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I am trying to diagnose why my LM386 (250mW version) audio amplifier is crackling when the volume is more than a few mV. I created the circuit according to the diagram on the datasheet with pin 7 and 8 floating and no variable resistor (signal from 3.5mm mono cable directly into pin 3). I did the following:

  • Used a "star grounding pattern" with all grounds connected to a common point next to the ground pin of the chip
  • Used a shielded 3.5mm mono cable
  • Tried adding a 250uF and 1000uF power supply decoupling capacitor
  • Tried changing the 250uF decoupling output capacitor to 1000uF
  • Tried a different speaker (4 ohm and 8 ohm)
  • Tried a different power supply
  • Tried removing the Zobel RC Filter
  • Power Supply is 12V / 120W

None of the above made a difference. Only when playing audio at very low (almost inaudible) volume does the crackling disappear. This is the output from the speaker:

enter image description here

This is the prototyped board:

enter image description here

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  • \$\begingroup\$ What's your supply voltage? How much current can your power supply offer? \$\endgroup\$ Commented Sep 12, 2021 at 22:48
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    \$\begingroup\$ Ceramic decoupling cap? \$\endgroup\$
    – Mattman944
    Commented Sep 12, 2021 at 23:09
  • \$\begingroup\$ @MarcusMüller 12V, the SMPS can supply 120W. \$\endgroup\$
    – tgun926
    Commented Sep 12, 2021 at 23:17
  • \$\begingroup\$ @Mattman944 electrolytic for both. Only the Zobel capacitor is ceramic \$\endgroup\$
    – tgun926
    Commented Sep 12, 2021 at 23:18
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    \$\begingroup\$ Looks like there might be an unintended stray copper strand (i.e. a short) between pin 6 (Vs) and 7 (bypass), but it’s hard to make out clearly. \$\endgroup\$
    – StarCat
    Commented Sep 13, 2021 at 6:10

1 Answer 1

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Place a 47 nF capacitor across the IC's power supply as the spec sheet says, fig. 22.

enter image description here

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    \$\begingroup\$ The AC coupling input cap Cc shown in this diagram would probably also be a good idea. \$\endgroup\$
    – brhans
    Commented Sep 13, 2021 at 0:08
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    \$\begingroup\$ The supply bypass capacitor should be placed very close to the chip, directly from pin 6 to pin 4. \$\endgroup\$
    – glen_geek
    Commented Sep 13, 2021 at 0:31

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