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Trouble building lo-fi mics....

Discussion in 'Electronic Basics' started by Chris, Sep 6, 2005.

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  1. Chris

    Chris Guest

    Hi, I don't know if this would be the right forum but I'm going to try
    anyways. Right now I'm putting together two homemade mics using a)a
    telephone and b) a cb radio. I've already taken the phone, pulled apart
    the reciever and wired it to an input jack. I run it through an amp but
    I only get sound at really low volumes. I'd like to boost the sound but
    I don't know how. I notice when I put my finger on the black wire it
    greatly increases the volume. I'm very new to electronics and all I've
    got is a soldering gun, some supplies, and a lot of ideas. Also with
    the CB, I've opened up the cable and found a white wire, a black wire,
    and a bare and red wire wrapped around each other. How would I go about
    hooking this up to a mono telephone jack? Any help would be greatly
    appreciated, as well as links to some sites that could help me this.
    I'm obviously the novice's novice. Thanks -(hris
  2. tempus fugit

    tempus fugit Guest

    It's possible that the mic elements you are using are electret condensers.
    If this is the case, they need to have a voltage applied to them to work. I
    would think the CB is for sure, but I don't know about the phone.
    I doubt the phone is a condenser, but the problem no doubt lies in the fact
    that the mic itself outputs a very low voltage. The amp is expecting a
    higher (in the 1-3v range) input signal and so the resulting output from the
    amp is also very low. What you need is a preamp before the amp stage. What
    kind of amp are you using?
  3. Chris

    Chris Guest

    Oh I was just using what I have, a tiny marshall guitar amp.
  4. Jasen Betts

    Jasen Betts Guest

    depending on the age of the phone it could be a carbon microphone
    (if it's quite large) they need a power source to operate. (typically phones
    of this age had real bells inside them)

    or it could be an elecctret microphone which is small and also needs a
    power source,

    or it could just be be that the microphone isn't suited to the input you've
    connected it to..

    I had succcess connecting the earpiece part of an old telephone to the
    micriphone input of a small tape deck.
    I think the white and black go to the switch and the red and bare carry the
    signal, connecct red to the tip and bare to the sleeve.

  5. Jasen Betts

    Jasen Betts Guest

    I've used a guitar amp with a 600 ohm dynamic microphone and it worked
    fairly well, but if you have electret microphones they'll need a little
    power circuit to activate them, usually 0v goes to the shield and the inner
    conductor gets the voltage via the resistor, but if you hook it up
    backwards it's unlikely to damage anything. a single AA cell should get you
    a few weeks of continuous operation.

    | ||
    to mic +----||-------------------
    ||10uF to amplifier

    if you hear a "click" from the microphone when hooking it up to the above
    circuit you have a dynamic micriphone and it should be direclty compatible
    with the guitar amp you have.

    if you have a carbon microphone use a lower resistor, say 100 ohms, and
    expect to need batteries more frequently,

    The carbon microphones can power a small speaker directly just give them more
    voltage (say 12V) and a high impedance speaker (50 ohms) and hook them
    up in series. don't expect high fidelity.

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