Feedback problem

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  • 03-08-2005, 07:26 AM
    20to20K
    Feedback problem
    Last Saturday my wife took the kids to the movies so I had a rare 4 hour window when I could crank my music as loud as I liked (which is pretty loud). While listening at a pretty high level I started to get severe resonant feedback coming from my left speaker. It would start off quietly (barely noticble) then would gradually build up to loud, booming hum until I turned the volume down slightly. Then it would disappear completely...until I slightly raised the volume again.

    I've had this setup in my basement now for about 8 months and never heard this before...but granted I've never played it this loud for this extended period of time before either.

    I thought I read something once about how the dimensions of your room can cause feedback. Something like if the width, height, and depth are multiples of each other? My listening area is approxiametely 22'deepx15'widex10'high. The speakers occupy the corners of the width portion(nudged between the sidewalls and the entertainment center) and opposite of the speakers is only about 25% closed off (just the staircase leading back upstairs). I have a rather large circular sofa in the listening room that comes within 5 feet of the left speaker on the long end.

    My guess is the sofa causing it...but I guess it could be the room dimensions/acoustics as well. Any suggestions? Can room treatment help me?
  • 03-08-2005, 08:24 AM
    Pat D
    Quote:

    Originally Posted by 20to20K
    Last Saturday my wife took the kids to the movies so I had a rare 4 hour window when I could crank my music as loud as I liked (which is pretty loud). While listening at a pretty high level I started to get severe resonant feedback coming from my left speaker. It would start off quietly (barely noticble) then would gradually build up to loud, booming hum until I turned the volume down slightly. Then it would disappear completely...until I slightly raised the volume again.

    I've had this setup in my basement now for about 8 months and never heard this before...but granted I've never played it this loud for this extended period of time before either.

    I thought I read something once about how the dimensions of your room can cause feedback. Something like if the width, height, and depth are multiples of each other? My listening area is approxiametely 22'deepx15'widex10'high. The speakers occupy the corners of the width portion(nudged between the sidewalls and the entertainment center) and opposite of the speakers is only about 25% closed off (just the staircase leading back upstairs). I have a rather large circular sofa in the listening room that comes within 5 feet of the left speaker on the long end.

    My guess is the sofa causing it...but I guess it could be the room dimensions/acoustics as well. Any suggestions? Can room treatment help me?

    What source are you using? I can see feedback, strictly speaking, if you play records. Feedback requires that your player be able to pick up vibrations from the speakers and feed them back through the chain. This can happen with turntables and cartridges but not with CD players and tape decks. Moving the turntable away from the speakers usually helps.

    You may, however, be describing a room resonance problem, but it's hard to say. You can calculate the approximate resonant frequencies by dividing 565 by the room dimensions in feet, but since your room is open to the rest of the house, this probably is not too useful.
  • 03-08-2005, 08:38 AM
    20to20K
    You may have hit the nail on the head...
    Quote:

    Originally Posted by Pat D
    What source are you using? I can see feedback, strictly speaking, if you play records. Feedback requires that your player be able to pick up vibrations from the speakers and feed them back through the chain. This can happen with turntables and cartridges but not with CD players and tape decks. Moving the turntable away from the speakers usually helps.

    You may, however, be describing a room resonance problem, but it's hard to say. You can calculate the approximate resonant frequencies by dividing 565 by the room dimensions in feet, but since your room is open to the rest of the house, this probably is not too useful.

    I was listening to both vinyl and CD's...but mostly vinyl. I thought (but I'm not 100% sure) that it was occuring while using both sources. Having said that, my turntable is right next to my left speaker and it was only occuring in the left speaker. Plus while I was using the CD player the turntable was still turned on. I think you may be onto something.

    I'll try and throw the wife and kids out next weekend as well and try moving the turntable further away from the speaker and see if it makes a difference.

    Thank you for your input...
  • 03-11-2005, 03:40 PM
    hermanv
    Tubes and feedback
    Turntables are a good source of feedback but so is vacuum tube equipment. All tubes are microphonic to some degree.

    I had a name brand pre-amp that mounted the tubes on a fairly large PCB supported on the edges so when I got exactly the right note the system would feedback.

    Accoustic damping material, isolation feet and more solid shelves all helped as did tube sox.

    I still love the sound of vacuum tubes but they do have their issues.