4 poles to 3.5mm audio

Discussion in 'The Projects Forum' started by Yemto, Apr 10, 2015.

  1. Yemto

    Thread Starter New Member

    Apr 7, 2015
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    I have a circuit board which have four poles to audio out, R+ R- L+ L- is there a way to combine L- and R- into ground so it would be possible to solder it into a 3.5mm plug?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    You want a three pole audio out signal like, LEFT, RIGHT, GND. It occurs in multiple place that I'm aware of so it must be OK.
     
  3. Yemto

    Thread Starter New Member

    Apr 7, 2015
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    Yes, but I want to know how I convert L- and R- to GND. From what I know it's not as simple as just solder them together.
     
  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    AFAIK (I'm not an expert), it depends on the amplifier. Some have differential outputs, and connecting the grounds together may shut down the amplifier. This link is the story of someone who tried to connect the grounds together and could not get their amplifier to work.
     
  5. ebeowulf17

    Active Member

    Aug 12, 2014
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    It depends on whether those outputs are balanced outputs or not. With unbalanced outputs, the L- and R- would both already be tied to ground and would be totally safe to join together. With balanced outputs, the L- and R- would be signals with reversed polarity relative to L+ and R+. You wouldn't want to tie those together, although I don't think it would damage anything; it would just reduce your stereo width.

    Odds are good that it's unbalanced, but I thought I'd throw this out there just in case.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    You have to show us the schematic diagram of your board. It would also be helpful to know what you are connecting it to.
     
  7. Yemto

    Thread Starter New Member

    Apr 7, 2015
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    Isn't there a circuit I could make that could convert the outputs into R, L, and GND? or would I need a small computer (Raspberry Pi?) to convert the signals into what I want?
     
  8. Yemto

    Thread Starter New Member

    Apr 7, 2015
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    The board comes from a broken pair of wireless headphones, but the board itself still functions properly. But it means I don't have a schematic diagram for it. My idea is to install a 3.5mm female plug/jack so I could use it with any non-usb headset. I know I could just buy a new wireless headset, but I like to re-use what I can from broken electronics.
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    We can't answer your question without seeing what you have. A circuit might be necessary or it might not, we just can't tell from the inadequate verbal description. Maybe you could create a schematic by looking at the board and tracing out the runs.
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    measure the resistance betweeen the two - terminals. if they are very low resistance, it should be ok to connect them together for a common.
     
  11. ebeowulf17

    Active Member

    Aug 12, 2014
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    Agreed! This is the simplest solution. If the resistance reading is not very low, then things get more complicated!
     
  12. RichardO

    Well-Known Member

    May 4, 2013
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    The simplest solution might be a transformer?
     
  13. ebeowulf17

    Active Member

    Aug 12, 2014
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    A pair of transformers I think... but that's still more work than just soldering the two negatives together. If the OP has a multimeter and a few seconds to use it, transformers may turn out to be unnecessary.
     
  14. RichardO

    Well-Known Member

    May 4, 2013
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    Oops. I meant the simplest solution for the situation where L- and R- can not be tied together. Sorry for the confusion.
     
  15. ebeowulf17

    Active Member

    Aug 12, 2014
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    Ah, understood. In that case, I think you're probably right.
     
  16. Yemto

    Thread Starter New Member

    Apr 7, 2015
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    Sorry it took some time before I replied, I bought a multimeter which measures 0.5 ohms between L- and R-
     
  17. tranzz4md

    Member

    Apr 10, 2015
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    I come more from the "try it" side than the "sketch it" side of things. 0.5 ohms from L- to R- is interesting, but I'd be reading all angles of those points. If everything reads under 1 ohm for example, you're probably learning more about your multimeter and how you've set it than about your circuit being tested. If that L- to R- reading is noticeably lower than the other combinations, you may have found something. You could connect R- and L- through a fairly low value resistor, to limit current, and if you get good results try it with a much lower resistor, or none at all, and test again.

    Also don't get the impression that "ground" is ground. It's usually only a common point within an isolated system. And I mean isolated! "ground loops" are a lot of fun for analog audio guys (if any others of us still exist) :( .

    With the info we have here, we're all guessing at this. Our guess is that "insufficient information" isn't a reply you'd like or have much use for. Most of us like this "blind man and the elephant" game.

    Best of luck!
     
  18. Yemto

    Thread Starter New Member

    Apr 7, 2015
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    I did some other tests and gets in ohms
    L- R+ = 3.7M
    L- L+ = 3.8M
    R- R+ = 3.8M
    L+ R- = 3.7M
    L+ R+ = 13.4M
    L- R- = 0.5

    Granted the values jumped around quite a bit. But L- R- was always the lowest and R+ L+ was always the highest.

    EDIT:
    I just notice something, when I was taking pictures of the circuit to post here. It seems L- and R- already is connected on the board.
    https://dl.dropboxusercontent.com/u/4506031/Pictures/Internet/board.jpg
     
    Last edited: Apr 12, 2015
  19. ebeowulf17

    Active Member

    Aug 12, 2014
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    Ha! That's awesome and not surprising. So you don't even have to join them, cause it's already done. That's great. Just hook your trs sleeve to the shared negative, ring to right, and tip to left.
     
  20. Yemto

    Thread Starter New Member

    Apr 7, 2015
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    What I think is odd is how they where separate on the board, even when the are connected. But oh well.
     
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