Audio ampifier noise

Discussion in 'The Projects Forum' started by AVOLLC, Feb 4, 2015.

  1. AVOLLC

    Thread Starter New Member

    Feb 4, 2015
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    Hello, everyone. I am attempting to make an audio amplifier for a microphone. I am having problems with noise in the output. The noise is about 1/10 the signal level and would clearly by audible. The microphone in use is my POM-3535P-3-R. It has a SNR of 68dB which should be more than enough for a quality signal. I have uploaded a schematic to this link. Any help is appreciated.
     
  2. #12

    Expert

    Nov 30, 2010
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  3. AVOLLC

    Thread Starter New Member

    Feb 4, 2015
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    Thank you for your reply #12

    I have attempted to replace the input amplifier with an lm833 (4nv/Hz). However, this has not fixed the issue. In fact, it has more noise now. Any idea as to why this would be the case?

    --AVO
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You used an 1K resistor to bias the mic.
    Electret mics seem to work the best when biased with a current of about 1 mA.
    Make the 1K resistor 10K and try again.

    Also the first transistor depends now on the same resistor.
    Bias the transistor seperate and put a capacitor between the mic and transistor.

    Bertus
     
    #12 likes this.
  5. #12

    Expert

    Nov 30, 2010
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    What you connected to the first transistor base in the first drawing should connect to the left side of the 1uf capacitor with no problems.

    Notice I said, "should".
    I've been fooled so many times that I get cautious. :(
     
  6. AVOLLC

    Thread Starter New Member

    Feb 4, 2015
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    Bertus, thanks for replying. I am however a little confused about your response. No DC should flow through the cap, so why would it need to be separated from the bjt?

    Thanks again!
     
  7. AVOLLC

    Thread Starter New Member

    Feb 4, 2015
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    I just discovered that the his vanishes if I touch the end of the mic with my finger. Could it possibly be picking up random air currents?

    --AVO
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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    How far is the mic from the transistor?
    Is the signal wire shielded?
    Is it battery operated or does it have a power supply.
    Do you have any decoupling caps?
     
  9. bertus

    Administrator

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  10. AVOLLC

    Thread Starter New Member

    Feb 4, 2015
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    Bertus: Thank you for the link. This appears to use a jfet amplifier instead of my BJTs (radoshack model). I heard jfets are superior for audio, is this the case?

    ronv: The microphone is powered by a 9V battery, I have a 440uF decoupling cap between the rails of the supply.
     
  11. bertus

    Administrator

    Apr 5, 2008
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