What is a "ground loop isolator" and why do I need one to stop the hum coming from my subwoofer in a system that never had a hum until I hooked up the sub? The speaker company says it is a cable company (Comcast) problem. And if I do need it, how do I hook it up (I'm technically challenged) and do they really cost about $60.

heh heh, otherwise I'm happy.....

Your speaker company is full of s**t. Cable would have nothing to do with the problem.Are you using line level or speaker level connections to the SW....Picture would be helpful.Are you using a HT reciever with a LFE output?

the sub is hooked up through the pre-amp mono l&r outputs into the subs line input. it's not an ht rcvr - when i leave the volume and low pass filter on the sub down, the hum goes away...

Best way you can single out if it's the cable or not is to disconnect the audio output of the cable box from your system, and try using another source. If you still get the hum, I'd look at the amp on the sub.. try flipping the plug over.. if the hum is gone, I would say it is the cable... I've got a cable ground problems myself, and I have proved it in this way..

i'm using a cd through the cd outputs and also tried my dat deck and the hum was still there. i assume that is what you mean by trying another source...

just gotta say how new i am to this stuff and i'd be doing this by myself (and probably throwing stuff by now!) but it is great to have a few guys helping like this....thanx!

Did you physically disconnect the cable box from the preamp?

Does the sub have multiple inputs? If you have unused inputs you might try installing grounding plugs on those. Worked for me.

Did you physically disconnect the cable box from the preamp?

yea, just did. still hummed like no tomorrow as soon as i gave it a bit of boost. no difference from previous.

Does the sub have multiple inputs? If you have unused inputs you might try installing grounding plugs on those. Worked for me.

yea, another set or 2, forgive me for my stupidity (and i am stupid about this stuff :tears:) but what are grounding plugs?

I had a terrible problem with hum and disconnecting the cable (tv) fixed it. It was also greatly improved by grounding the cable to the electrical ground. That said, I got a new TV recently and now I use the TV sound out to drive the receiver audio in, ground noise gone.

Disconnect your cable - if the hum persists, it's not the cable.

Thanx for all the help folks. I'm gonna take a nap and shower and then out for a quiet new years eve with the wife and friends. I'll pick up this problem tomorrow....again thanx!

Disconnect the " line in "cables- from the sub --does the hum persist ?

If they think a ground loop eliminator is needed, then it could indeed be a ground loop. Don't spend $60 on one though!!!!!!! All you need is a box that connects the signal from one RCA cable to the signal of another but does not connect the shield. Put one RCA from receiver to box, and another RCA from box to sub. This way, one half of your cable's shield is grounded at the sub and the other half is grounded at the receiver. If I am understanding the concept of "ground loop" correctly, it has just been eliminated because there is no longer one piece that is grounded at both ends.

Poor grounding of the CATV feed is the #1 cause of hum in a home theater system.

If there is any interconnection between the incoming CATV feed, even if it's through another component such as the TV or STB, it can cause a hum.

This is usually not a problem so long as all your gear is of the 2-prong power cord variety. Usually the problems start when the TV or cable box is connected to the audio system and a component with a grounding power cord is added to the audio chain. It's the addition of the second ground added to the system that closes the ground loop and results in the hum.

The first thing to do is check near where the CATV feed comes into the house. There should be a grounding block with with a heavy wire clamped to the same ground rod/wire that grounds your service entrance panel.

If it still hums after that, then we go to step two.

If it still hums after that, then we go to step two.

Which is?

Grounding my cable at the splitter coming into the house helped, but some times it will suddenly start back a bit.. then go away... I can stand to know what step two is... please..

Cut all the grounding prongs off my other power cables? :D

Ok. here is where i am so far.
1. disconnecting line in stops the hum
2. i believe there is a "grounding block" outside where catv comes in
3. disconnecting tv from pre-amp didn't stop the hum
4. i put 2 "fake" plugs into the outer set of rca jacks (lfe in/out), didn't help
5. DISCOVERYthere are 2 other knobs on the back of the sub. one is "volume control" and the other is "low-pass filter". the less i move these up, the hum goes away, but the sound coming from the sub is minimal. do i have some sort of phasing or incompatibility problem?

