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Thread: Why does the audio shield's headphone jack use a virtual ground?

  1. #1
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    Why does the audio shield's headphone jack use a virtual ground?

    This isn't about a problem I'm having, but I'm curious why the line-out on the audio shield uses capacitors and a true ground, but the headphone jack uses no capacitors and a virtual ground? It seems like it would be safer to isolate the headphone jack in case someone plugs it into the wrong aux-in?

    I found an interesting article about designing headphone circuits, but I don't fully understand it and I'm not sure how it applies: https://www.electronicproducts.com/A...eadphones.aspx

  2. #2
    Senior Member PaulStoffregen's Avatar
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    The reason is the large capacitance that's required to pass the lower part of the audio spectrum when the load is the relatively low 32 ohm impedance of headphones.

    These days, many chips which drive headphones use this or similar approaches so you can avoid those bulky components.

  3. #3
    Senior Member oddson's Avatar
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    Quote Originally Posted by skybrian View Post
    ...would be safer to isolate the headphone jack in case someone plugs it into the wrong aux-in?...
    Has anyone done this?

    How much damage?

  4. #4
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    Interesting. The thing is, I expect most people who aren't audio enthusiasts haven't heard of this, and would just plug a headphone jack into a random aux-in without thinking about it? I'm wondering how consumer gear being sold to the masses handles it. Maybe most aux inputs have capacitors on that end?

  5. #5
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    What's a "wrong aux-in"? Are you saying that the audio board can get smoked if its phone jack is plugged into some "wrong" amplifier?
    The phone jack on my audio board just it just stays plugged into my active speakers. I never used the "Line out".

  6. #6
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    On the Teensy audio shield, it says "Caution: do not short VGND to GND". The headphone jack uses VGND. There is a risk that if you connect the headphone jack to an aux input, that equipment might have the ground wire in the aux-in jack connected to ground, shorting it out. But that's going to depend on the circuit in the other equipment.

    I'm new to this, but I think it won't matter if you're on battery power? Also, even if were a true ground, you might still have trouble with audio quality due to creating a ground loop?

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