Lack of standards for line and mic levels is quite confusing!

Discussion in 'General Electronics Chat' started by jenks, Apr 24, 2008.

  1. jenks

    Thread Starter New Member

    Apr 24, 2008
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    Have any of you found when designing audio circuits that deal with microphone and input levels that there is a lot of misleading and sometimes contradictory information out there?

    I've discovered lots of articles contradict each other when it comes to altering impedance and level of a line signal to that of a microphone signal!

    I've read that line-level ranges from 0.3 to 3 volts, I've also read that it is +4dBu which corresponds to a sine wave of 1.228Vrms, and I've also read that it is -10dBv!

    I've read as well that the impedance of line signal is 600 ohm in some articles but 50 kilo ohm in others!

    I've also read that microphone level is in the micro-volts range or -30 dBv. And that a typical microphone input should 'see' an impedance of 150 ohm, but others say 3 kilo ohm!

    All this doesn't help in my quest to build a D.I.! I just want to know how many turns my transformer should have in order to alter my active bass guitar signal so that it will be suitable for the input of a microphone input on a mixing console!

    Any attempt to demystify all this stuff surrounding impedance and level of different signals will be greatly appreciated!

    Cheers.
     
  2. jenks

    Thread Starter New Member

    Apr 24, 2008
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  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You can use a low impedance (600 ohms) dynamic microphone that has a magnet and a voice coil like a tiny speaker.
    You can use a medium impedance (3 k ohms) electret microphone.

    You can whisper into a microphone 10cm from your mouth and its output is 1mV.
    You can scream (or sing very loud) into a microphone that you are eating and its output is 200mV.
    So a mic input has a level control to adjust for the differences in level.

    A guitar pickup is usually fed to a preamp with a very high input impedance like 3M ohms (a vacuum tube or FET transistor), not the low input impedance of a transformer.

    You say your guitar signal is active. Then it already has a high input impedance preamp. If you want to attenuate its signal down to mic level then you need to know its output impedance and level.
     
  4. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Hello jenks.

    The microphones and pick-ups have an assortment of impedances from brand to brand that can drive most crazy. Same with the input impedances of equipment.

    Just observe the specifications for each, and if posible place a sticker stating the impedance on every item; that will give you some peace.

    Modern equipment inputs have high enough impedance to discard all your worries and is probably a reason to not have to care about an standard.
    Miguel
     
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