I fixed a hum problem involving a sub by changing the power connector on the sub to a non-grounded plug adapter (the kind that let you plug a 3 wire plug into a socket that has only 2 prongs).

No guarantees, but it worked for me. Good luck.

Which is?

Grounding my cable at the splitter coming into the house helped, but some times it will suddenly start back a bit.. then go away... I can stand to know what step two is... please..

Cut all the grounding prongs off my other power cables? :D

Step 2 is usually when you disconnect everything and start reconnecting until the hum comes back...then go from there.

I know, that's not nearly as dramatic as you had hoped for. :boring:

And yes, there are many people who use "cheater plugs" with great effect and without incident, but I don't advocate that.

Ok. here is where i am so far.
1. disconnecting line in stops the hum
2. i believe there is a "grounding block" outside where catv comes in
3. disconnecting tv from pre-amp didn't stop the hum
4. i put 2 "fake" plugs into the outer set of rca jacks (lfe in/out), didn't help
5. DISCOVERYthere are 2 other knobs on the back of the sub. one is "volume control" and the other is "low-pass filter". the less i move these up, the hum goes away, but the sound coming from the sub is minimal. do i have some sort of phasing or incompatibility problem?

OK, just we are 100% on the same page...

Are you saying there is absolutely nothing connected to the incoming CATV feed that is also connected to any of your audio gear? The easiest way is to disconnect the catv feed itself from the TV and or STB.

How many of your components have grounding (3-prong) power cords?

Have you simply tried some different cables?

Is all of your equipment on the same circuit? You might be on two different circuits or phases. That makes the connection much longer between the two systems which is worse. Grounding the cable to the outlet that your equipment is connected to has helped for me. Actually I grounded the case of the amp to the outlet.

Changing the filter just pushes the response out past the 60hz (or 50hz) noise that you are hearing.

ok, i'm going to take everything apart and reconnect until the hum comes back. whoaru99, i did disconnect right from the cable box into system and tv.
i have 4 components with 3 prongs (2 amps, pre-amp, sub) and 4 with 2 prongs (DAT, CD, DVD, LD). pastro, yes, i believe that it's all on the same circuit. what confuses me is that there was absolutely no hum until i connected the sub, and that is where the hum comes from.

ok, i'm going to take everything apart and reconnect until the hum comes back. whoaru99, i did disconnect right from the cable box into system and tv.
i have 4 components with 3 prongs (2 amps, pre-amp, sub) and 4 with 2 prongs (DAT, CD, DVD, LD). pastro, yes, i believe that it's all on the same circuit. what confuses me is that there was absolutely no hum until i connected the sub, and that is where the hum comes from.

hmmm, I mean humm... The hum is probably inserted in the sub and not from the preamp. You might try grounding the rf cable to the subwoofer ground wire. Basically, getting all the grounds to a common point should help.

At the end of the day, you could add a line isolator like the jensen vrd-1ff. Hate to suggest you spend $$s though.

http://www.jensen-transformers.com/prices.html#vrd1ff

ok, how do i ground the rf cable (what is it?) to the subwoofer ground wire? and if i bought the line isolator - how do i hook it up? boy, am i new to this stuff! thanx for all the help!!!!!!!!

ok, how do i ground the rf cable (what is it?) to the subwoofer ground wire? and if i bought the line isolator - how do i hook it up? boy, am i new to this stuff! thanx for all the help!!!!!!!!

RF cable = cable tv coax input.

If you have an amp or a splitter with a metal case and a screw on it, connect a wire from this to the center terminal screw terminal on the outlet that the sub plugs in to.

The line isolator is an in-line device that is connected on one end to the cable tv coax, and on the other to the tv or to a splitter or amp. It would be best to put this isolator before any amp that connects to the ac line.

