I am amplifying and audio signal with ac levels of around +-0.3V. This signal also has DC level of 1.2V my circuit I used 2 DC block capcitors because I saw the input should be ground referenced. The output of this circuit that I measured was for some reason also DC bias [even though I used a dc block capcitor aswell] but the sound was awful and I think it is because the output has lost all its negetive sound . Also first time I noticed this amp doesnt have -Vsupply but ground. I know it is also got truncate but I doubt thats the reason . output meausre I know there is FFT on the right side but this is not FFT of the audio its in time plain .

I thought maybe to lose the DC blocks in the input and to put 1 or 2 diode in series to lower the DC but I dont think it will end up well. Also thought to replace LM386 to LM370 but the input resistance is 2M . any solutions How can I get the negative part ?

  • Where did you get the idea that you need those 220uF caps on the input? They are almost certainly the source of your trouble. – brhans Sep 27 '16 at 17:20
  • Show the polarity of that output capacitor on your diagram. – brhans Sep 27 '16 at 17:22
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    And while you're about it - turn the schematic around so that its "the right way" with inputs on the left, outputs on the right, ground symbols pointing down, etc. You're asking for help, so make it easy to help you by not making your schematic more difficult to read. – brhans Sep 27 '16 at 17:24
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    Look up the datasheet for the LM386, then build the circuit as shown. Read the text while you are at it. It helps to understand what is going on. – JRE Sep 27 '16 at 17:27
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    You have 'reverse-biased' an electrolytic capacitor, so it's no longer a capacitor ... That's why you're getting a DC offset on your output. Take that cap out & throw it away, then put a new one in the right way around (+ towards LM386's output pin). – brhans Sep 27 '16 at 18:18

The short section of audio shown looks OK. It is suffering from clipping on both the bottom peaks AND the top peaks too. Clipping indicates that you're applying too much signal at the input. Such overdrive can result in your "awful" distortion.
Don't forget that this chip has a minimum gain of 20 dB. And don't forget that any input signal extending more than 0.3v below ground is outside acceptable territory.
Those 220uf input caps are overkill, and might cause strange effects. I'd suggest reducing them to 0.33uf non-polarized types.

  • I didnt thought the clipping will effect the quality so badly . How can I know what capcitors to use for what DC bias ? – Maor Sep 27 '16 at 18:14
  • @Maor LM386 input circuit is internally self-biased. Capacitor value shouldn't really matter, but strange things happen when you drive the input transistors into non-linearity with a big signal. Your basic circuit should work OK. Observe proper polarity of C21. + side should face LM386. – glen_geek Sep 27 '16 at 18:21
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    @Maor ...internally biased with 50k resistors to Ground. Large electrolytic capacitors leak and upset the bias point, especially if they are connected backwards. 220uF gives a cutoff frequency of 0.014Hz, which means it could take several seconds for the bias point to stabilize after a change in DC input voltage. – Bruce Abbott Sep 28 '16 at 0:36

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