Changing ground reference of general electronic devices.

Discussion in 'General Electronics Chat' started by coinmaster, Feb 29, 2016.

  1. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    So I have a dedicated DAC that I want to plug into one of my amplifiers but I want to direct couple it.
    The input of the amplifier is referenced at -220v.
    The DAC uses opamp servos to eliminate the DC offset at its output so what happens if I connect the ground pin of the DAC to a -220v supply?
    This should effectively shift the ground reference to match the input bias voltage of the amplifier but I wonder is there some potential danger to this?
    The DAC is quite expensive and I do not want to attempt this unless I am absolutely sure.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    I see much smoke in your future, along with some spitzen und sparken.
     
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  3. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Can you explain why?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    It's virtually impossible to evaluate what you are doing without seeing a schematic of it.
     
  5. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    There is no schematic. I'm asking if there is a danger to changing the ground reference of an electronic device from wall ground to -220v by connecting a -220v power supply to the ground pin on the power plug.

    This should allow the dc offset at the output to be referenced to the new ground without blowing up.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    I feel reasonably certain that this is a bad idea. Others have taken a more nuanced approach. I guess you should just try it out and see.

    Full disclosure: I really don't care what happens to the DAC, but I bet you do.

    If you want a definitive answer I would get busy and work up a schematic of the DAC output stage and the amplifier input stage.
     
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  7. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    I was hoping for a more comforting answer:p
     
  8. WBahn

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    Mar 31, 2012
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    Surely you could sketch something up!

    Remember that "ground reference" can mean different things. You always get to pick an arbitrary node and call it whatever voltage turns your fancy and use it as a reference. Is that the kind of "ground reference" you are talking about? Or are you talking about an equipment earth ground? Or are you talking about the neutral wire? Or what?

    A sketch sure goes a long ways.
     
  9. Sensacell

    Well-Known Member

    Jun 19, 2012
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    While it's possible to "float" equipment grounds at a high voltage level, it's a dangerous and highly undesirable approach.

    If you want a real solution, you need to fully explain and document what you are trying to accomplish, guessing is really boring and unproductive.
     
  10. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    This is the closest thing to a schematic I can give
    http://www.audio-gd.com/Pro/dac/NFB12015/NFB12015EN.htm

    So it uses opamps to eliminate DC at the output aka direct coupling. The problem is, I want to direct couple it to one of my amplifiers that has -220v bias voltage on the input.

    Normally the DAC would be plugged into the wall and it would use the ground reference of the wall to determine the ground reference point of the DAC circuitry and to determine what 0vDC is for the opamps that are eliminating the DC at the output.

    What I want to know is, if I reference the ground pin of the power plug to -220v instead of the ground outlet of the wall will I regret it?
     
  11. WBahn

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    Mar 31, 2012
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    Which pin are YOU referring to when you talk about "the ground pin"?

    Is this a three-prong plug?

    The answer depends on the details of the circuitry -- but I'd be willing to bet that you would let the magic smoke out of something (and risk getting tangled with HV circuitry yourself in the process).
     
  12. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Yeah, a 3 prong plug.
    Can you throw me some potential reasons why/how this would happen? From my understanding the dac should not be able to tell the difference.
     
  13. WBahn

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    So a three prong (single phase) plug has a hot wire, a neutral wire, and a ground wire. Which wires are you planning to change and apply -220 V to? And is -220 V DC? Relative to what?
     
  14. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Ground
    Wall ground. DC voltage.
    The amplifier is still referenced to wall ground so the -220v input bias I'm trying to couple the DAC to is -220v below wall ground.
    What I want to do is make them 0v to each other.
     
  15. WBahn

    Moderator

    Mar 31, 2012
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    In almost all applications, the neutral and the ground wire are expected to be at or very close to the same potential. So if you go and try to have the (safety) ground of your DAC box be 200 V below the it's neutral, then you are likely to get fireworks (or worse, since the safety ground if often tied to the chassis which could allow you to get tangled with it).

    A quick test you could do would be to just take an ohmmeter and see what the resistance is between the neutral and the ground prongs on the plug. It may or may not be informative. Again, whether this is even conceivable depends on the circuits in the boxes -- there is NO way that we can possible determine that it is possible and/or safe without those details.
     
  16. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Wait I thought neutral and hot were near the same potential? Aren't they just the entry/exit point for the AC voltage while the ground pin sets the ground reference point for the circuit?

    Wait I'm starting to see my folly, if hot and neutral were the same then there would be no potential. What is the purpose of ground vs neutral?
     
  17. Sensacell

    Well-Known Member

    Jun 19, 2012
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    We are talking about audio signals, correct?

    Capacitor or transformer coupling is typically utilized to eliminate inter-stage DC voltage offsets.
     
  18. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Those are the things I'm trying to avoid.
     
  19. WBahn

    Moderator

    Mar 31, 2012
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    The ground (often called the "safety" ground for a reason) is a wire that is not intended to carry any current and that goes back to an earthed reference (I'm talking U.S., the details are a bit different in other places). The neutral also goes back to an earthed reference (at the service entry point -- essentially the breaker panel). The hot wire is a voltage relative to the neutral and the load current flows in the hot and the neutral conductors. Depending on the appliance, the safety ground is connected to the equipment chassis so that if it ever comes in contact with the hot wire, directly or indirectly, the chassis will be maintained at near-earth potential. This often results in enough current flowing in the hot-ground path to pop the breaker.
     
  20. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Ah okay, that clears up a lot. So I guess I would connect the -220v to the neutral pin and I should be okay leaving the ground pin on ground right? Assuming I were to do this.
     
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