lm 1875 amp problem

Discussion in 'The Projects Forum' started by TIP31, Dec 14, 2014.

  1. TIP31

    Thread Starter New Member

    Aug 17, 2013
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    I need help with an amp using an lm1875. I am using a 15 volt wall wart as a power supply. But i am getting 6 volts of dc through my speaker. Below is the schematic I am using. I forgot to order a 220k resistor for the one near the ic so I had to use a 22k. Could that be the reason? 20141211_171939.jpg
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You are using single supply at 15VDC, correct? Then you cannot directly couple the speaker to the amplifier. Put a 2200-4700uF capacitor in series with the loudspeaker.
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    MrChips pretty much nailed the reason you are getting 6 volts across your speaker.

    The 220K resistor you replaced with a 22K will lower the gain of the amplifier by a factor of ten.
     
  4. TIP31

    Thread Starter New Member

    Aug 17, 2013
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    Like this? Or do I need to switch the polarity?
     
  5. TIP31

    Thread Starter New Member

    Aug 17, 2013
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    Thank you for explaining the resistor les. I was wondering why it was a little quiet.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    You need to switch polarity. Negative lead of the capacitor goes towards the loudspeaker.
     
  7. TIP31

    Thread Starter New Member

    Aug 17, 2013
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    That's what I meant. And does it matter which cap I use? I have both a 4700 uf and a 2200uf.
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,638
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    Hello,

    When I look at your last picture, it looks like you are connecting the speaker to pin 1.
    Pin 1 is one of the inputs of the amplifier, pin 4 is the output.
    See this example schematic from the datasheet:

    [​IMG]

    Bertus
     
  9. TIP31

    Thread Starter New Member

    Aug 17, 2013
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    No I have pin one as input and pin 4 is output.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Try both of them and listen to the bass response. If the 2200μF cap sounds good to you then go with that. You can even compare it with a 1000μF cap.
     
  11. TIP31

    Thread Starter New Member

    Aug 17, 2013
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    I'm wanting to make a 3 band eq for high mid and low. How should I go about putting that in there? Also I am putting an lm386n pre amp on it.
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    A LM386 is not a preamplifier.
    It is an 1 - 3 Watt power amplifier, depending on the model.

    If you are looking for a 3 band tone regulation, have a look at the following schematic:

    [​IMG]

    Bertus
     
  13. TIP31

    Thread Starter New Member

    Aug 17, 2013
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    Thanks and what would be a good ic for a pre amp?
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do we need to present everything on a golden platter?
    A quick search on this forum will give you this:
    Op Amps

    Bertus
     
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