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  1. #1
    Junior Member
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    How to remove noise from DC line?

    I decided to remove the 5v regulator that powers the DAC chip on my squeezebox and use the 5v line off my ELPAC power supply to power the sb and the DAC at the same time. When I did this, I have a bad humming noise from the speakers, but the sound has improved. What do I do to eliminate this problem? Should I use a DC line filter? Thanks for your time in this matter.

  2. #2
    Senior Member
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    The problem is most likely a ground loop. You need to make sure that each component connects to the ground terminal of the supply, not to a common wire that runs to the ground terminal.

    TD

  3. #3
    Junior Member
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    I tried your suggestion, but I still get the hum. When I use a separate regulator, then the hum goes away completely. Thanks for your help in this matter.

  4. #4
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    Run separate power and ground lines for each device back to the terminals on the power supply. Sharing either the power or ground lead can result in hum due to the voltage drop down the length of each wire.

    Take a look at Rane's web site for app notes on grounding in audio systems. Lots of good info there.

    TD

  5. #5
    Senior Member
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    Is the hum that you are hearing at 50/60Hz? I'm assuming that the ELPAC PSU is supposed to be a highly regulated and quiet unit. If it is 50/60 then you may have a difficulty with the PSU.

    One thing that you may need to look out for is the difference between a digital earth and an analogue earth. They are not always the same.

    I don't follow the comment about a separate regulator. Do you mean that if you use an external regulator between the ELPAC and the SB then the hum goes away. If this is true then two things strike me. The ELPAC would have to be variable voltage otherwise there isn't enough voltage to run a regulator. If it is variable voltage and you are running it higher to drive the regulator then it isn't surprising that the hum is reduced since the hum is a varying voltage on a line which is being removed by the regulator doing it's job. This could also indicate that the ELPAC doesn't have good output regulation and in which case I wouldn't use it in that configuration. The regulator would also change the source impedence as well as possibly contain various filters/capacitors.

    Designing analogue circuits and clean power to provide good sound reproduction is highly skilled and subject to lots of compromises and sometimes you have to do things which seem counter intuitive to make them work.

    By the way if the ELPAC does have poor output regulation then the reason that it may be sounding better is that you could be feeding the SB with a higher voltage essentially making the output louder.

  6. #6
    Junior Member
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    The Elpac power supply has 5v 2a and 12v .5a outputs. The voltage rated at 5.12 isn't any higher than the stock sb power supply. When I run a 5v regulator off the 12v line bypassing the switch supply, the sound is much cleaner and the hum is nonexistent. When I attempt to run the 5v supply to power both the dac and the unit, the hum is there. My plan is to run a 5v and 3.3v regulator off the 12v line. Is there enough current to power the regulators. One is rated for 50ma 3.3v while the other is 500ma 5v. From looking off the datasheet, the PCM1748ke dac has a limit current of +/- 10ma current.

  7. #7
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    That explains why the regulator is quieter. The 5V line doesn't seem to be particularly clean. At 12V the ripple (which is translated into hum) is so far above the regulator threshold that it is ignored.

    Watch out for your power dissipation in dropping 12V down to 5V. Don't use a regulator too close to the current level limits as they are degraded at high voltage differences. I'd also suggest running the 3.3V off of the 5V you just produced as it will stress the 3.3V regulator less.

  8. #8
    Junior Member
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    I'm sorry to reopen this thread but I too am having a hum problem and don't know how to solve it. When I connect the SB receiver directly to a DC power source, (ie battery) the hum goes away but if I use the AC/DC converter, I ust can't seem to make the hum go away. I've used a ground lifter on the amp and tried reversing the AC plug polarity. This is happening on each SB receiver I own so it's not a defect.

    Is there anything I can but to fix this? Do they make something that lifts the ground at the RCA jacks? Or something I can plug the AC/DC converter into?

  9. #9
    Senior Member iPhone's Avatar
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    Quote Originally Posted by mskvarenina View Post
    I'm sorry to reopen this thread but I too am having a hum problem and don't know how to solve it. When I connect the SB receiver directly to a DC power source, (ie battery) the hum goes away but if I use the AC/DC converter, I ust can't seem to make the hum go away. I've used a ground lifter on the amp and tried reversing the AC plug polarity. This is happening on each SB receiver I own so it's not a defect.

    Is there anything I can but to fix this? Do they make something that lifts the ground at the RCA jacks? Or something I can plug the AC/DC converter into?
    Sounds like you have a ground loop in your system. When you use a battery, there is no path going back to a wall outlet. When you plug the OEM (I am assuming we are talking OEM and not after market SB3 PS) wall wart in, that provides a path for the loop in your system. Many times this issue shows up with how equipment is connected and from where its powered from. So if the SB3 is connected to a pre-amp, it could be the Pre-amp or anything connected to it. A simple 120/120 isolation transformer should eliminate the problem or finding what is causing the loop (if that is what it is) is the best solution.

    Could you list your audio setup? Also if you have a separate amp (not a Receiver or Integrated Amp) you could run the amp directly off the SB3 to see if that eliminates the hum. Make sure to turn the SB3 volume all the way down on the unit and as well in SC before attempting this test. You will be controlling the volume with the SB3 remote.
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  10. #10
    Senior Member Mnyb's Avatar
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    Speaking of ground loops the most common one is cable-tv, try to unplug the cable ? I have to use isolation transformers my cable for mine as most of hifi has real ground conections

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