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Static hiss... help!

Discussion in 'AV Receivers & Amplifiers' started by BrowneR, Dec 28, 2004.

  1. BrowneR

    BrowneR
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    I have just purchased a basic (£20) amp from ebay. It is an old 30W stereo amp branded Tensai (possibly from the 70's.)

    I have made up a cable with a 3.5mm jack plug leading to two RCA plugs which i use to connect it to my PC soundcard. The jack plug obviously had one common "ground" terminal and one other for each channel. This leads me on to some interesting interference i've been noticing...

    When I switch on I hear hiss (sounds a bit like 50Hz mains) even on the very lowest volume setting. The problem is intermittent and after a while usually there is a click and it is gone… for a while.

    I thought it may be static so I earthed the unit (it only had a two core plug). The problem remains.

    The interesting thing is that if I take the “neutral” terminal on the 3.5mm jack plug (or one of the others) and touch it on the case the hiss goes instantly. I don’t understand why this is?

    Can anyone else understand why earthing one of the inputs to the case removes the interference? I have tried removing all the cables form the amp (except speakers) but the noise remains so I can rule them out. Maybe a dry joint somewhere?

    Any help/ideas would be much appreciated as the amp works fine otherwise and sounds ok when the hiss has decided to stop. I'm not afraid to bodge something either....
     
  2. BrowneR

    BrowneR
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    come on someone round here must know something...
     
  3. Tejstar

    Tejstar
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    Sorry this is over my head, maybe somebody else can help? :confused:
     
  4. Andy Dee

    Andy Dee
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    Sometimes you get an earth loop, could it possibly be this?

    I used to get this a lot with setting up DJ equipment and speakers. We used to make sure that only one mains fired unit in the Rack was earthed.
    For example..

    If the amps were earthed then we would remove the earth wire from the Mixer (undo the plug and disconnect the earth wire.)



    I think this is quite common practice, however...use with caution!
     
  5. Andy Dee

    Andy Dee
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    Heres the idea.

    "First try plugging the units in to the mains side by side (in the same double wall socket or power strip). This often eliminates the hum, but if not, I can only relate what I would then do myself - I am not recommending it to anybody else because my insurance forbids it.
    The simplest solution is to remove the mains earth (disconnect the green/yellow earth wire inside the plug) from one of the units. Make damned sure that wire can't touch any other pin - insulate it very carefully or cut it off very short, unless you want to meet your maker when you next touch the unit. This should stop the hum. If there are several units in your system, each earthed, you may have to repeat this until the hum stops. It is worth checking with a "Continuity checker", or "Ohm-meter" that all units are still earthed via the one remaining earth, which you must retain for safety.
    I repeat again, I am not recommending this procedure, it's simply what I do myself in such a situation.
    The other answer is to employ a "balanced" signal interconnection between the units, obviously only possible where both units have true balanced XLR sockets, as this doesn't rely on linking earths between the units, thereby breaking the dreaded loop"



    Taken form here
    http://www.walrus.co.uk/tech/hum.htm
     
  6. BrowneR

    BrowneR
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    Thanks for all your suggestions people!

    However as the amp is used on its own (no recievers, decks, mixers etc) and wired to my pc (over 2m away) and still suffers the hiss when every other electronic item in the room is unplugged this rules out any interfearance from other devices. Also as the hiss was there before i earthed the unit (i earthed it to try and remove the hiss thinking it was static on the case?) i dont think it is caused by the ground loop either.

    The thing that worries me most is how it comes and goes, so sometimes it will be silent and then 5min later there will be a click from the speakers and the hiss returns.

    I'm going to get the case off and see if removing all the dust and dirt will do the trick or at least expose a problem...
     
  7. BrowneR

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    given the unit a good clean with a paint brush and hoover :)

    plugged it in and thought i'd solved it... but no it wasnt long before it came back.

    so i just soldered a wire running from the ground terminal on the phono inputs to the case (earth) and that has removed the hiss - no idea why but it works

    if anyone has some insight into why this works i'd love to hear it :)

    ...oh and thanks for all your help!
     
  8. BrowneR

    BrowneR
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    one more thing thats not connected but i dont want to start another thread...

    I am using 4ohm speakers with the amp which is designed for 8ohm versions. I persume this will pull more current and so stress the unit? i was thinking of putting a 4ohm resistor in series but i'm not sure if thats a good idea - is there any point?
     

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