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Thread: problems with aux sound

  1. #11
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    That's what we call a ground loop. Or it may be - your description is a bit ambiguous, else I amn't awake.

    Funny - I was going to reply "So what ground did you cut?" to your previous
    Quote Originally Posted by settra View Post
    no its not a ground loop...
    The question I have is whether fuses/protection will work if you have a ground fault on your PC PSU, though I think it will.


    For those interested in inverter "earthing" (and dc-dc earthing since a dc-dc converter is also dc-ac but with its output converted back to dc), keep reading.
    Otherwise don't.


    Normally ground loops are fixed by breaking the signal ground - ie, breaking the (pair of) shields in an audio signal path.

    But it may be that what you have done is bond a noisy floating chassis to the vehicle ground. That's normally done by the signal cables to the PC (chassis being signal return/shield) or by adding bypass capacitor(s) from PSU chassis to vehicle ground.


    FYI - normally inverter AC outputs are totally isolated - ie, floating with respect to input DC +12V and ground.
    The inverter output AC "earth" (ground) should be connected to its L2 output. [ IE - to imitate domestic AC distribution with 3 pins (active aka hot or L1, neutral aka return or L2, earth or ground) where the neutral and earth are bonded together at the power-source end, the inverter output's L2 is joined to its output earth/ground. ]
    That can mean to totally floating AC system between the inverter and PC PSU.
    Or it could be vehicle-chassis connected if the PC has its PSU's chassis(/earth) joined to the motherboard ground (via audio or other connections to vehicle ground), or otherwise if the inverter's output AC earth is connected to the inverter chassis which then touches the vehicle ground.
    Easy eh?

    Unfortunately it isn't always simple or obvious. The only "definite" should be that inverter AC outputs L1 & L2 aka active & neutral are totally floating (isolated) from the DC input EXCEPT when deliberately wired otherwise - eg, by the user. (Or maybe authorities requiring output earth to inverter chassis, and then maybe output earth(/chassis) to output L2/neutral.)
    Hence the output AC might be floating which may be good from a safety point of view, but it means a floating AC chassis or antenna that might need bonding to ground.
    Ah, the fun and games.

    Otherwise an inverter is essentially the same as a dc-dc converter - they both convert to dc.
    But the dc-dc converter may have its fully internal ac shielded by its chassis which is itself connected to vehicle chassis on typical automotive dc-dc converters (by both input & output for negative ground vehicles; noting that on typical dc-dc converters, the outputs are NOT isolated from the input unless they use a separated 2-winding transformer instead of an inductor or auto-transformer; most simply use inductors).


    I'm going back to bed.

  2. #12
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    Quote Originally Posted by OldSpark View Post
    Otherwise an inverter is essentially the same as a dc-dc converter - they both convert to dc.
    But the dc-dc converter may have its fully internal ac shielded by its chassis which is itself connected to vehicle chassis on typical automotive dc-dc converters (by both input & output for negative ground vehicles; noting that on typical dc-dc converters, the outputs are NOT isolated from the input unless they use a separated 2-winding transformer instead of an inductor or auto-transformer; most simply use inductors).


    I'm going back to bed.
    hope you slept good (time to wake up!)


    while the principles of invertors, and dc-dc's are the same, the applications, and switching frequencies are different(as you know).. dc-dc's use a very high, switching freq. while most inverters are limited to a very-audible 60hz...

    i had a similar problem using a car subwoofer in the house using a pc psu. i had to ground the outside of the rca cable to the ground of the ac power supply to get rid of the hum that it had..

  3. #13
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    Oldspark dude.. Sry but i didint understood a s@#t from what you just discriped i dont even know what the dc-dc /ac-dc diferences have to do with that post:>

  4. #14
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    Quote Originally Posted by soundman98 View Post
    ... dc-dc's use a very high switching freq. while most inverters are limited to a very-audible 60hz...
    Not quite...
    Inverter output is mains frequency (50 or 60Hz), but internally they are high frequency.
    For example, the 1200VA inverter I once spec'd used 50kHz switching.

    Whereas dc-dc is say 12Vdc - ac - 20Vdc, inverters are 12Vdc - ac - 150Vdc which is then "stepped" out as maybe 80V, 120V,80V,0V,-80V etc for 120VAC.

    A true-sine inverter would convert to above 120VAC x root-2 (the peak voltage) = 170V (say 180Vdc) and then high-frequency modulate (PWM) to get the sinewave.
    (Not that I described the -ve half of the cycle.)

    So both use high frequency conversion.

    Even if inverters only had their mains output (50/60Hz), if stepped wave, it would have "infinite" harmonics - ie, frequencies of 3x, 5x, ... 157x, 159x ... the "fundamental" frequency (eg, 60Hz). [ Or is it 2x, 4x, etc - a square wave is a summation of all (even or odd) harmonics to infinity....

    Fun stuff eh?

    Hence why I say inverters = dc-dc converters.

  5. #15
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    ye ye, but all that stuff are still irelevant to the post. if you can explain why the problem was solved the way i told in the first page, then plz do it, else, dont comfuse people up..

  6. #16
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    oldspark did cover that (and schooled me on inverter powering )-- you had a floating ground between the ac-dc psu and the inverter. connecting the ground wire from the ac-dc to the car put all the power supplies on the same ground plane, so your noise went away, because there was no longer a voltage potential between your carpc, and radio...

    this is also why i typically recommend connecting a wire from the negative part of the rca to the vehicle ground as a first step to trouble shooting-- it is really easy to do, and will correct floating grounds like what you had..

  7. #17
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    You broke a ground loop, or you grounded a floating ground (or both).

    Thanks soundman.


    PS - mi first lines 4 the benfit of thoes I comfused up.
    Last edited by OldSpark; 04-14-2011 at 09:47 PM. Reason: PS...

  8. #18
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    i dont know... my psu was not touching any metal part (was in wood) and i did not grounded its negative part, but the "middle" one wich we here can ground... i dont see any way that before was a ground loop...

  9. #19
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    I presume "middle" one is the PSU chassis earth connection - typically a green or green&yellow wire?

    So YOU know that in your system, the PSU chassis is NOT connected to PC -ve (ground), and that the inverter outputs (any of the 2 or 3 lines) are not connected to the inverter chassis or its -ve (ground)?


    Cool. Then it wasn't a ground loop involving those components. You therefore merely grounded a floating radiator.

  10. #20
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    yiap. it was green and one of its ends was in the psu chassis (whic combined with a desktop chassis makes a good enough electic sinc..)
    So the exact diagram is:
    Car battery -> (possitive cable) ->inverter . the negative cable then goes to ground.
    then a normal (european) cable connects the inverter and psu.
    What i did was cut that cable and connect the litle green one (going from the psu towards the inverter) to a car grounding... tbhat solved and i 'm damned if i know why

    (also if u can tell me, must i connect the ather part (inverter ->psu) as well to the grounding?)
    the two chassis are not touching meta parts...

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