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Thread: WM8731 headphone output

  1. #1

    WM8731 headphone output

    I am using the headphone output on the WM8731 codec for line out or optionally headphone out. I want to detect the difference between a mono 1/4 plug (line out) and a stereo (headphone out) when the connector is plugged in. If I ride a DC level on the AC headphone output, it is hard to detect the difference between a shorted ring to shield (mono plug) and a 16 ohm headphone. Any suggestions?

  2. #2
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    Adding DC to the headphone out isn't advised, it'll risk pushing the transducer cone to the end-stops.

    Why do you need to detect the difference? Is it to set different levels or some such? Or to disable the right channel
    to avoid overloading the output stage?

  3. #3
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    Quote Originally Posted by bdoan View Post
    If I ride a DC level on the AC headphone output
    What do you mean by "ride"?

  4. #4
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    It means add. I've heard this terminology especially when discussing oscillograms

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    Weird, yeah don't add a DC offset to any speaker. That's a bad idea.

  6. #6
    When I am talking about adding DC, I am talking through a large value resistance and sensing the voltage division (or shorted to ground). The current through the transducer would be very small. The problem is detected the difference between 16 ohms to ground and a short using a 10 bit ADC.

  7. #7
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    So are you asking how to measure a tiny voltage? Or something else?

  8. #8
    Asking for any other ideas other than trying to accurately measure a tiny voltage. The point is to protect the headphone amp by not running it into a shorted load.

  9. #9
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    The datasheet for the WM8731 says nothing at all about any limit on the audio outputs, nor about short-circuit
    protection, about the only clue is that the internal output drivers are portrayed as opamps. Its pretty
    universal for opamps to have indefinite short-circuit handling without issue - other than increased current
    draw.

    Its certain that the chip tolerates short-term short-circuiting due to the use-case in a phone, as TRS connectors
    are a ghastly design and short the signals to ground during insertion/removal. That's the basic problem, a
    Victorian era design of connector...

    Whether the chip can gracefully handle extended output shorts is a question for the manufacturer, but if so there
    isn't a big problem.

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