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Audio device ground hum

Discussion in 'Audio' started by ikodel, Jun 9, 2015.

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  1. ikodel

    ikodel

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    Jun 9, 2015
    Hi everyone just joined.

    Ive done a lot of troubleshooting into a problem I have with a hardware sampler audio device picking up mains ground noise.

    I would really appreciate specific information for the question below rather than troubleshooting steps.

    Would a capacitor and resistor inline on a ground wire for an audio device be sufficient to filter noise on the circuit that cause ground hum on the audio device?

    If not what could fix this for a single device at the power source not the audio source?

    Removing the ground wire in the plug solves the problem but I dont want to do this for safety reasons.

    Any help really appreciated.

    Thanks.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
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    Nov 17, 2011
    Welcome to electronicspoint.

    A resistor in the ground line is not a good idea as ground typically needs a low impedance to work properly. With a high impedance any curent through ground will create a voltage drop which in turn will create audible distortions.
    In addition, designing a filter for 50 Hz that leaves audio above 50 Hz intact requires considerably more effort than a simple first order RC filter.

    Sometimes it helps to simply turn the mains plug of one of the devices by 180° in the socket.
    If not, an audio transformer will isolate the two ground potentials from each other while allowing the signal to cross. This is a well proven method for suppressing mains hum.
     
  3. ikodel

    ikodel

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    Jun 9, 2015
    Thank you.

    I'm trying to avoid anything that works at the audio source as I have multiple audio outputs from the sampler and costs start to mount. Don't have a great deal to buy multiple specialist devices.

    Anything I can make to treat the incoming power besides removing ground completely?

    Thanks for your help.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
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    Nov 17, 2011
    If you don't want to add audio transformers on the audio side, add an isolation transformer on the power supply side.
    The backside is that you may still need to connect the chassis of the audio input device to protective earth for safety reasons. If the chassis is electrivally connected to the internal ground, this may again couple hum into the system.

    A safe way is to use a completely isolated external power supply at the low voltage level used internally by the audio input device. If the audio device has no low power voltage input (e.g. a 12V plug), this will require major modifications - if possible at all (depends on the power supply circuitry within the audio device).
    If the sampler could be operated from batteries that would be ideal: no connection to mains, no mains hum.
     
  5. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi ikodel,
    I had the same problem with my mobile disco. Laptop chargers would hum and drive me mad. Disconnecting them fixed it, but for obvious reasons, I couldn't run the laptops all night on batteries.
    I found that these worked beautifully. I bought two and inline from laptops output to mixer inputs. A lot chaps use these for car audio systems too.
    I don't know if you are an audiophile, but playing my system at 6k watts sounds great.
    So a simple £10 Ground loop isolator worked for me.

    [​IMG]
     
  6. ikodel

    ikodel

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    Jun 9, 2015
    The sampler is the size of an old server, about 4U rack height so definitely doesn't take batteries.

    What you have said about an isolated circuit...

    I have a few variable voltage transformers that use two prongs on the mains.

    If I can match the same volts and amps that the sampler is fed by it's internal PSU, are you saying that a transformer wired to the sampler would prevent this hum?
     
    Last edited: Jun 9, 2015
  7. ikodel

    ikodel

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    0
    Jun 9, 2015
    I would need a few of these hence why Im trying to solve the problem on the power rather than audio side in order to save costs.
     
    Last edited: Jun 9, 2015
  8. Martaine2005

    Martaine2005

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    May 12, 2015
    This is probably a really stupid question...
    But some high end audio equipment have a GROUND LIFT switch for that exact reason.
    But I am not sure if safety regulations still allow this.
     
  9. ikodel

    ikodel

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    Jun 9, 2015
    Not stupid, but unfortunately not in this sampler.

    Its weird that only this device out of all my synths causes this squeel noise when hooked up to the audio interface.

    Funnier still, when I plug the sampler into the inputs of say one of my synths that can take a sound source and add a nice vocoder effect to it, no squeal.

    Its some sort of problem between the EMU E6400 sampler and the M-Audio Firewire 1814 interface.
     
  10. Martaine2005

    Martaine2005

    2,713
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    May 12, 2015
    I presume the unit is rack mounted?
    If it is, is it mounted in the same rack with other equipment? Could try rubber bushes to isolate chassis from rack.

