Can't isolate hum in our HT

Discussion in 'AV Receivers' started by Michael Mohrmann, Feb 16, 2004.

  1. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    It appears that we have been bitten by a ground loop problem. Or have we? We have been hearing a hum coming from both of our ACI Titan II LEs subwoofers. The hum appears to be coming directly from the amplifier backplane and not the woofers themselves. It can occur whether the subwoofer is active or when in "wait" mode.

    No other hum or buzzing occurs in the system. I already have an isolator installed for the cable TV system that we have been using for over 2 years, and I have also tested this out with the cable TV line disconnected at the point where it reaches our outside splitters. The hum remains when it is "active". Let me explain that last statement further.

    Here's a scenario that played out this evening, one that has occurred quite often the last two weeks, and see if anyone else has an idea of what is going on.

    We leave our home around 5:30pm to check out a few TVs and have some dinner. At this time, there is very little hum coming from the subwoofers, only heard when real close to the amplifier backplane. It is not noticeable at any listening position.

    We arrive back home around 7:30pm. Right away, we can tell that the hum level has increased, easily heard from 4-5' away with the room being quiet. I attempt to try a long, heavy-duty extension cord and connect one of the subwoofers to another room. Still have the hum. I try turning off and/or unplugging numerous items throughout the house. Still have the hum.

    After about 45 minutes of this (it is now around 8:15pm), I head upstairs to do some more searches on my PC. The rest of the family is also upstairs. Around 9:30pm, I decide to head downstairs to check the system out. Yep, the hum is mostly gone again! BTW, the evening seems to be a common time when this problem occurs.

    This is driving us crazy. If it were a typical ground loop problem, I figured we would have the hum at a fairly constant level at all times during the day. And yet this is not the case. The electrical system in the house is about 10 years old, the circuit to the familyroom is 20 amps, and no other devices are connected to that circuit. We have no dimmer switches or halogen lights.

    Any ideas?

    Michael
     
  2. Ed Moxley

    Ed Moxley Cinematographer

    Joined:
    May 25, 2003
    Messages:
    2,701
    Likes Received:
    1
    Location:
    Eastern NC
    Real Name:
    Ed
    Try it in a neighbor's house or on a friend's system.
    If it does it in another house, I'd say it's the subs. May be a known problem with that brand. Have you called or sent an e-mail to that company about it? Try it somewhere else first, to make sure and take it from there.

    Good luck!
     
  3. JohnSer

    JohnSer Stunt Coordinator

    Joined:
    Dec 4, 2002
    Messages:
    198
    Likes Received:
    0
    Could be the amps, but wondering if you are having high or low voltage problem, from your power service. If so, going to a neighbor, might have the same issue. Do you have an AC voltmeter to take a reading? If so, take readings when its a problem and when its not, and compare.
     
  4. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    Good suggestion, as I do have a voltmeter, but I haven't used it yet. I'll try that next when the hum returns.

    I believe our problem is more of a "cause and effect" nature rather than an actual defect with the subwoofers. We have owned both subs for almost two years, and we have not had this problem until recently. Also, the odds that BOTH subwoofers would somehow fail or become flawed at the same time seems kind of slim.

    Just to add to the information (overload?), when the hum starts, I can unplugged either one of the subs and the other one continues to hum. Also, I can unplugged everything in the familyroom except for one sub and it will continue to hum when present.

    Michael
     
  5. Wayne A. Pflughaupt

    Joined:
    Aug 5, 1999
    Messages:
    6,098
    Likes Received:
    33
    Location:
    Katy, TX
    Real Name:
    Wayne
    It could also be a problem with the subs and a ground loop. It could also be a bad cable somewhere in the system. Next time it starts acting up I’d disconnect everything in the system a piece at a time (including the CATV feed) and see if you can isolate the problem to a specific component. If you end you with everything disconnected and you still have the problem, obviously it is the sub(s).

    Oh, if you’re using a power processor – line conditioner, surge protector, etc. – check there, too.

    Regards,
    Wayne A. Pflughaupt
     
  6. Chu Gai

    Chu Gai Lead Actor

    Joined:
    Jun 29, 2001
    Messages:
    7,270
    Likes Received:
    1
    To what Wayne and others have added, I'd check for the presence/absence of the hum with the TV completely disconnected from the rest of the audio system. So pull any connections that you've got that hook up your TV in any way from the rest of your audio system.

    Since you've got a VOM, might as well check the voltages but I would think if you had a general overvoltage/undervoltage situation it would manifest itself in other areas of the home. Consider picking up an outlet tester at Home Depot. They're about $4 and you can use them to check on your wiring. They can detect miswiring, absence of ground and the like. Further, give some thought to any activities that occur at a specific time when this hum is most noticeable like the dishwasher, washing clothes, cooking. Try to pin it down.
     
  7. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    Just so I get the message straight, I have disconnected everything in our familyroom, including our TV and the cable TV coax on the outside of the house. I still have the hum even when doing all of that. BTW, when I left for work this morning, I didn't have the hum. Like I said, it is very intermittent.

    There doesn't appear to be a specific cause for the hum. Like I mention in my first post, we came home from dinner without a large appliance running, other than the refrigerator being plugged in, and we had the very audible hum.

    I was discussing this problem at work when I thought of something else. Can cordless phones or cell phones cause interference that can be picked up through the electrical system? Also, can the transformers being used for the cordless phones in the other rooms in our house be a source of the problem?

    Michael
     
  8. Baldemar Garcia

    Baldemar Garcia Stunt Coordinator

    Joined:
    Feb 4, 1999
    Messages:
    141
    Likes Received:
    0
    Are you sure you don't have any flourescent/halogen (?) type lamps in use? Try cutting power off to the other rooms one by one to see if you can isolate the source. Just an idea.
     
