Subwoofer Hum w/ Long Coax Cable

I was hoping I could get a little help on a subwoofer hum issue. The sub is a Polk PSW505. Up until yesterday I had my subwoofer sitting in front of my TV stand, connected with a cheap, VERY thin, 6' composite cable. It was only a temporary setup until I could route a cable up the TV wall, thru the attic, and down the back wall. This would allow me to place my subwoofer on the back wall where I have room for it.

So yesterday I finished running the wire, which was a huge PITA. It's approximately 60' long. It's a solid copper RG6 tri-shield (60%) coax with RCA compression connectors I got from Home Depot. They are DataShark brand. I get it all hooked up. With everything connected and the receiver off, I can hear medium volume hum if I turn the volume up to 100% on the subwoofer. It was something I didn't recall hearing with my temporary setup.

I then begin experimenting with all sorts of configurations. I first bring the sub back next to the receiver and use the cheap store-bought composite cable and listen for a hum. There is a very faint hum with the sub volume at 100%, but you almost need the house dead silent and your ear just inches away. Out of curiosity I made a 6' cable with the RG6 coax and HomeDepot/DataShark connectors and the hum was no different than the cheap store-bought composite cable; VERY quiet hum.

So then I decide to isolate the sub from the receiver. With power to the sub and the 65' coax cable connected to the sub, but NOT to the receiver (this end just loose, laying on the ground), it makes the same medium level hum at 100% volume. As soon as I remove the 65' coax cable from the sub the hum gets so quiet, it's almost nonexistent. Same goes with the 6' store bought composite cable. As soon as I remove the composite cable from the sub, the hum goes from vary faint to almost non-existent.

Do you think the cable is picking up interference along with length? It just seems the longer the cable I attach, the louder the hum.

I know there are hundreds of sub humming threads on here and I read many, just couldn't find a solution that worked for me. Based off what I saw in other threads, Im going to list a few things ahead of time that may be asked. I do not have cable or satellite. I have an OTA antenna in the attic. The receiver and sub have 2-prong power cords. I think they are on the same circuit, but will need to confirm this.

Ideas? Thanks!

A cheaper plug is a 3 prong to 2 prong adapter, correct? The sub has a 2 prong plug on it already.

No, I did not have a power extension cord from the same outlet as the receiver, but that did cross my mind as something else to try and I will when I get home from work. I am also not 100% sure the 2 different outlets are on the same circuit, but I think they are. This too I will have to confirm when I get home.

But my question is does it really matter about the receiver and where its getting it power since this happens without the sub even connected to the receiver?

Lets image I don't even have a receiver. I plug the sub into wall outlet "A" and turn the sub volume up to 100%, hum is VERY VERY quiet, almost nonexistent. I then attach only 1 end of the cheap composite RCA cable to the LFE IN port on the sub. The hum gets slightly louder, but still very quiet. You would need your ear withing a foot to hear it. I unplug the cheap cable and then plug the 6' coax cable I made and there is no change to the volume of the hum compared to the cheap 6' cable. I remove the 6' coax and move the sub to the back of the room and plug the power into wall outlet "B" and there is almost nonexistent hum (just as it was when powered by wall outlet "A" with no LFE cable attached), but as soon as I attach the 65' coax to it, its hums at a medium level. The other end of the 65' coax is connected to nothing, its just going up the back wall, through the attic, down the TV wall and laying on the floor.

Oh, I wanted to add, the hum stops completely, from the best I can hear, when in standby mode.

I have read all your replies and appreciate them. I'm going to mess around with it tonight and get back with what I find.

DonH50, I now understand how testing the cable connected to the sub and leaving the other end open is pointless. I believe connecting the other end to the receiver only reduces the hum slightly, and does not eliminate it, but I need to confirm this.

A power line conduit runs down the back wall parallel to the sub-woofer cable, about 6 inches away. There is fiberglass insulation and one 2x4 wall stud between them. It may also be in contact perpendicularly with other electrical conduits in the attic. In those cases I can re-route it. But the cable iteself and where it comes down the back wall are not changeable.

I didn't think quad vs tri-shield made much a difference. I know it doesn't for TV (OTA, cable, or satellite) But this home theater stuff is all new to me. Running a new cable is pretty much out of the question. Running this one was extremely difficult. If I would have know this before had, I would have bought the best cable available, I just didn't think it mattered. I thought the coax I had was of decent quality and sufficient for a sub-woofer cable, therefore I used it.

Well using the existing cable to pull a new cable would make it a lot easier, but then it just goes from VERY VERY hard to Very hard, haha. My biggest worry is that if I do replace the cable, that the new one is no different.

Okay, I have done a little more experimenting. 1st off, let me start by saying when connecting the RCA coax cable end to the receiver, the hum does decrease slightly, but it's still evident.

2nd, I wanted to try something to determine if the home's 60hz electrical was interfering with the 65' subwoofer's coax cable, especially that segment that runs parallel to it down the back wall. Of course I couldn't turn off all the power to the house because then I would have nothing to power the sub, but I found a 50' power extension cord and plugged it in an outlet are far away from the subwoofer coax cable as possible and turned off all the circuit breakers except the one with the extension cord powering the sub. This of course cut the power to that segment where the coax and electrical conduit ran parallel to each other. Doing so made no difference in the volume of the hum. Based off that, wouldn't it be safe to say the hum isnt being caused by the homes 60hz electrical? Maybe the hum wouldn't disappear completely, but I would expect a significantly quieter hum at the very least. Thoughts?

Lastly, this is what I found most interesting and maybe someone can explain it to me. While unplugging and plugging the RCA connector at the receiver's subwoofer pre-out socket, to test how much less hum there is when it is connected, I noticed that when the metal outer barrel of the RCA plug makes contact with the barrel of the receiver's socket, the hum completely goes away. And then as you further push the RCA plug onto the receiver socket and the center pin of the RCA plug is fully seated (making contact inside the socket), the hum starts. I hope I clearly described what I am trying to get across.

This may not be necessary, but I am unsure my 3rd paragraph in my post above was clear. Hum based on 0 being none and 10 being the loudest. Thanks again for all the suggestions and help.

Hum volume 6


Hum volume 4


Hum volume 0


Hum volume 0

Using a 14ga solid copper wire I was able to do sub chassis to AVR chassis, sub chassis to barrel part of the RCA plug attached to the AVR, and AVR chassis to the barrel part of the RCA plug attached to the sub and all 3 did the same thing. All 3 made the hum louder.

I am not able to do barrel of RCA plug on AVR to barrel of RCA plug on sub because I have no means to securing it. I would need a 2nd person to hold it on. I could test that tomorrow if it would be helpful.

But what does it mean that the hum got louder when I did what you described?