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  • Music Input

    Any of you avionics gurus help me out?

    I am making up my own radio/intercom harnesses - I know, I know - but actually, it's going quite well ..... but I have a small issue.

    I have a Guardian iPad Mini mount with USB power. That end of it is fine - wiring diagrams on their website. However, it also has an audio cable with a 3.5mm plug. All looks simple - 4 rings - from the tip - left audio, right audio, ground, mic. So I don't need the mic so I can just terminate it and connect up the other 3. But when I use my tester to check which wire is which, I come up with a problem. Both audio leads fine - found them. Ground ring doesn't give any connection to the remaining 2 wires. The mic ring gives 6.7k resistance to one wire - nothing to the other (black - so I assume ground). I can see that the mic input might need some pull-down resistor which is built in so no problem. But the resistance across the remaining 2 wires is also 6.7k - ???????

    So my question is this - where has my earth (sorry - "Ground") gone? Do I need to roll my own jack and cable - or is that going to blow something if I just use 3 wires and no resistor?
    Last edited by PaulSA; 03-12-2019, 11:46 AM.

  • #2
    Google TRRS wiring diagrams. Mono plugs are called TS, stereo are TRS, stereo plus mic are TRRS. It gets complicated with TRS and TRRS. Sometimes you can plug one into the other and work, sometimes not due simply to the geometry and length of the rings on the connectors. Plus, there are two standards for wiring them.

    I had no intention of ever learning any of that.

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    • #3
      Can you post a link to their manual or an image of the diagram?

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      • #4
        I have Googled and know what it should be - it's just not as I expected. The wiring diagrams are kept on-line at Guardian's site. The power side of thing is fine, but there is nothing concerning the audio - which SHOULD be simple.

        Again, the intercom (PAR200A) expects audio left, audio, right and audio lo. The cable has audio left, audio, right, ground and mic - this should be the configuration from the jack tip inward. The 2 audio channels are fine (red and white). Ring 3 doesn't seem to be connected to anything. Ring 4 is connected to purple with 6.7k resistor. The final wire (black) gives 6.7k across the purple wire but nothing on anything else. I can't even work out a wiring that exhibits these parameters!! I have also found a note on the PAR200A diagram saying NOT to connect Audio lo to ground as it is a differential music input. Well that's fine because neither of the "spare" wires is grounded - but where DO I connect Audio Lo?

        I'll see if I can get something from Guardian or PS support and/or the VANS forum (traitor!! - but they do have some seriously knowledgeable avionics types)

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        • #5
          OK, got it thanks - must have been going mad last night.

          In correct nomenclature - Tip is red - Audio L, Ring 1 is White - Audio R, Ring 2 is in fact connected to the green wire (not purple......) - Audio Lo. The remaining wire is black but does not appear to be connected to the sleeve. Doesn't matter - for the iPad this is Mic and I don't need it.

          So, good to go!

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