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inductive capacitance???

Discussion in 'General Electronics Discussion' started by ahnakel, Jan 29, 2011.

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  1. ahnakel


    Dec 14, 2010
    I have recently put an LM386 chip on a breadboard and I have a 1000uF capacitor from the chip output to the speaker. The speaker is very crackly and not a clear pitch. I hooked up an LED and have noticed that if I touch the top of the capacitor then the light gets brighter. I noticed that it does also gets brighter before I even touch the capacitor. I know from reading about the Theremin that the hand has about 2 pF capacitance. Is this capacitor using inductive capacitance to boost it's own capacitance level?
  2. Laplace


    Apr 4, 2010
    So just where did you hook up the LED? If you put a Faraday shield around the input section of the breadboard circuitry, does the effect diminish?
  3. ahnakel


    Dec 14, 2010
    I hooked up the LED to a 1000uF capacitor from pin 5 which is the speaker output. This is the capacitor that is affecting the lighting .

    I have not tried a shield.
  4. duke37


    Jan 9, 2011
    If a small capacitance affects the output, it looks as if the amp is oscillating at high frequency. Have you by-passed the power supply properly?
    You should try a Zobel network (Wkipedia) across the output, perhaps a 100n capacitor in series with 10 ohm to correct the phase of the load and damp the oscillations, this type of network is often used on audio amps.
  5. ahnakel


    Dec 14, 2010
    I was just wondering if this is a sign of a cheap capacitor.
  6. Resqueline


    Jul 31, 2009
    Nope, it's most likely as duke says. I'm still just as wise as to how/where you hooked up that LED btw..
  7. ahnakel


    Dec 14, 2010
    I have the positive end on the capacitor going from pin 5 on the amp and then plugged into the ground. It is all on a breadboard. I am getting a screeching sound when I plug in a speaker. After trying the same chip in a different application i am sure that it is the chip. It just isn't working the way it should. I have ordered a replacement.
  8. davenn

    davenn Moderator

    Sep 5, 2009
    hi ahnakel :)

    time for us all to see your actual circuit schematic so we can better visualise what you are doing :)

  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It sounds more like your amplifier is oscillating.

    I doubt it's dead unless you've connected the power backwards or it's running so hot it burns your finger.

    1000uF for the output cap sounds high. The datasheet only shows 250uF

    I'm also wondering if you are using it in a breadboard and the otherwise disconnected pins 1 and 5 (for gain control) are getting feedback somehow which is inducing these oscillations.
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