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I am working on an audio amplifier PCB circuit, where I have used LM386 as an amplifier, an aux jack for audio output, and a small speaker of headphones (30 ohm). I have two 220 uF molded and one 470 uF Alu. electrolytic capacitors parallel to the speaker and aux. You can understand from the figure.

I am facing some problems with audio amplification when the Aux is attached to the external MP3 player or Bluetooth MP3 player. Audio is not amplified specially low frequencies.

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I can see a total of ~1000 uF (22 + 2x 220 + 470) capacitance.

This amount of capacitance, at 1 kHz audio frequency, will show a reactance of \$1/(2\pi \ 1000 \ 930\mu) = 0.17 \Omega\$ which is basically short circuit. At higher frequencies this reactance will get even lower. So, what you are doing is effectively shorting the output speaker so nothing can be transferred to it.

I'm not sure if LM386 can survive on a continuous shorted output, you may need to replace it.

The R21 (10R) will be the LM386's load. Since it's able to drive 8R loads the LM386 shouldn't get damaged. Even if you remove the large caps this 10R along with the speaker impedance will form a voltage divider so the output power transferred to the speaker will be lower. If you need to reduce the power you should do this by changing the input level or the LM386's gain.

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You see from the schematics that you can put a huge capacitor in parallel with speaker or audio output.

The problem is, it makes no sense to do that, if you actually want to hear the audio.

Remove all the parallel caps from the audio path.

The next thing that makes no sense is the 15pF capacitor for coupling in audio to the amplifier. The cap is so small and it makes high pass frequency so large that not even bats can hear it.

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  • \$\begingroup\$ Should I also remove the R21 (10ohms) or that is fine? \$\endgroup\$ Oct 3, 2023 at 6:41
  • \$\begingroup\$ @NoorHassan I don't know why you designed R21 there. That's up to you if you want to keep it or remove it. Please bear in mind you have headphones. They usually provide ear-deafening sound pressure levels at measly 1 milliwatt, will burn up at 100mW, and LM386 depending on power supply voltage can push out far more power into 32 ohms than 100mW. \$\endgroup\$
    – Justme
    Oct 3, 2023 at 7:00

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