75 Ohm Coax Cable for Subwoofer?

[shawnster]

I have been plagued with subwoofer hum in two different houses. It started in our old house, soon after I purchased a Denon and Paradigm system from my local shop. I tried everything to eliminate the hum, based on information widely available on the web, but ended up bringing my Paradigm sub back and basically trading it in for another one. My current is a Paradigm PDR-10 v.3, auto on and off, with spring loaded speaker input jacks and one Low Level Input RCA jack.

Same problem. It comes and goes, sometimes it's great, sometimes its awful. This is definitely not a low-level hum, but more of a loud, feedback sort of hum. Sometimes rearranging cables works, sometimes it gets worse. Sometimes just plugging and unplugging or turning the system off and on will help but it always comes back. I've tried to isolate it by unplugging one component at a time, including my cable tv coaxial, but there's no clear association between any of the components and the hum.

I'm using a Monster Clean Power HTS3000 for all electrical, which is then plugged into a grounded outlet so I don't see where I would be causing a ground loop. And trying to keep AC and signal cables apart doesn't seem to work either.

All of a sudden, I looked at the subwoofer cable that my guy sold me with my system 3 years ago- it's marked "75 Ohm Coaxial."

When I do a search for "subwoofer cable" it seems to me that most of the subwoofer cables out there are not 75 Ohm Coaxial, they are simple audio cables with RCA jacks on either end. Could it be that there's something with the 75 Ohm cable?

 

[jonnythan]

That's the type of cable you should be using, but it wouldn't hurt to try another.

 

[markw]

75 0hm cable itself is not llikely the cause of your problem.

Virtually all audio and video interconnects nowadays are made of 75 ohm cable.

Your problem almost sounds like a bad ground connection on at least one of the ends of the cable, though.

 

[zumbo]

I use a 3 to 2 plug reducer on my sub to lift the ground. It's a bad thing, but I have a work-around. I run a complete system ground from my receiver phono ground to my surge supressor. This grounds every component connected. Hum is gone.

 

[Frugal]

balanced cable

Longer cable runs should be balanced, that is the resistance on both conductors should be excactly the same, to reduce hum. Professional XLR and phone plug connectors use three wires, one for shield and one for each conductor (balanced). Unfortunately RCA has only two contacts which works well for coax. Coaxial cable uses the sheilding for one conductor and the core wire for the other causing a slight difference in resistance (unbalanced) which is fine for short runs but can cause hum on longer runs. Twisted pair shielded cable uses the shielding and one wire for one conductor and one wire by itself for the other conductor (unbalanced). Make your own cable using twisted pair shieded wire and leave the shield disconnected on the recieving end. I wish I could buy them made that way.

 

[shawnster]

hey, zumbo

I went to Best Buy today and purchased an Acoustic Research subwoofer cable. I have the arrow pointing towards the subwoofer. It sounds better to me now, just low-level, electrical hum (probably normal.)

Do many of you simply turn your subs off when listening at low- level "background" music in order to avoid the hum?

If all of my components are plugged into the Monster HTS3000 and that is in turn plugged into a grounded outlet, isn't the ground wire from the receiver to the Monster HTS3000 redundant?

 

[zumbo]

I am using a 3 to 2 prong a/c reducer to lift the ground on the sub power cord. Therefore, it is not grounded. The ground is what picks-up the hum from whatever the source is. Now, we need to be sure the sub is grounded, because we have just taken the ground out of the power cord. So, if the power cord is connected to the surge supressor which is grounded to the receiver which is connected to the sub by a patch cord, the sub is grounded by the patch cord. As is any other piece of equipment that has the power connected to the surge supressor that is connected to any other piece of equipment by a patch cord that is somehow connected to the receiver. This is a total system ground. Should be in the surge supressor manual. It was in mine. That's how I learned it. Hum is gone.

I also have a cable isolator. It worked on it's on before I added things over time and picked-up the hum again. This time it wasn't the cable. Couldn't find it. I changed the wall outlet, breaker, and checked every cable one-by-one. Couldn't find it. So, this is how I got rid of it for good.

 

[shawnster]

my sub...

doesn't have a grounded 3-prong plug, it's just plain.

 

[westcott]

doesn't have a grounded 3-prong plug, it's just plain.

Start here and let us know what works.

http://www.audioholics.com/techtips/setup/avhardware/groundloopcableTV.php

Good Luck!

 

[Sheep]

There is no hum with my subwoofer. I listen to it at any volume level.

SheepStar

 

[Hi Ho]

I went to Best Buy today and purchased an Acoustic Research subwoofer cable. I have the arrow pointing towards the subwoofer. It sounds better to me now, just low-level, electrical hum (probably normal.)

There shouldn't be any hum at all.

 

[j_garcia]

I have never had hum on any of my subs either. Definitely read the AH article on hum & buzz linked by westcott, it's got good info. I've also seen systems use 50ft of 75 Ohm coax for a sub with no hum. If you disconnect the wire from the sub and the hum goes away, it is not the sub. If you disconnect the cable/sat connection from your system and the hum goes away, that is what it is and it can be relatively easily solved.

 

[Hi Ho]

I had hum problems when I bought my first sub and connected it using a cable made of lamp wire. That was before I new any better. I quickly remedied the problem by buying a length of RG-6 cable and terminating it with RCA connectors. I'd say it's definetly the most cost effective subwoofer cable one can come up with and it works just as good as the "subwoofer cables" you buy in the store.