Tech Square

Posted by Bruce from DC (B) on March 23, 2004 at 07:03:13

I kid you not. Last night, after my "record break" I turned off my system and noticed that the right speaker only was still humming softly with what sounded like a 60 Hz hum.

My amp shuts down to a "standby" mode, so, just to be sure, I killed the main power switch. This completely disconnects the amp (both legs of the AC power) from the power line.

Still humming away. When I short the speaker wires, the hum stops.

The speaker wires for this speaker are not near any AC cords. The wire feeding this speaker is a Goertz "ribbon" cable, about 3 meters long.

Only remaining test is to see if the hum persists over a long period, in case it takes a while for the power supply caps in the amp to fully discharge.

I live near TV transmitter towers, about 2 km away.

Any ideas as to cause or cure from you tech types?

TIA.

• Mag fields - Jim Hagerman 23:16:46 06/28/04 (0)
  In Reply to: Speaker still hums with amp shut down: cause? cure? posted by Bruce from DC on March 23, 2004 at 07:03:13:

  Are you running a tube amp? Could be stray magnetic field getting into the output tranny.

  jh
  
  
  

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• Re: Speaker still hums with amp shut down: cause? cure? - j.bradley 05:10:48 04/23/04 (0)
  In Reply to: Speaker still hums with amp shut down: cause? cure? posted by Bruce from DC on March 23, 2004 at 07:03:13:

  hiya
  Dont mean to be flippant but
  try swapping the positions of the speakers
  if the source of the noise is still in the same place then its the central heating pump under the floor boards - Viola!
  (yes i know i spelt viola)

  j.bradley
  
  

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• WHOA BRUCE!!!!!!! VERY IMPORTANT YOU READ THIS...CAN ANYONE CALL HIM?? - jneutron 15:31:24 03/26/04 (3)
  In Reply to: Speaker still hums with amp shut down: cause? cure? posted by Bruce from DC on March 23, 2004 at 07:03:13:

  BEFORE YOU DO ANYTHING ELSE WHATSOEVER!!!!!

  CHECK YOUR HOUSE GROUND...EVERYTHING...CHECK GROUND TO NEUTRAL...

  If there are ground currents running around, for example, through the radiator, that could mean that something in your house is causing currents to run through the radiator, or through ground somewhere..

  If that is the case, you may have a very big safety issue.....

  I'll e-mail you also, if I can.

  John
  
  
  

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  • Re: WHOA BRUCE!!!!!!! VERY IMPORTANT YOU READ THIS...CAN ANYONE CALL HIM?? - Bruce from DC 15:56:36 03/26/04 (2)
    In Reply to: WHOA BRUCE!!!!!!! VERY IMPORTANT YOU READ THIS...CAN ANYONE CALL HIM?? posted by jneutron on March 26, 2004 at 15:31:24:

    Got it, John. Will check for voltage potential to ground. I have a VOM.
    
    

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    • Still walking and talking - Bruce from DC 06:37:29 03/29/04 (1)
      In Reply to: Re: WHOA BRUCE!!!!!!! VERY IMPORTANT YOU READ THIS...CAN ANYONE CALL HIM?? posted by Bruce from DC on March 26, 2004 at 15:56:36:

      It does not appear that my radiator is electrified. Hum is gone; but I am still investigating. . .

      thanks for your concern "jneutron"
      
      

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      • If I am in error... - jneutron 07:19:44 03/29/04 (0)
        In Reply to: Still walking and talking posted by Bruce from DC on March 29, 2004 at 06:37:29:

        I am doing so, hopefully on the side of caution..

        I'm personally not so fraid of the electro-stuff...but I fear mightily for my children..It would be very difficult for me if something I casually tossed aside as no problem hurt one of them..

        As for your problem..

        I wasn't so worried about the potential voltage on the radiator, but that was a good thing to check...

        For hum to be induced, there would have to be a 60 (50) hz magnetic field present, that from a loop of AC current. Since all house wiring has the hot and neutral very close to each other, they really don't broadcast a huge magnetic field very far. So, the only thing I could think of was a larger loop, and that would mean typically a ground current, perhaps flowing through the heating system.

        One such scenario was this: your AC to the house comes in...and near the panel, a water pipe is connected to the ground of the AC mains..

        Later, a plumber (or DYI'er) installs a whole house water filter..Mine had a plastic body...

        THIS ACT in itself, is hugely dangerous...it breaks the house plumbing from hard ground...

        If something in the house suddenly has a hot to ground short, it could conceivably electrify the entire house plumbing system, or even worse, part of it...

        One of my bathrooms has the baseboard heating on one wall, the shower on the other, toilet in the middle...When sitting, I can touch both...

