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Problem with new microphone preamp.

Discussion in 'General Electronics Discussion' started by DominicNewbie, Jul 6, 2016.

  1. DominicNewbie

    DominicNewbie

    7
    0
    Jul 6, 2016
    Hi guys.
    I have just designed my new microphone preamplifier basen on two BC549C transistors.
    I used scheme found in Internet. Many people have successfully made this circuit and it works for line level.
    95_1187900973.gif

    I made one too, but it doesn't works. I'm looking at scheme and amplifier and wondering what I have done bad. I can't see any reason.

    Capture.PNG
    I can hear only regular "buzzes".

    Hope we will fix it :)

    Also, check mp3 probe:
    http://www.speedyshare.com/CUTTU/probe.mp3
     
    Last edited: Jul 6, 2016
  2. BobK

    BobK

    7,645
    1,663
    Jan 5, 2010
    I simulated it with LTSPICE and it works, and has a gain of 230. Input is 2mV p-p output 560mV peak to peak.

    It should work fine as a microphone preamp. It should not work at all with a line input since it will be clipping like crazy with that gain.

    upload_2016-7-6_18-36-37.png
     
  3. DominicNewbie

    DominicNewbie

    7
    0
    Jul 6, 2016
  4. duke37

    duke37

    5,254
    726
    Jan 9, 2011
    The plot shows a gradual change in the middle which indicates something to do with a capacitor.

    Is the input capacitor the wrong way round? The base of the transistor will be a little positive and a dynamic microphone will be connected to earth.
     
  5. DominicNewbie

    DominicNewbie

    7
    0
    Jul 6, 2016
    Yes, I connected microphone input to + of capacitor.
    Should I try?
    Untitled.png


    I remember this circuit above was working fine.
    Moreover I use standard 2.2k electret Microphone.
     
    Last edited: Jul 7, 2016
  6. DominicNewbie

    DominicNewbie

    7
    0
    Jul 6, 2016
    Updated
    Cicruit with one capacitor at input (main scheme) works. My voice can be heared, when I speak into electret microphone capsule. Anyway it's clipped.

    How can I change gain in this circuit ? :)
     
  7. BobK

    BobK

    7,645
    1,663
    Jan 5, 2010
    You can put a resistor between the emitter of the first transistor and the existing resistor/capacitor pair. The gain will be approximately 22K (the collector resistor) divided by the emitter resistor. So 220Ω would make the gain around 100.

    Bob
     
  8. DominicNewbie

    DominicNewbie

    7
    0
    Jul 6, 2016
    Do I think properly?
    95_1187900973.gif
     
  9. AnalogKid

    AnalogKid

    2,307
    649
    Jun 10, 2015
    Not quite. What you did will reduce the gain of the first transistor stage, but it also will shift significantly the DC operating point, probably leading to asymmetrical distortion. Referring to the schematic in post 1, break the connection between the first transistor emitter and the 1K/100u node, and insert the 220 ohm resistor there. This also will shift the DC operating point, but not by nearly as much.

    ak
     
  10. DominicNewbie

    DominicNewbie

    7
    0
    Jul 6, 2016
    I have modified this circuit due to your tips. Now it has large self noise. I have to blow into microphone capsule, to see amplitude changes.

    Hmm, I checked twice. First transistor is nearest microphone input.
    I moved circuit into mounting plate.
     
    Last edited: Jul 8, 2016
  11. DominicNewbie

    DominicNewbie

    7
    0
    Jul 6, 2016
    Updated.
    Thanks for help. I just had to reduce value of capacitance capacitor.
    @BobK I looked at LT Spice scheme and did it.
    Current amplification is quite good, anyway I have to reduce input wire because it's like an antenna.
    :)
     
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