LM386 Problems

Discussion in 'The Projects Forum' started by shredability, Aug 26, 2012.

  1. shredability

    Thread Starter Member

    Apr 1, 2012
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    [​IMG]

    I built this circuit on a breadboard using my 12V supply. When I transferred it over to my proto board it all stopped working. My 8ohm speaking will make a loud gargled frequency and when I check with the scope there is an unstable 5VDC at pin 5.
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The first thing I would check is the polarity of the 250 μF cap.
     
  3. shredability

    Thread Starter Member

    Apr 1, 2012
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    Yea that's all good
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Maybe if you post a photo or two, someone could spot a problem. Here's the one I built.

    Hey, I just noticed this was my 1000th post.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    That sounds like an oscillation. Did you use any bypass caps?
     
  6. shredability

    Thread Starter Member

    Apr 1, 2012
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    I just built when the schematic showed me, where would one put the bypass caps? and why would it work on the breadboard but not the protoboard?
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Oscillations are very much a function of layout and wiring, especially if you have no bypass caps. See Tracecom's schematic. He did it right.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    If the amplifier is powered from a little battery then a pretty big (470uF to 1000uf) supply bypass capacitor is needed to stop the supply voltage from jumping up and down with the output signal. The jumping creates a motorboat sound.

    12V is a very high supply voltage. The max output will be only a little higher than if a 9V supply is used and the extra power just makes the amplifier get very hot.
     
  9. shredability

    Thread Starter Member

    Apr 1, 2012
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    So when you say bypass capacitor, to what are you referring to? the 250uf in my schematic? I was told that's called a decoupling capacitor? or a cap across the +12 and gnd pins?
     
  10. shredability

    Thread Starter Member

    Apr 1, 2012
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    Alright guys, thanks for the help all I needed was a cap across the power pins I guess my supply was shaky.
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The most important by-pass capacitor is between the power and ground pins. Try adding a .1 μF cap as close to pins 4 and 6 as possible. The next step is to add a .1 μF cap from pin 7 to ground.

    And as I previously suggested, photos (top and bottom) of your assembly might help someone to spot problems.

    Oh, and the 250 μF cap in your schematic is a coupling capacitor. It is there to connect the output of the amp to the speaker. It allows the AC (sound signal) to pass, but blocks the DC.
     
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Good. As you saw, it was not included in the schematic in the datasheet. That is because it is standard practice to include it and the publishers of the datasheet took it for granted that you would do that.
     
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