  9. Chu Gai

    Chu Gai Lead Actor

    Joined:
    Jun 29, 2001
    Messages:
    7,270
    Likes Received:
    1
    These problems become so vexing that even without knowing the answer to your questions I'd wind up unplugging the phones and the fridge. The fridge FWIW, is supposed to be on its own outlet anyways according to NEC code if I recall but strange things have happened in homes. One more thing I'd try Michael is to find an inexpensive subwoofer cable that's directional. IOW, the shield is only connected at one end and see if that makes matters any better. Something that I could return later if it didn't work out.
     
  10. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    Before I forget, I want to thank everyone for their suggestions. Based on your feedback, I have several more ideas to try out when I get home from work. Thanks. [​IMG] Of course, with my luck, the subs won't have the hum! [​IMG]

    I am already using one, but since I have the hum with the subwoofer's cable disconnected, that isn't the issue.

    Michael
     
  11. Wayne A. Pflughaupt

    Joined:
    Aug 5, 1999
    Messages:
    6,098
    Likes Received:
    33
    Location:
    Katy, TX
    Real Name:
    Wayne
    Michael, that pretty much tells me right there that problem is the subs. That and the fact that you said it’s primarily coming from the amp, not the driver itself.

    It may be that they have bad transformers or something. Perhaps some one more familiar with electronic circuitry can comment.

    Regards,
    Wayne A. Pflughaupt
     
  12. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    Wayne,

    It is starting to look that way. Having a small amount of transformer hum from a standalone or subwoofer amp is not uncommon. Having the hum level rise and fall without an obvious cause would seem to point to an issue with the amp, which is what you are saying here. I would have to agree with your assessment of our situation.

    I have sent Mike D. at ACI an email asking him this question. The ACI web site has a page that mentions that the transformer in their Titan and Force subs can shift slightly, affecting the hum level. They list instructions on how to open up the sub and readjust the tension on the transformer mounts. Given what Mike says, I may just do that.

    If only one sub were affected, I might believe that the latter will help. But both subs? [​IMG]

    Michael
     
  13. Chu Gai

    Chu Gai Lead Actor

    Joined:
    Jun 29, 2001
    Messages:
    7,270
    Likes Received:
    1
    Well maybe you were moving them around recently?
     
  14. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    Chu,

    My first reaction was to say no, but then I realized that my wife has cleaners come to our house once every 2 weeks. One of the ladies is very thorough, going so far as to move our china cabinet to clean the hardwood floor underneath.

    I have given explicit instructions not to move the speakers in the middle of the room (after finding that they had been moved previously). I wonder if they think the subs are okay to move? If the one lady can move a china cabinet, an 70-80lb sub may not be a problem either.

    Swell, but that would explain why both subs would be affected.

    Michael
     
  15. Chu Gai

    Chu Gai Lead Actor

    Joined:
    Jun 29, 2001
    Messages:
    7,270
    Likes Received:
    1
    Like Tom Hanks said...sh*t...it happens.
     
  16. Kevin Magee

    Kevin Magee Agent

    Joined:
    Aug 26, 2000
    Messages:
    44
    Likes Received:
    0
    If both subs are exactly the same age and have the exact same usage, it is possible for the both to fail around the same time. You may have not notice the hum untill it was being produced by both subs. I've owned certain brands of electronics where I and other seem to experience the same problems at about the same usage point with the product. If the type of hum you are experience is known to be problem for that brand, that's likely what it is. Have you tried just using a cheater plug to defeat the ground and see it that solves the problem?
     
  17. JackS

    JackS Supporting Actor

    Joined:
    Jan 17, 2002
    Messages:
    634
    Likes Received:
    0
    Try physically moving the connecting cable during the hum. Does this movement increase/decrease hum? I had the same problem a few years ago although intermitant, moving the cable did work for a short time. The problem has since dissapeared completely and I'm not sure why. Maybe I changed cables but can't remember .
     
  18. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    I have already determined that it is not a faulty subwoofer cable. I get the hum with NOTHING else connected in the room except for the subwoofer to the AC outlet. Everything else is unplugged and disconnected.

    Michael
     
  19. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    While my wife and I wait for ACI to respond to our email, I decided to do a few more tests this evening. At 4:00pm we had no obvious hum from the subwoofer. Somewhere between 6:30pm and 7:00pm, the hum returned. It is still here at 8:45pm.

    During the last 1:45 I have: unplugged the refrigerator, unplugged the water dispenser, unplugged all of the cordless phones, the answering machine, all of the PCs and the garage door opener. The hum remained.

    During this time I tested each and every socket in the house, inside, outside and in the garage. Each one was properly wired and grounded. Just for yucks, I also tested all of the surge protectors and power strips in the home. Again, the wiring and grounding were correct.

    I have taken voltmeter readings from the familyroom for the last hour and a half. The meter readings have ranged from 121.6 to 122.7. At least during the time of the hum we are not lacking volts. I will repeat this later this evening when the hum (hopefully) disappears before calling it a night.

    Michael
     
  20. Michael Mohrmann

    Michael Mohrmann Screenwriter

    Joined:
    Dec 30, 2001
    Messages:
    1,394
    Likes Received:
    0
    Morning update:

    Just like clockwork, the hum is gone in the morning. The meter readings from last night had the familyroom at 122.4 to 122.7 volts (the upstairs readings were about 0.8-1.0V lower). This morning, the readings were 121.5 to 121.7 volts, about 1.0V lower. I can't imagine that one volt would make a difference. The problem has to be either some sort of interference in the AC or amps in the subs themselves.

    Michael
     

Share This Page