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Driving a headphone amp off a phantom power mic line

Discussion in 'Electronic Design' started by Piglit, Feb 16, 2007.

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  1. Piglit

    Piglit Guest

    Trying to devise a way of efficiently driving a phone amplifier off a
    48 volt phantom powered mic line. The acitve lines (2) are current
    limited typically by 6k8 resistors, and allowing for an additional 1k
    on each leg to bridge into the amp, that gives around 12ma short
    circuit, 6ma at 24 volts - the point at which maximum transfer power
    is available. I would like more current and less voltage (Nat Semi etc
    have some nice efficient little class D amps that work on 5 volts that
    are ideal for driving headphones). Hence I need a DC/DC converter. The
    problem is that all the buck regulators in the TI, Nat Semi offerings
    loose efficiency drastically below 100ma or so, thus defeating the
    purpose of the exercise. None of the switchcap regs work at 24 volts.
    Any other suitable devices out there ?.
  2. Generate the headphone signal at higher voltage and lower current, then
    use a small audio transformer to step the impedance down. It will give
    you isolation too.

    Unless the headphone amp is running in Class A, you are going to have
    terrible problems preventing the variable loading on the power rails
    from causing trouble to the mic circuit. You might have to finish up
    using a battery instead.
  3. Phil Allison

    Phil Allison Guest

    "Adrian Tuddenham"

    ** Excellent suggestion.

    A common audio op-amp ( ie NE5534) may be used from a + 24 volt ( zener
    stabilised) supply and drive a cap coupled transformer matched to the
    headphone load - typical headphones have near constant impedance across
    the audio band.

    An electro cap across the +24 volts will allow current peaks over 12 mA with
    a 6 mA average value - no problem for most op-amps.

    The optimum load presented to the op-amp is about 10 mA at 10 volts = 1000

    Power max = about 50mW = damn loud !

    ** I dearly hope no poor mic is expected to work on the same input !!!!!!

    ......... Phil
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