    I also presume that all your cables are fairly decent and shielded? Have you swapped them around?

    Have you tried both BALANCED and UNBALANCED? Some people immediately use balanced.

    Have you tried a firewire to usb? Sorry, don't know how you have your connections.

    Have you tried the firewire interface and sampler on a different computer?

    I do suspect that this is much simpler than you think.

    I wouldn't use a two terminal supply (isolated) for your sampler either. A fault could potentially leave the chassis live.

    I apologise for the trivial questions but even experienced sound engineers over look the obvious.
    EG; Nah, can't be the lead it's brand new..
    I once had a situation with 2U power panel that had a flickering neon light. It caused all sorts of noise issues.
    We cut out the neon and problem solved. Now that is a situation I cannot explain..

    Sorry to bleat on.
    Martin.
     
  11. ikodel

    ikodel

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    0
    Jun 9, 2015
    Thanks for your reply.

    Yes everything is rack mounted in the same rack bar the audio interface as it is not rack mountable so just sits on my work table.

    Ive tried balanced and unbalanced cables. Initially thought that the problem to be the sampler having balanced outs and the interface unbalanced ins, but found the interface to actually have two balanced ins also but the problem persisted when using them.

    I've spent a frustrating evening dismantling my set up, removing the sampler from the rack and laying it and the audio interface on the floor.

    Lo and behold, no more noise.

    The sampler was racked improperly. Not using rack ears but a metal rack cage (slides on to sampler, but has no bottom so screws hold it to the device).

    It was not a good fit, only using two screws to hold it to the sampler. I just wonder if this was acting as a kind of noise antenna.

    First purchase is proper rack ears, but I may need to purchase a new audio interface as I seem to have blown two of the analogue inputs (any ideas how to replace ADCs?).

    I don't know how though but I did accidently unplug the FireWire cable when the computer and the unit were both powered on and the manufacturers explicitly stated not to do this. Though this was to prevent blowing Firewire ports on either computer or device.

    So £250 I hadn't planned on spending.

    Then again that would be on a completely different device with better ADCs (there are many reports that the M-Audio 1814 has poor converters but not sure my ears are good enough to tell).

    I could still use the device with the remaining 6 analogue ins and substitute the dead pair for digital SPDIF but now I'm worried when the others may go and I really must get on with making some music.

    Anyway..I digress A LOT.

    Thanks again for your replies.
     
    Last edited: Jun 10, 2015
  12. Martaine2005

    Martaine2005

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    May 12, 2015
    Are we saying it was something simple???
    Gosh, what a surprise.
    Check the obvious first....
    Martin.
     
    Arouse1973 likes this.
  13. Martaine2005

    Martaine2005

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    May 12, 2015
    Somebody will be along to assist you..
     
  14. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Replacing ADCs will require you to disassemble the unit and identify the component first.
    Hopefully it's a simple component swap, but we can say for sure without a service manual, or some detailed pictures of the board.

    As far as hum is concerned, it's usually picked up one of two ways:
    -Electrical noise. (ie. poor shielding or cables)
    -Current flow. (This usually happens when the equipment is either poorly grounded, or on different AC circuits. The 'ground' of one unit is not the same as the 'ground' of another, so current ends up flowing along the signal wires to correct the imbalance. The item posted by Martaine2005 works extremely well for correcting this and is ONLY required on the signal line(s) that have the current flow.)
    In addition to Maraine's ground loop isolator, and your troubleshooting it usually best to try to isolate which signal wire the hum is coming from.
    For example: 3 sources, a mixer and an amplifier. If all 3 sources have the hum, assume the issue is the signal path between the mixer and amplifier and verify. It's also incredibly handy to use a cell phone or MP3 player as a signal injector. Unlike other audio equipment these things have no ground, and are not connected to mains ;) This means they are more reliable for injecting audio in without risk of causing hum from current flow. You could plug an mp3 player directly to the amp, and if the hum continues, troubleshoot the amp. If it stops, the issue is upstream (mixer, or one or more sources)
    Best of luck in your future endeavours, and if you're feeling brave get us some details on the unit model#s and a service manual. (Or detailed pictures) We can take a look and determine what you will need to replace, but also what else might have gone.
     
    Martaine2005 likes this.
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