Electret mike - connection to plug

Discussion in 'General Electronics Chat' started by atferrari, Apr 12, 2014.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    I soldered the red/white wires to the mike's +/- terminals and the shield to the metalic housing.

    A) At the plug end, the (-) and the shield were soldered together going to the common of the preamp. Is this reasonable?

    B) Found the shielded plastic jacketed bipolar cable quite hard to bend. Could I use single shielded wire (red only plus shield), soldering the shield to the housing and negative terminal? I do not see any difference since in both cases signal is unbalanced.

    C) Used it to test on the bench my just-built pre + filter + amp. It seems to work but I hear some hum (most probably 50 Hz from mains). Should I do something else on the mike + wire + plug or could I expect that a metalic enclosure could cure it?

    Thanks for any reply / comments.
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. panic mode

    Senior Member

    Oct 10, 2011
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    electret mikes need power. in general they are either 2 or 3 terminal devices (one of the terminals is the mic casing, this is the GND).

    one of the terminals is the audio output.

    if the third terminal is present, that is power. on 2-terminal mics, power and audio out are combined on same terminal.


    connections for 3-terminal mic:

    1. gnd to casing (shield)
    2. audio out (use cap to block DC component)
    3. power (some 1.5V, 0.5-1mA), if using larger voltage, add series resistor...




    connection for 2-terminal mic:

    1. gnd to casing (shield)
    2. audio out (use cap to block DC component). supply power through resistor. since mic needs some 1.5V, 0.5-1mA, choose suitable resistor (can be 2.2-3.3k if supplied by 3V, 5.1-10k if supplied by 9V)

    if you have datasheet, you can use manufacturer recommendation...
     
  4. atferrari

    Thread Starter AAC Fanatic!

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    Sorry to you both but my mike is already duly powered and up to the amp, everything is working as I say in my OP.

    My question is just about shielding.
     
  5. panic mode

    Senior Member

    Oct 10, 2011
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    use shielded cable and make sure there is a cap across supply
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I'm guessing it is quiet when the mic is not plugged in. If so all you can do is keep the length of exposed leads as short as possible. A grounded metal box will probably fix it.
    Really high gain?
     
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