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Capacitance across an audio signal

  1. Oct 9, 2012 #1
    Looking at this datasheet:
    http://www.ti.com/lit/ds/symlink/lm386.pdf
    Most circuits near the bottom have a 250uF cap in line with the output.
    Would changing this cap affect the audio signal or would it just further smooth out the waveform?
     
  2. jcsd
  3. Oct 9, 2012 #2
    Just took a very quick look. I think if you were to make the signal smoother then you are effectively attenuating the AC content of the signal, so if you were to increase the cap size then the audio would become quieter.

    If you wanted to improve the audio quality replace the cap with a polypropylene or polyester type one.
     
  4. Oct 9, 2012 #3
    I don't really have access to polypropylene/ polyester caps.
    I'm just wondering how accurate I have to be with the cap size.
     
  5. Oct 9, 2012 #4

    berkeman

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    Staff: Mentor

    That is a DC blocking cap, since the opamp is being run off of a single power supply voltage (instead of split power supplies symmetric about ground). It needs to be big enough to pass the lower frequencies in your audio signal.

    You would calculate the value of the capacitor based on the lowest passband frequency and the impedance of the load (like the speaker).
     
  6. Oct 9, 2012 #5
    Alright, I'm just having trouble finding a 250uF, I'm looking at abra electronics (already ordering from there) and the don't seem to have one in any type.
     
  7. Oct 9, 2012 #6

    berkeman

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    You can certainly go to the next biggest size. Use a 330uF or 470uF with a voltage rating that is at least twice the amp's power supply voltage.
     
  8. Oct 9, 2012 #7

    berkeman

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    Oh, and be sure to place it with the polarity correct -- the + side faces the amp, and the - side faces the ground-referenced load.
     
  9. Oct 9, 2012 #8
    I figured that with the polarity.
    I'll try to get as close as I can while staying above,
    Thanks.
     
  10. Oct 9, 2012 #9
    If you look at the -3db cut off assuming 8Ω speaker

    [tex] f=\frac 1 { 2πRC}\;=\; 80Hz[/tex]

    If you need to get more bass, you should use bigger cap. A 470uF will bring the cut off freq down to about 40Hz.
     
  11. Oct 9, 2012 #10
    Sounds good, I'm going to be looking for a more bassy (bassish?) sound, 470 will be perfect.
     
  12. Oct 9, 2012 #11

    dlgoff

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    They probably chose 250uF to get the cut-off above 60Hz in order to minimize the "60(or 50)hz hum" picked up from stray a.c. power lines. The 330uF cap would put the -3db cut off right at 60Hz.

    With the 470, the OP may get a little more "bassy (bassish?)" than he wants. :biggrin:
     
  13. Oct 10, 2012 #12
    Bass is good, if your speaker is still intact (and not burnt out), there isn't enough. :D
     
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