The line isolator costs about $42, and most people don't have any issue with channels not working since it's bandwidth is to 1.3ghz, but you never know you might have issues. It's an expensive item to just try out.

Go buy a $3 cheater plug (3 prong to 2 prong) and see if it eliminates the problem. If it does that means you have a ground loop problem.

ok - thanx guys. the cheater plug is my first try.

Yea! Here's my story, It's sad but true, about a subwoofer I once knew....
Well, Radio Shack didn't have any cheater plugs, Home Depot didn't either (they didn't even know what I was talking about!) so I figured aw, what the heck, go next door to the local Wal-Mart. Yup, they had one (actually 2 in the pack) for $1.27. Ok, ok, I know, the suspence is killing you. Yup, NO HUM. NO HUM. NO HUM! Am I happy? You bet. and the system sounds GREAT! Thanks to all that helped with suggestions etc.! It really was a big help! Now, I'm gonna shut this laptop off and go listem to some serious music. Great Forum and great people! Thanx!

Audio Advisor has a device called the HUM-X that takes out that pesky hum. Worth a try but about $99.00 for the solution. You got 30 days to see if it works.

Yea! Here's my story, It's sad but true, about a subwoofer I once knew....
Well, Radio Shack didn't have any cheater plugs, Home Depot didn't either (they didn't even know what I was talking about!) so I figured aw, what the heck, go next door to the local Wal-Mart. Yup, they had one (actually 2 in the pack) for $1.27. Ok, ok, I know, the suspence is killing you. Yup, NO HUM. NO HUM. NO HUM! Am I happy? You bet. and the system sounds GREAT! Thanks to all that helped with suggestions etc.! It really was a big help! Now, I'm gonna shut this laptop off and go listem to some serious music. Great Forum and great people! Thanx!


I think the chances of a problem arising by using a cheater plug are small but just so you know, some equipment uses the case ground for surge protection. That won't work without that wire. Also if there was a weird failure inside your amp that somehow shorted to the amp body, it would create a potentially dangerous situation. That said, I would still probably sit back and listen to the hum free music..

I think the chances of a problem arising by using a cheater plug are small but just so you know, some equipment uses the case ground for surge protection. That won't work without that wire. Also if there was a weird failure inside your amp that somehow shorted to the amp body, it would create a potentially dangerous situation. That said, I would still probably sit back and listen to the hum free music..


I've got the sub plugged into the cheater and the cheater plugged into the surge protector. Does that help my situation at all. The surge protector is plugged directly into a wall outlet, NOT into the ac line enhancer that everything else is plugged into. Thanx for all the info. I'm learning by the minute.....:thmbsp:

Yea! Here's my story, It's sad but true, about a subwoofer I once knew....
Well, Radio Shack didn't have any cheater plugs, Home Depot didn't either (they didn't even know what I was talking about!) so I figured aw, what the heck, go next door to the local Wal-Mart. Yup, they had one (actually 2 in the pack) for $1.27. Ok, ok, I know, the suspence is killing you. Yup, NO HUM. NO HUM. NO HUM! Am I happy? You bet. and the system sounds GREAT! Thanks to all that helped with suggestions etc.! It really was a big help! Now, I'm gonna shut this laptop off and go listem to some serious music. Great Forum and great people! Thanx!I figured a ground loop was the problem, it usually is. I had the same thig when I first bi-amped my system. The components were on the wall to my left and all plugged into one outlet. The amps were at the front of the room, one in each corner next to the speakers, each plugged into their own outlet. Sure enough, 60hz buzz like crazy. I bought 2 cheater plugs for the amps and the problem was solved. Dead quiet now.

I figured a ground loop was the problem, it usually is. I had the same thig when I first bi-amped my system. The components were on the wall to my left and all plugged into one outlet. The amps were at the front of the room, one in each corner next to the speakers, each plugged into their own outlet. Sure enough, 60hz buzz like crazy. I bought 2 cheater plugs for the amps and the problem was solved. Dead quiet now.

Brent71, thanx for the suggestion and help. :thmbsp: