Help with mic preamp

Discussion in 'The Projects Forum' started by zell12, Sep 23, 2011.

  1. zell12

    Thread Starter New Member

    Jul 15, 2010
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    We're trying to build this intercom,
    http://www.zen22142.zen.co.uk/Circuits/Misc/intercom.htm and the speaker amplifier section and relay switch works just fine but im having a hard time troubleshooting the mic preamp.

    We used 2terminal electret mic instead of the 3terminal shown on the schematic, and an MPS2222a transistor instead of a BC109C. I've compared the datasheets of the transistors but i've no idea what to consider first.
     
    Last edited: Sep 23, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
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  3. zell12

    Thread Starter New Member

    Jul 15, 2010
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    ive connected the 4.7u electrolytic (going to Q1) and 2.2k in parallel w/ the positive terminal of the condenser

    can that higher gain be tolerable?
    btw thank you for the reply sir.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Well, I'm still not quite sure how it's hooked up. Schematics make a big difference.

    Do yo have any 2N4401 or 2N3904 transistors? They would be better than the MPS2222; higher gain.

    Have a look at the attached. I increased the resistance to the mic to compensate for the higher voltage supply. With the pot VR1 turned all the way up you'll get a gain of about -160. That may not be enough gain; the BC109C has a good bit more than either the '3904 or '4401.
     
  5. zell12

    Thread Starter New Member

    Jul 15, 2010
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    oh sorry, i misread, so the mps2222 cant make up much of the required gain.

    should i stick with the transistorized amp or is it much recommended if i replace it with an amp chip? i have a spare lm358 here and ive searched some schematics, tried it on aperf board and its working fine . i just cant compare it yet w/ the transistorized one

    Thanks again sir.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    LM358 opamps are pretty old and slow. Do you have any TL071, TL072, TL081/82 or LF351/LF353 opamps around?

    Have a look at this:

    [​IMG]

    The 470k pot should actually be a 500k audio taper or log pot
     
  7. zell12

    Thread Starter New Member

    Jul 15, 2010
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    maybe youre right sir. cause when i connected the lm358 preamp before the relay section it exhibits lots of noise not tolerable for a lengthy wired intercom, though when connected directly through the speaker amp it's just fine.

    is the schematic above applicable for all the chips listed? ill try to look for one sir.
     
    Last edited: Sep 25, 2011
  8. SgtWookie

    Expert

    Jul 17, 2007
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    The schematic will work with any of the opamps that I listed. TL071 opamps would be the most preferable, as they have the lowest noise.

    Please remove the link to the site where your image is hosted.
     
  9. zell12

    Thread Starter New Member

    Jul 15, 2010
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    Done.

    Offtopic. How can I shift from mobile into classic version of the site? Thanks
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Thanks.

    I really do not know. There was a thread about how the mobile version changed months ago in this forum:
    http://forum.allaboutcircuits.com/forumdisplay.php?f=11
    but since I don't use a cell phone to access the site, it didn't apply to me - and I've never seen the mobile version.
     
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