Subwoofer issue

I recently added a ML Decent sub to my system. The cable runs about 30' from the pre in the basement to the sub on the first floor. I notice with the Decent sub (purchased used) that it has a hum pretty continuously noticeable when the system is shut off. If I unplug the signal cable it stops. It did not have any hum at the place I bought it from that I noticed. Once I unplug it and re-plug the signal cable it usually stops the hum but it returns. I am not suspecting anything wrong with the sub itself but more likely something with the signal or signal cable. Electrically the sub is plugged into an old surge strip (which I no longer need as the house is surge protected now).

Any ideas what I should be investigating? Better signal cable, remove surge suppressor, other ideas?

Thanks in advance for your ideas!

Hola and welcome. I think that you have a ground loop problem. Just to make sure this is the case, remove the coax cable from your tv or from your cable box for a test. If the noise is gone, then it is a ground loop problem. Please tell us your findings, and to solve this problem is fixed adding a filter attached between the coax cable input to the cable´s tv box. This problem is caused by the potential difference of ground cables of your house and the outside electrical post. Happy listening!

by coax cable you are saying the subwoofer cable or the actual cable TV feed? If I unplug the subwoofer feed cable it immediately stops. Only returns after it is plugged back into the sub. What is the "filter" you are talking about - something on the sub feed line or something for the electrical feed to the sub itself?

No... it is the antenna cable, the input of your cable box. the signal from your system is contaminated by this cable through the ground...just remove the cable (you will not have any tv cable for this test)

Got it- will try that now. I did notice that what ever it is, it is happening AFTER the receiver. I turned the volume up and down and the hum did not change at all in volume level (something I read on an online article). Will try that now.

Thanks Roberto!

Wow - dramatic reduction. Although there is still a hum it is greatly reduced - like 95%.

and re-conntected the cable antenna to the cable box and it came right back. So there is an isolator filter for this?

Ok... good to know. Yes, Rata Shack has it...or most at any hardware store. Just say that you want an antenna ground loop suppressor. Just keep in mind that you will need an extra little coax cable for this transformer (ground loop suppressor device) because at both ends are female F connectors. Just follow the signal arrow at the device.

Now to suppress all you hum, and answering your question before, you need a better coax cable. Also route it where there is no electricity cables or any AC 117V mains. Do not place this cable near any AC cables!

I have being using a RG-11 coax cable with excellent results, just measure what cable length that you need, and tell the salesperson that you need it terminated in male´s RCA connectors. Perhaps this will isolate all the hum that you are having. Happy listening!

Outstanding and thank you for the responses this morning! I actually have a cable that was made for me by a local high-end audio shop so I was not thinking the cable was the issue.

Hola. Just wanted to know if the sub RCA cable was near to an AC line and because this proximity, a 60 Hz was induced and you can pick up some this unwanted hum. Long runs of RCA cables are not recommended. They could cause hum easy. A very good shielded coax cable might work, and can terminated the cable with RCA F connectors. Happy listening!

yes - and it is also about a 30' run to the sub. Got a ground break device to hook on the cable box between the box and the cable input line and it killed all the hum.

Thanks again

new to Martin Logan, yet very pleased… 1st time quality

I just recently purchased a secondhand (as new) MLT-2 6 speaker surround system including sub. After perusal I noticed that some subwoofers here have pet names but mine did not come with one. Speakers seem fine but sub having an issue. Seller wants all or nothing for partial refund - shipping, and I've been at this a month. 5 satellites working flawlessly. I'm trying to keep this item off the floor to prevent problems with downstairs neighbors (down firing). I realize my problem may be that this unshielded item is 1 foot from my TV. On auto/standby, I have this occasional scratching evident using all functions of my AV receiver. Unit takes extra time to power down due to same. Occasionally almost sounds like there's a build up, like static from am. radio, going on to break the typical silence. Was trying to figure out a way to make sure I didn't have a loose wire within but cannot seem to get the grill off in front and didn't even try to dig in from underneath. Does anyone have any suggestions if it's the shielding, what to look for, how to attack it, as otherwise this unit works just fine and pumps out as appropriate.