Noise from power supply - help pls

Discussion in 'The Projects Forum' started by LaZyLuke, Apr 5, 2010.

  1. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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    Hi.

    First I want to say that i have a car audio deck hooked up to my laptop as an audio system. Everything is fine when the laptop doesn't have the dc converter hooked to it. But when I plug it in to charge the laptop a background noise shows up in the speakers (humming noise- goes louder when power supply heats up)- so i thought the dc converter is not working right- I got one that have an option to hook it up to 115V of wall socket or a 12V DC (car cigarette lighter)- the noise dissappears from the audio when I use the PC PS to provide the 12V DC (the same PS that the radio is hooked up to)- however then a "noise" shows up on my external monitor thats hooked up to the wall.... it shows up as horizontal lines moving vertically through the screen - like a ripple... not quite sure what to do.... any ideas? Please let me know as its really frustrating...

    I tried putting the magnetic ring on the audio cable before as SAYAL staff advised, but it didn't work- same ring is on the power cable for the laptop....
     
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    It's probably a Ground Loop ('hum loop').

    As long as only one item (ie. the amp) is connected to something external, the only signal on the audio cable is the audio you want.

    Once you connect the laptop to power as well, you have the possibility of externally induced currents, which get superimposed on the audio.

    This is commonest with two seperate items that both have AC supply grounds, but can happen due to capacitive coupling within power supplies etc.

    The simplest way to remove the problem is probably to use an audio transformer in the audio cable to the amp.

    More info on the problem:
    http://www.aqdi.com/groundloop.htm

    Audio isolating transformers at Radio Shack:
    http://www.radioshack.com/search/index.jsp?kwCatId=&kw=ground isolator&origkw=ground isolator&sr=1
     
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  3. Engr

    Member

    Mar 17, 2010
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    Is it possible to eliminate the noise by just plugging the supply of both components to a same socket? If they have a same ground plane, I think it can eliminate the noise.
     
  4. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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    I have tried that already....

    rjenkins thnx for answer

    not sure what to do still since the "amp" is the cd deck which only uses mini jack connection (conversion from mini jack to RCA and RCA to mini jack?)... beside with that noise canceled device would I loose quality? (that's the whole reason for running this setup)

    P.s.
    I found this one eBay:

    http://cgi.ebay.ca/PAC-SNI-1-3-5-GR...emQQptZLH_DefaultDomain_0?hash=item3efe43e9ec

    but the build quality is not really promising (most expensive eBay has to offer)
     
    Last edited: Apr 6, 2010
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    It's not noise cancelling, it's just coupling transformers that eliminate the direct connection between the two items, so there cannot be a ground loop.

    The one on ebay looks worth a try, I'd not worry about the casing as the specs look fine.

    I generally use modem transformers (like off scrap 56K PC modem boards). These have extremely good frequency response and low distortion. It's quite possible these are also used in the commercial units as they are very cheap and excellent qualty.
     
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  6. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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    Can you possibly make a picture of one from the modem board? (or describe how it looks? - I work at the lab in the university and they throw out bunch of old computer equipment all the time- i might be able to scavenge something....
     
  7. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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  8. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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    k so i hooked the laptop converter to the PS, and lifted the ground from the monitor- as far as i know ground is to there to protect the device from surges - is there anything else I should worry about when the ground is lifted?
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    If the problem is resolved by lifting the ground, you have proven it is a ground problem. Now you can get a ground isolating device to decouple the laptop and keep the safety of the ground.
     
  10. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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    yes, that would be correct resolution if the ground loop isolators were reasonably priced- and they still maintain a bit of the noise as far as the reviews I read online are correct.... the device im talking about is some humx product that costs $70 - now I wont mind paying that much if it was some fancy electronic - however it appears to be fairly simple build so the item is way overpriced.... if you can advice me a source that I can find it for a fair price I wont mind buying it.... thanks


     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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  12. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    I have used them on studio instruments and amps with great results.. But I cannot say for SURE it was ground loop hum and not AC line noise. But one of the bass amps I use has a ground-lift option that did kill the noise in the amp, and after I used the isobar, I didn't need the ground lift anymore.
     
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  14. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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    Ok, thank you for the info- I will try it regardless since this is defiantly worth the price looking at all the reviews and sturdiness of the item. Thanks!
     
  15. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    ever wonder to find out how the connections are made by the OP?
     
  16. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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    would be nice to know....
     
  17. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    tell us or show us how you made the connections.
     
  18. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
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    I thought you were going to enlighten us ...
     
  19. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I'll like to know if it is you termination that is producing the hum
     
  20. retched

    AAC Fanatic!

    Dec 5, 2009
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    Rifaa was asking to see your (LaZyLuke) wiring layout for your components. If your connections are improperly terminated, you can be picking up much interference.
     
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