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Line-out replacement for piezo speaker

Discussion in 'Electronic Design' started by Arild P., Jun 6, 2006.

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  1. Arild P.

    Arild P. Guest

    I have this electronic talking toy with a "ceramic speaker" (or
    whatever it's called) and want to create a line out socket so I can
    connect it to an audio system, but I'm having problems getting a good
    sound.
    The "speaker" looks similar to this picture I found by doing a google
    search: http://www.mcld.co.uk/oddmu/tincanbass/piezotransducer.jpg

    and is marked as follows: Murata VSB41D25-07AR

    I've searched on the web for it, trying to get some specs, but to no
    avail (I did find Murata.com, but that part is most likely outdated and
    not available any longer since the toy is from the early 80's).
    I've tried measuring the resistance across the "speaker", but nothing
    comes up on my multimeter! Must have an infinite resistance or
    something strange is going on ;-)

    Then I disconnected the speaker from the toy's circuit board and
    connected a jack socket in its place. The signal was way too loud, but
    I got the right level by connecting 440K Ohms worth of resistance (two
    220K Ohm resistors) along one of the signal paths. Still, the sound
    isn't quite too my liking. I believe I need to filter out some bass. I
    might possibly also need to raise the treble a bit, but I'm not sure
    about this yet. But I definitely have to remove some lower frequencies.
    I know that a filter involves resistors and capacitors, but other than
    that I'm clueless and have spent a lot of time playing around with
    different combinations, but not really getting anywhere, so what do I
    do?
    This is what I've done so far:

    ORIGINAL CIRCUIT:

    ----------------->------
    audio |
    out O piezo-ceramic speaker

    |
    ----------------->-------


    NEW CIRCUIT:

    ----------------->-------------------------------------------------->

    audio line
    out out

    ----------------->---------------o---xxxx----o---xxxx-------->
    220K 220K
     
  2. Roger

    Roger Guest

    Your best bet would probably be to use an op-amp 'integrator' circuit.
    Take a conventional inverting op amp circuit and place a capacitor in
    paralell with the feedback resistor. the input resistor should be
    connected to the speaker wire via a capacitor (use +/- supplies on the
    op amp. Use 100K for the input resistor and vary the feedback resistor
    to get the desired gain. The integrating capacitor should be such that
    C * Rf is appox 0.0001

    Once you have the gain about right by juggling the resistors (should be
    1V p) you can juggle the capacitors. A smaller coupling capacitor gives
    less bass. A bigger feedback capacitor gives less treble.
     
  3. Guest

    Problem with that, it relies on the impedance of the input of your amp
    along with your series resistors to drop the voltage down. Plug it into
    a different amp or other input, and the volume level may be different.

    High side 220K 10uF
    ----------------->---------------o---xxxx----o o--||----->
    | |
    50K x |
    audio Audio x<----| line
    out Taper x out
    |
    ----------------->-------------------------------o----------------->
    Ground side


    Try that before trying to alter the frequency response.

    I'm having a hard time getting that to display correctly. Signal side,
    to 220K resistor, to one side of a 50K audio taper potentiometer. Other
    side of the potentiometer to ground/common. Center of potentiometer to
    10uF capacitor, other side of 10uF cap to amplifier input.

    Look up piezo speaker on Google for more info. DC resistance is
    infinite, it acts a bit like a lossy capacitor.
     
  4. Arild P.

    Arild P. Guest

    OK, I've redone the circuitry, but one question regarding what you
    said: will the output volume change only if I connect it to a different
    input/amp, or will the volume sometimes change even when it's connected
    to the same input?
    The sole purpose for adding this output is for sampling (recording) the
    toy's audio with my computer.
    Anyway, I've followed your example and also played around with
    capacitors to change the sound's bass/treble. This is what it looks
    like now:


    High side 220K 10nF
    ----------------->---------------o---xxxx----o o--||----->
    | |
    50K x |
    audio Audio x<----| line
    out Taper x out
    |
    ----------------->-------------------------------o----------------->
    Ground side

    Note that I'm using a 10nF capacitor instead of 10uF as you suggested.
    At least I think that's what it is, if I haven't misinterpreted my
    digital multimeter. The capacitor is marked "103".

    I tried out many different values, but found something between 10 to
    15nF to give the sound I wanted.
    Well, very close anyway. If I used say a 6nF capacitor (or lower) the
    sound became very "thin", filtering away almost all the bass. If I used
    a 40nF capacitor (or higher) the sound became too "bassy" and "harsh".
    But if I want to do this really well I also want to filter out some of
    the digital noise from the toy. If I use a higher value capator this
    goes away, but that'll also change the overall sound giving more bass,
    which I don't want.
    Are there other ways I can create the filter? I would like to try
    something like a 12 or 13nF capacitor, but I don't have any with those
    values. Besides, based on my experimentation with all sorts of
    capacitors I doubt it'll remove the digital noise without removing the
    frequencies I want as well.
    Is it possible to create a "narrow" filter of some sort in addition to
    the above filter?
     
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