Speaker hum - potentially a ground loop problem?

Discussion in 'The Projects Forum' started by Soundfish, Apr 5, 2012.

  1. Soundfish

    Thread Starter New Member

    Apr 5, 2012
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    I have an issue with a loud buzz or hum coming from a speaker and am searching for a component that can reduce this noise.

    There is mains power to an ac/dc converter powering the amplifier. Audio input to the amp is by an mp3 player that is also powered by the mains. When the ac/dc converter is plugged into the mains we get a loud buzzing from the speakers. This is fixed by powering the amp with a battery but this isn't ideal as we need constant power. However, if the mp3 player is plugged into the mains we still get the noise when the amp is powered by the battery. I think this might be a ground loop problem, have seen a product called hum x advertised but this is only available for us mains power. So am looking for a solution for the uk. Have seen ground loop isolators with red and white phono plugs but this would only work between the mp3 player and the amp for the audio input, can't find a device that would work for the ac/dc adapter.

    The specs are as follows:
    Ac/dc adapter: laptop charger 16v, 3.5A, 56W
    Amplifier: Kemo 40W, 16V, 4-8 ohms http://www.maplin.co.uk/40w-power-amplifier-module-37737
    Mp3 player: plugged into usb adapter to mains for charge
    Speaker: UW30. 30W 8 ohms http://www.lubell.com/UW30.html

    Please can you advise any solutions to this problem?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I can provide a solution if u show me what I need to see.

    I need pictures of ur connection and the inter connect cables.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    First thing to check might be that the supply you are using has proper isolation from the mains. If not, it is not only a nuisance but potentially dangerous.

    Then check that the adapter is decently smoothed when not delivering much current. If it is designed mainly as a charger for a laptop, it may not be much good when only delivering the idle current of an amplifier when silent or at least not playing loud.

    Next, you could check that you have connected the screen ( or "cold" or "common" ) lead correctly at both ends of the link from the player to the amp. Are you using good quality screened cable, no longer than it needs to be? Are all the connectors good?

    After that, you might need to see that there is no shared current path between the power and the signal, particularly that DC is not passing in the "cold" side of the audio link.

    This kind of problem can be really challenging if you are dealing with microphone level signals of maybe a few mV, but frankly at half a volt or so should not be rocket science to fix unless you require really a really superbly low hum level, with speakers having superb bass response.

    Finally, at this point it occurs to me that you might be using a microphone level input on the amplifier with the CD output turned very low. Prefer to use something like a 600mV line input on the amplifier (if available), as the hum may then be relatively much lower.
     
    Last edited: Apr 5, 2012
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Update : I have looked at those speakers. My first reaction is that a laptop power supply should not be feeding anything designed for swimming pool use without some kind of additional grounding protection on the output. This is really a matter for a qualified electrician fully au fait with whatever version of regulations apply nowadays. Laptop supplies are typically floating isolated, which may not be good news for this.

    The second thing I have to say that if you are getting loud hum on these speakers with good response only down to 100Hz, then you really do have quite an issue here. Do you have access to an oscilloscope?

    Finally, I would second R!f@@'s request for diagrams, a picture really is worth 1000 words, as they say.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ajduster....Why do u even bother with words..?

    U know photo's cannot lie or be wrong.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I have a tendency to spray out many words, that is my way. Also, I am stuck somewhere I would very much rather not be, and time hangs heavy.

    Realising that this fellow is getting heavy hum on swimming pool speakers though, perhaps a strong safety message is justified. I do not like the idea of anyone not 100% familiar with electrical safety setting up such things.

    Frankly, if I were at home my first response might have been a lot shorter: unless the OP favours us with a reply, I think this has gone as far as it can.
     
    Last edited: Apr 5, 2012
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I know..or it could be poorly shielded cables.
     
  8. Soundfish

    Thread Starter New Member

    Apr 5, 2012
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    Hi there,

    Sorry I didn't upload my photo and circuit diagram yesterday, I did try when I first posted but my internet connection was having issues (I am currently based at a remote fish farm in Scotland). Hopefully it will attach this time.. *** Sorry it won't let me attach anything.

    Sorry for my ignorance with this set up.. I am a zoology PhD student so need to use electronics for my playback work with fish, unfortunately I don't have much background in this area! Any comments at all are gratefully received!

    Let me try and address your suggestions:

    1) Power supply having proper isolation from the mains.. please could you explain what this means? As we are in the UK we are using 240V mains power 3 pin plug. One of which is earth. There are 3 cables going into the adapter from the mains and only 2 cables coming out of the adapter and going to the amp. The audio input to the amp is separate to the power input from the adapter and also has 2 cables.

    2) Check that the adapter is decently smoothed.. would you mind explaining this a bit further? There is a ferrite on the cable between the amp and the PSU which I thought was for this purpose.

    3) Re the screen between the mp3 player and the amp.. the screen is connected to the sleeve terminal of a 3.5 mm mono jack at the mp3 player end and to the -ve in on the amp. NB the white wire from the PSU also connects to the -ve on the amp. Please take a look at the specs of the amp on this page for details of how its connected as I can't post a picture.. http://www.maplin.co.uk/40w-power-amplifier-module-37737. The connections all seems good. I'm not sure about the quality of the cable.. I also have a 10k log potentiometer between the mp3 player and the amp. Am about to try building the circuit without the potentiometer in.

    4) "After that, you might need to see that there is no shared current path between the power and the signal, particularly that DC is not passing in the "cold" side of the audio link." As I said, the -ve from the PSU connects to the amp where it reads -ve in. The screen from the mp3 player also connects to the same place.. could this allow shared current path?

    5) "I have looked at those speakers. My first reaction is that a laptop power supply should not be feeding anything designed for swimming pool use without some kind of additional grounding protection on the output. This is really a matter for a qualified electrician fully au fait with whatever version of regulations apply nowadays. Laptop supplies are typically floating isolated, which may not be good news for this." As I said I am a phd student working with these speakers in fish tanks, doing the best I can in this situation :). Can you offer any suggestion for how to add grounding protection on the output?

    6) I do have access to an oscilloscope.

    Sorry again I can't provide pictures.. but would welcome any further comments.

    Thanks for your help!
    Sophie
     
  9. Soundfish

    Thread Starter New Member

    Apr 5, 2012
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    Ah my attempt at a circuit diagram has uploaded! I hope this is of some use.
     
  10. Soundfish

    Thread Starter New Member

    Apr 5, 2012
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    By the way, my cables aren't shielded at all.. I am trying wrapping them in tin foil.. doesn't seem to have helped so far.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    The cable that feeds the input of an audio amplifier MUST be a shielded audio cable (not ordinary wires with tin foil). The shield will block the pickup of mains hum if the shield is connected to 0V of the amplifier.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U will need properly shielded cables to avoid noise pickup.
     
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