What type mic is my P5B-E mb expecting?

Discussion in 'Asus' started by Bert, Sep 2, 2013.

  1. Bert

    Bert Guest

    I'm using this headset with the HC-6 mic element:


    I just tried to connect this headset to my old P5B-E motherboard and
    find that the mic audio level is very low. The mic works fine with my
    ham gear and my old R51 Thinkpad laptop, so it's working.

    No software adjustments will improve the audio, although I can reduce
    the level even more.

    The mic is dynamic, with a nominal impedance of 600 ohms. I don't see
    any mention in the manual of what sort of mic the MB expects.

    Is this just the wrong sort of mic, are there some jumpers or BIOS
    settings I've missed, or is something broken?
    Bert, Sep 2, 2013
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  2. Bert

    Paul Guest

    The motherboard expects something other than a moving coil microphone.

    Moving coil (dynamic) might be 2 to 3 mV or so. The AC'97 or HDAudio chips
    don't have enough gain for that, even if the 20dB boost is engaged.
    There is even one HDAudio chip, where some idiot "forgot" proper
    boost altogether (no evidence of good gain range in the
    datasheet - could be a datasheet error). I'm not aware of
    later designs repeating that error.

    A ceramic microphone (piezoelectric), can put out about 100mV.
    That should be OK.

    But what the motherboard is really designed for, is electret
    microphones. Computer audio has a DC bias voltage source (somewhere
    in the 3V to 5V range). The bias source connects via a limiting resistor
    (1K to 2K), and that is the power source for the microphone. The idea is,
    such a power source is not supposed to upset the other kinds
    of microphones. (In theory, there may be a register in the
    HDAudio chip to turn off the bias source, but in practice
    it isn't exposed so a user can do that. Older designs, like
    the AC'97 era, the bias source may have been external to
    the chip, and not under software control. Such a source might
    be 5V.)

    Even stereo microphones are handled this way, with DC bias on
    both left and right. So it's not like a three contact 1/8" plug
    has power on one pin, and microphone signal comes out on the other.
    In fact, on a mono microphone, the microphone end shorts left and
    right together, so any bias source is connected to the output signal

    +----------------- L-ADC
    | +--------- R-ADC
    | |
    Vbias | Vbias | GND
    | | | | |
    1K | 1K | |
    | | | | |
    +---+ +---+ |
    | | |

    Now, the puzzling part, is the last time
    I looked in a catalog, there were some
    electrets that seemed to need 5V. And some
    of the audio chips don't have 5V as an option.
    Yet you don't hear a lot of complaints, which
    means the electrets used in computer mic/headsets
    must be selected to work with the lowest common

    There is a picture of the typical electret
    setup here. The DC blocking capacitor, already
    exists on the inputs of the HDAudio chip. (Everything
    on HDAudio is AC coupled.) I didn't bother to show
    that cap in the above ascii-art.


    Since the Heil document I downloaded, doesn't
    have any technical tidbits, I can't translate
    what they're telling me, into anything useful.

    The input impedance on the motherboard, for
    microphone in or for line in, should be 10K ohm
    or greater. So not a problem when dealing with
    a 600 ohm source.

    If the microphone doesn't work when plugged into
    your stereo, your stereo probably won't have a
    bias source on it.

    Paul, Sep 2, 2013
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  3. Bert

    Bert Guest

    Thanks. Looks like a little change in plans is called for.
    Bert, Sep 2, 2013
  4. Bert

    Paul Guest

    I can't really be sure what kind of microphone is
    involved there. There seem to be too many adapters.

    Paul, Sep 3, 2013
  5. Bert

    Bert Guest

    The adapters are there just to match the seemingly endless variety of
    microphone and headphone connectors used on various makes and models of ham
    radio gear.

    There are only two types of microphone elements, dynamic and an electret.
    Mine has the dynamic element.
    Bert, Sep 3, 2013
  6. Bert

    Paul Guest

    If so (really is dynamic, moving coil), you might benefit
    from a separate amplifier.

    And picking an amplifier, isn't exactly easy. This is why,
    an ideal audio product, would already have a preamp built in.

    It might actually be easier to just switch to another microphone.
    If one of these boxes costs $100, it might be easier to just find
    an electret microphone for computer usage.


    Be aware, that even so-called "reputable" computer product
    makers, make crap. I have a Logitech headset with microphone,
    where the only way you can get audio output from the microphone,
    is by scratching the microphone surface with your thumbnail.
    It is *that* insensitive. It cannot possibly be used for voice

    My best microphone, is an Apple Plaintalk (oval shaped) one.
    There is an Apple microphone which is a circular disc, which
    is terrible. The oval shaped one has a separate 5V power
    supply, to run a tiny 4 pin amplifier chip inside the microphone.

    This won't work on a PC directly, due to the weird plug on the end.
    I think the 5V goes on the end contact.


    An Andrea Superbeam is another option. It's a stereo microphone,
    that with an appropriate driver, uses beamforming to do more
    directional recording of a mono source. Some motherboard
    sound drivers, used to have a provision to work with one
    of these. This is two electrets and stereo wiring.


    They also sell those with a USB adapter, bypassing the
    computer audio path entirely, and making it a USB microphone.


    Paul, Sep 3, 2013
  7. Bert

    Bert Guest

    Exactly so. There are lots of headsets made specifically for this
    application. They're cheap and easy to find.
    Bert, Sep 3, 2013
  8. Bert

    Paul Guest

    See if you can find the "Boost" tick box for Microphone
    and maybe there will be enough gain. They like to
    use creative techniques to hide this tick box from the


    For SoundMax (and perhaps your P5B-E), it would
    look like this.


    Paul, Sep 3, 2013
  9. Bert

    Bert Guest

    Tried that, wasn't enough.
    Bert, Sep 4, 2013
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