Another annoying hum thread

Discussion in 'Hi-Fi Stereo Systems & Separates' started by Ugg10, May 4, 2018.


    1. Ugg10

      Ugg10
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      So, I rearranged my system and added a second sub woofer and now i have an annoying hum from both subs (wharfedale sw150).

      here's what I know -

      1) using a mobile app the hum is around 200hz
      2) it is still there if I disconnect the line-in to the subs from the preamp
      3) it is on both subs and does not change when either is turned off
      4) subs are connected to two different wall sockets
      5) I have plugged the subs into three different loops on the house, one only installed last year, no change
      6) when subs are turned off and the amps/main speakers are on, it is not there (same double plug as one of the subs)
      7) it is a constant hum not an occasional pop/click
      8) room is approx 10ft sq with the subs firing across the width
      9) the sound is constant irrespective of what volume the preamp is set at
      10) moved one of the subs to another socket/another room, no change, changing the volume or frequency on the rear of the sub makes no difference. the noise is not so noticeable so may be getting a bit of enhancement in the new location but it was still there.

      any ideas?

      Edit

      11) turned central heating, fridges, freezer off, no change
       
      Last edited: May 4, 2018
    2. Ugg10

      Ugg10
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      spectrum close to speaker

      [​IMG]
       
    3. Ken T

      Ken T
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      Are you saying that with nothing connected to the subs, save for electricity, you have a Hum?
       
    4. Ugg10

      Ugg10
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      Yep. That’s correct and it is on both subs. So it is either a mains hum or a “feature” of these subs. Whichever way it is annoying.

      A quick calculation give the length and width resonant frequencies of my room just below 50hz so if there is a light sensitivity to mains then this may be emphasised by the room now I have the subs firing across it rather than under my desk.

      and walking about the room there is a definite null about in the center of the room equidistant from the subs.
       
      Last edited: May 4, 2018
    5. Ken T

      Ken T
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      short the phono inputs and try again.
      By short I mean get a phono cable - insert one end into the phono/line inputs of the speakers, and use a bare wire to connect the inside connector and outside connector (short) of the other ends together.
       
    6. Ugg10

      Ugg10
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      thanks ken, with the sub on or off ? the sub has left/right in and out, do i do it to all ?
       
    7. Ken T

      Ken T
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      No no no.
      I meant plug the cable into the sub - no amp , no second sub (switch all else off).
      I just want to make sure the sub is OK.
      On the sub, plug a stereo interconnect into both left and right, and short both other ends.
      centre to outer.
       
    8. Ugg10

      Ugg10
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      Thanks, yes, got that bit, sub isolated from system just with rca cable hanging out from the back, the subs are active and can be on or off, I am guessing if they were on then you would get feedback/crackle, was thinking this was a way to discharge a capacitor or something that may need it.

      so sub plugged in and switched on or not whilst I make the contact ?

      For info I can do a frequency/discrete frequencies sweep from a spotify playlist with the balance left or right (main speakers disconnected, balance on pc and preamp) and all seems OK, both subs go from 30hz up and the volume knobs on both work fine. So basically they work fine other than the hum.
       
    9. Ken T

      Ken T
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      First short one ends, then plug in to sub.
      on or off, makes no difference.
       
    10. Ugg10

      Ugg10
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      No ghosts. :clap:

      tried what you suggested, no noise when the plugs are shorted out. Is this right ?

      What should have I been listening out for ?

      As i am using some stereo cables for each sub I have put insulating tape over the loose ends, no change. Will be ordering some mono rca cables now I know how long they need to be.
       
    11. Ken T

      Ken T
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      So the speakers are good.
      Now try to connect the interconnects (no shorts) to the amp pre-amp outputs to one sub only.
      With both amp and sub switched on, and nothing playing, no speaker cables connected to the subs, see if there is any hum.
      Remember, amp on, sub on, nothing playing, only phono interconnects between amp pre-amp outputs to sub line inputs.
       
    12. Ugg10

      Ugg10
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      I only have rca cables plugged into the left-in on each sub form the respective l/r out from the preamp. Preamp has two outputs, makes no difference which I used for the poweramps and the subs.

      Couple of observation whilst trying what you suggested.

      Power amps off - pre amp on - left/right/no connection to each sub checked seperately - hum does not change on either speaker.

      Right does hum slightly more than the left but is not affected by volume control on sub. I could just about cope with the left hum but it is intrusive when both are on.

      Right sub is enhanced when I push it back into the alcove where it normally sits.

      [​IMG]

      Most interestingly, with the poweramps on, pre amp on (no sound), sub on - when i turn the sub off, 15s ish later I get a sound like a constipated dolphin out of the main speaker for about 2s. This happens even with the preamp off and is louder with that off ??????

      So,looks like the left hand sub is probably the normal level of hum but the right seems to have an issue somewhere , thoughts ?
       
    13. Ugg10

      Ugg10
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      just to add, change the poweramps over left to right and the dolphin follows the power amp ? and does not happen with the other sub.
       
    14. Ken T

      Ken T
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      looks like bad RCA cables!
      the earth wire is loose
       
    15. Ugg10

      Ugg10
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      many thanks, ken, was going to order some new ones so will get onto that right away.
       
    16. StanleyKubrick

      StanleyKubrick
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      Amazon Basics...£9.99 for 15m (more expensive for shorter lol)
       

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