re-wiring a speakerphone to a mixer input (very noob)

Discussion in 'The Projects Forum' started by nikošveikovsky, Jul 23, 2011.

  1. nikošveikovsky

    Thread Starter New Member

    Jun 3, 2010
    3
    0
    Hi,

    I work at a radio station, and I'm trying to integrate an incoming phone line into our mixer board. We have IP phones, so a converter box wouldn't help. Thus I ripped open one of our phones and hope to replace the original 45 ohm speakerphone speaker with a resistor of some value and an 1/4" TS jack:

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    Being completely clueless with electrical circuits, I have no idea how to go about wiring this besides the obvious of soldering the jack to where the speaker was with a resistor in line somewhere. I also don't know how to find out the line level of analog audio connections. I only know that this is theoretically possible.

    Some pointers as to what would be good reading? I've read a good deal of electronics textbooks but I don't know where to go from there. Sorry about the absolute noobness here.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    Use two electrlytic capacitors wired together in series to form a NON polarized cap(put the two negative legs together). I would suggest 10-22uF.

    Come off the speaker with one wire directly to the 1/4 inch plug and solder the non polar capacitor to the other speaker connector. Then connect a wire from the free end of the cap to the other connector of the 1/4 inch plug.

    You now have a direct to soundboard connection for your speaker phone, and you don't even have to disconnect the speaker from its circuit. That portion will continue to work as always. The cap will prevent any DC signal from getting into the soundboard.

    The gain knob for that channel will probably have to be set very low, if it needs to be used at all. The slide fader will control volume on your board just like any other signal at this point.

    Should the signal prove to be overly strong for the input jack you can add a 1k resistor in series with the cap to cut down the signal level, at which point you will probably need to use the channel gain control to find a proper input level for the signal.
     
  3. nikošveikovsky

    Thread Starter New Member

    Jun 3, 2010
    3
    0
    Clearly I came to the right place— thanks very much!

    But what can I do if I don't want the speaker in the series? I'm already using it in another project, and I really don't need it for this one, as the voice will be in the mix.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    replace it with a resistor(several 1/2 watt resistors in series, or one single 5 watt resistor). This will provide the amplifier circuit a proper load and prevent the audio signal from possibly becoming distorted. The amp driving the speaker would not be 'happy' if it doesn't see the load resistance it is expecting. It might function acceptably well without it for some time, but could decide to fail and probably when you are most in need of it working. :)

    Four 10 or 12 ohm 1/2 watt resistors in series would suffice to substitute for the speaker and will easily handle the power output of the driver amp.
     
  5. nikošveikovsky

    Thread Starter New Member

    Jun 3, 2010
    3
    0
    Thanks so much. I figured all it would take was the proper resistance in line with everything. I just didn't think about the tolerance last time and burnt out a resistor. But now it works, thanks!
     
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