face plates look pretty, but unless you're moving speakers often, I don't think they're worth the expense, especially when often they remain unseen.
You're introducing potential failure points all over the place and are now mixing cables as well.

I'd suggest replacing with a long enough piece of your own sub woofer cable, be that purchased new specific for the task, or created from shielded RG59 with your own RCA plug connectors applied.
At least in the interim confirm that you do, or don't get the hum when running a cable directly to the subwoofer. This gives you an opportunity to start nailing down where the issue is coming from.

Rikkitic

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#1400088 5-Oct-2015 10:55

eracode:
Rikkitic: Others here know a lot more than me about this but I can tell you my experience. Shielding is important for high-impedance inputs, which this is not. It shoudn't make a difference for this kind of thing. One thing I found that did make a real difference for me was grounding. Don't rely on any cable connections between the sub and your main amp. Find some wire (anything will do) and make a direct connection from the chassis of the amp to the chassis of the sub-woofer amp. This can make a real difference and it definitely did for me.

My original sub-woofer amp died and I had to replace it. After solving the ground issue I noticed I still had hum. I checked carefully and it turned out to be mechanical vibration from the transformer. I had to shock-mount the amplifier with rubber to solve it. I doubt this is the case for you, but it is worth checking that your hum is really electrical interference.

That rubber you used to mount the transformer sounds like handy stuff!

I posted about this elsewhere. The rubber came from car door frame weather moulding, which I got for free from a dismantlers. The rubber part is in the form of long tubing. I have used it for years in a variety of projects and it is the most useful repair and construction material I have ever found.

Plesse igmore amd axxept applogies in adbance fir anu typos

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richms

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#1400089 5-Oct-2015 10:56

If the sub has an earth on it could just be a ground loop too. Try an extension from the same power the reciever is on.

Richard rich.ms

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#1400139 5-Oct-2015 11:18

You need a shielded cable to start with. Is your rca cable shielded with 75 ohm impedance

Yogi02

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#1400191 5-Oct-2015 12:05

Last time I had something like that after moving house ended up being a ground loop problem with the receiver causing a low frequency hum from the sub.

I fixed it by running a single core cable from the Earth terminal on the back of the sub to a screw on the back of my TV - (The TV has a 3 prong power plug and therefore the metal surround is earthed)

No problems since and I would try this first if your receiver has an ground "GND" plug on back as its much easier than playing with connections

andrew027

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#1400219 5-Oct-2015 12:30

Hi all - thanks to everyone for the answers so far. To provide some more info:

Rikkitic: My original sub-woofer amp died and I had to replace it. After solving the ground issue I noticed I still had hum. I checked carefully and it turned out to be mechanical vibration from the transformer. I had to shock-mount the amplifier with rubber to solve it. I doubt this is the case for you, but it is worth checking that your hum is really electrical interference.

I'm pretty sure it is electrical. It certainly changes volume when I adjust the volume level using the control on the sub itself. Tonight I'll try moving the sub to the front of the room and plugging it directly into the receiver - I expect it would still hum if it's mechanical but not hum if there's some electrical issue with the extended wiring.

jonathan18: damn stupid they left only 4 inches, forcing the use of an extension cable ... But are you saying they used standard speaker cable for the sub cable?

Yes, those little bits of dangling cable are ridiculous and annoying, and yes, it's a standard two-wire speaker cable joining it all together, except the two "extension cables" I bought to connect the plugs to the receiver and sub, which are shielded.

Jaxson: as an aside, I'm quite against the faceplace phenomenon in this situation.

Face plates look pretty, but unless you're moving speakers often, I don't think they're worth the expense, especially when often they remain unseen.
You're introducing potential failure points all over the place and are now mixing cables as well.

True. And I realised after I had posted the original question that even with faceplates there would still be the same number of connections are there are now with the "extension cables" - just different ones.

joker97: You need a shielded cable to start with. Is your rca cable shielded with 75 ohm impedance

To be honest, I'm not sure about the "extension cables" I bought - I just got something at a retail store and no longer have the packaging. It's reasonable heavy duty. Of course, 80% of the length is not shielded at all...

richms: If the sub has an earth on it could just be a ground loop too. Try an extension from the same power the reciever is on.

Yogi02: Last time I had something like that after moving house ended up being a ground loop problem with the receiver causing a low frequency hum from the sub.

I fixed it by running a single core cable from the Earth terminal on the back of the sub to a screw on the back of my TV - (The TV has a 3 prong power plug and therefore the metal surround is earthed)

No problems since and I would try this first if your receiver has an ground "GND" plug on back as its much easier than playing with connections.

Hmmm, that's something I hadn't thought of. I'm pretty sure there is an earth terminal on the back of the receiver receiver but I'm not sure about the sub. I might check this option first... and then look at a single long shielded cable with no connections...

Yogi02

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#1400223 5-Oct-2015 12:39

It bugged me for weeks, nothing to do with the sub in the end. The interference was turning the sub on (auto-on amp on the sub) and humming occasionally.
Lots of googling and an accidental solution as it stopped when I touched the black of the TV (cramped behind there) with one hand while holding the new ground cable with the other.

Good Luck

eracode

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#1400281 5-Oct-2015 14:07

Rikkitic:
eracode:
Rikkitic: Others here know a lot more than me about this but I can tell you my experience. Shielding is important for high-impedance inputs, which this is not. It shoudn't make a difference for this kind of thing. One thing I found that did make a real difference for me was grounding. Don't rely on any cable connections between the sub and your main amp. Find some wire (anything will do) and make a direct connection from the chassis of the amp to the chassis of the sub-woofer amp. This can make a real difference and it definitely did for me.

My original sub-woofer amp died and I had to replace it. After solving the ground issue I noticed I still had hum. I checked carefully and it turned out to be mechanical vibration from the transformer. I had to shock-mount the amplifier with rubber to solve it. I doubt this is the case for you, but it is worth checking that your hum is really electrical interference.

That rubber you used to mount the transformer sounds like handy stuff!

I posted about this elsewhere. The rubber came from car door frame weather moulding, which I got for free from a dismantlers. The rubber part is in the form of long tubing. I have used it for years in a variety of projects and it is the most useful repair and construction material I have ever found.

Yes - sorry, I was just being a SA. Saw your earlier thread about the rubber and it struck a chord with me - I keep bits and pieces that "might come in handy later" too. Cheers.

Sometimes I just sit and think. Other times I just sit.

Yogi02

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#1402037 7-Oct-2015 21:49

How did you go?

andrew027

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#1402148 8-Oct-2015 09:01

Yogi02: How did you go?

Sorry - I have been flat out at work and doing some other things in the new house and haven't had much time since posting. I'll get to look at it on Friday evening and will post an update over the weekend.

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