        So, I was more concerned that there are AC currents running around that shouldn't be there..If there are, and they are going through the plumbing (for example), then there may be very little between you and danger, some connection there by accident, or by purpose..

        Again, I think worst case here...but it could very well be possible that the grounding bond at the water pipe infeed for your house may be saving your life as we speak..

        Please try moving the wire around to find where the most sensitive spot is..

        Thanks for listening..

        John
        
        

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• Re: Speaker still hums with amp shut down: cause? cure? - Rod M 08:35:55 03/23/04 (5)
  In Reply to: Speaker still hums with amp shut down: cause? cure? posted by Bruce from DC on March 23, 2004 at 07:03:13:

  I think you're on the right track. My first thought was the amp storing up power. An easy way to test it would be to turn off the amp with the source still running. I'll bet you'll hear the music continue for a fair number of seconds. Once it stops, then check for hum.
  
  
  

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  • Re: Speaker still hums with amp shut down: cause? cure? - Bruce from DC 09:26:44 03/23/04 (4)
    In Reply to: Re: Speaker still hums with amp shut down: cause? cure? posted by Rod M on March 23, 2004 at 08:35:55:

    Yeah, I'm going to shut everything down overnight -- then see if the hum is present the next morning. All the PS caps should be fully discharged by then.
    
    

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    • Nope, not the amp. - Bruce from DC 08:38:41 03/24/04 (3)
      In Reply to: Re: Speaker still hums with amp shut down: cause? cure? posted by Bruce from DC on March 23, 2004 at 09:26:44:

      with the amp powered down all night (to let the PS caps fully discharge), it's still there.

      Somehow, the speaker cable is picking up powerline hum. (When you turn lights on and off, the hum changes.) The cable passes fairly close to a large cast-iron radiator (not it's not electrically "hot"). Re-arranging the position of the cable slightly eliminated the hum.

      weird.
      
      

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      • Hmmmmm... - jneutron 15:26:09 03/26/04 (1)
        In Reply to: Nope, not the amp. posted by Bruce from DC on March 24, 2004 at 08:38:41:

        I know, I know..keep my day job...:-)

        Two possibilities exist..Either e-field pickup, or m field.

        E field would require very large fields to couple enough to make it audible..So I would rule that out.

        So, where is the M field?

        In the wall wires are usually parallel, non twisted, so they radiate a dipole field...strongest nearer the wire.

        The speaker has internal loops, like the woofer litz wires. But you said moving the wire changed it.

        The only way m field can couple to produce hum is if the amp impedance at the output terminals is not too high...that completes a loop...

        You're speakers must be pretty efficient..

        Do you have more than one speaker connected to the terminal?

        With the wire in the place position to produce the most hum, get an old speaker magnet or a good piece of magnetic iron...or even a good horshoe type magnet, like the ones to pick boat motors from the lake..

        Use the iron or whatever, to enhance the field near the wire...run the iron along the wire, listening for louder hum...If that happens, the source of the field is there.

        If you twist the speaker run tightly, it should stop hum pickup.

        I'm dyin to know what you find.

        BTW, I'd have seen this interesting problem earlier if ya posted on prophead..I don't normally look here..

        Cheers, John
        
        

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        • Re: Hmmmmm... - Bruce from DC 16:05:59 03/26/04 (0)
          In Reply to: Hmmmmm... posted by jneutron on March 26, 2004 at 15:26:09:

          There's no house wiring in the wall -- this is an outside wall. There's a wire under the floor that connects to a floor outlet. But it's gotta be 12 to 18 inches below the speaker wire, maybe more.

          Stupid question: if there's no current flowing through that house wire, would it still produce a magnetic field? I thought it wouldn't. The wire I'm thinking of terminates in this outlet; connected to the outlet is an incandescent lamp that is usually off.

          Amp is a little Krell integrated, probably pretty low output impedence.

          Speakers are Joseph RM-7 si's, not particularly efficient -- 86dB IIRC.

          I'll check for any voltage potential to ground from the radiator. Seems odd that there could be any; everything is tied together in the system to a water inlet pipe.

          Stay tuned . . .
          
          

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      • You must have gremlins.... - Rod M 17:49:16 03/24/04 (0)
        In Reply to: Nope, not the amp. posted by Bruce from DC on March 24, 2004 at 08:38:41:

        Does changing cables reslove it?

        This is crazy, but what if you ground the radiator to the system. Also, what if you mive the speakers out in the middle of the room so that the cables aren't near anything. And then try suspending them. Get the girls to hold them up. Is there power in the floor?
        
        
        

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