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Mixer circuit pre-amp

Discussion in 'Electronics Homework Help' started by Jamesmeister, Mar 10, 2013.

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  1. Jamesmeister


    Mar 5, 2013
    For Mixer part, we are required to have: Three (3) high impedance microphone-level inputs (unbalanced), two (2) high impedance line-level inputs.

    For microphone section:
    -Input impedance (Zin): 800 ohms
    -Level gain (Av dB): 42 dBu

    For Line input section:
    - Input impedance (Zin): 20 k
    - Level gain (Av dB): 25 dBu

    i have here a sample mixer circuit i saw on the internet. my problem is, how do i accomplish the requirements above. i have no ideaaaa :confused:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The first step is unserstanding the basics.

    If I were you, I would review my notes. I would also consider what the point is of those 1k trimpots.
  3. Laplace


    Apr 4, 2010
    I don't quite understand the specification as given: "-Level gain (Av dB): 42 dBu".

    Now 42 dB is a voltage gain of about 126 but dBu gives a signal level referenced to 0.7746 volts (1 mW @ 600 ohms) so 42 dBu is 97 VRMS or 274 volts P-P. Are you really expected to design a circuit for a 274 V peak-to-peak signal level? The LM3900 is not suitable for 42 dBu.
  4. Jamesmeister


    Mar 5, 2013
    ive done a little computation. 20log (x/1x10^-6 V (since ref is db(micro)) = 42
    therefore, i should have an output voltage of 0.12589 mV. as far as i know, some microphones have input voltage of 1 micro volts
  5. davenn

    davenn Moderator

    Sep 5, 2009
    standard dynamic microphones dont get supplied a voltage - they generate a voltage

    Electret condenser mics need a supply voltage but the preamp circuit you have shown is not set up for phantom power

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