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I'm trying to learn electronics on my own. I got this circuit from an old audio amplifier device I had lying around. I'm trying to understand how it works. I have some questions about it. Could you help me?

  1. This point has a DC Voltage of +6V, and it oscillates in the range of a few mV in response to Audio In. Then the DC offset gets removed by the directly next capacitor. Is all of this correct? What's the point of all of it?
  2. If I short this capacitor, the amplifier stops working. Why? Current through Audio In is still flowing, right? Is the capacitor there to remove some DC offset?
  3. I don't think I understand what exactly the op-amps are doing and how they do it. I assume the transistors simply take the heavy work of providing the current needed to drive the Speaker once the input signal is completely amplified.

Edit:

I have redrawn the schematic. Also, I think I found the explanations by experimenting in Falstad Simulator:

  1. The DC Offset at the output of OA1 is slightly above +6V because of R2; C3 removes that offset. The output of OA1 oscillates by more than a few mV. The job of OA1 is to increase back the voltage of the input, since the input got lowered its voltage by the load it is put through.
  2. Through AudioIn is the path of least resistance to ground for the output of OA1 to go through, so C1 is there to remove that DC offset.
  3. Explained in the comments: "OA2 and OA3 are driving the two outputs to be opposite from each other, plus they reduce the effect of the class B outputs."

schematic

simulate this circuit – Schematic created using CircuitLab

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    The circuit looks incomplete to me. Op-amp mid rail biasing is missing a resistor. – Andy aka Dec 19 '20 at 09:46
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    Perhaps the explanation would be easy to find if the circuit diagram was drawn in the conventional way... – Circuit fantasist Dec 19 '20 at 10:34
  • @Andyaka Do you mean another 20k resistor should be in the C section? I also think that. However, I have analyzed the circuit board several times and I don't think I'm missing any component. But I could still be wrong: I'll update all of this if I happen to find anything else. –  Dec 19 '20 at 17:46
  • You ought to show the power rails for the op-amp and mention what voltage they are at. Maybe the op-amps are running from +24 volts and 0 volts. – Andy aka Dec 19 '20 at 17:48
  • @Circuitfantasist Sorry about that. I drew the diagram many times and thought this looked OK. I'll figure out what I need to do to make it better. –  Dec 19 '20 at 17:48
  • @Andyaka Sorry about that. Each of the OP-Amps is running directly from the 12V source. I'll update the image. –  Dec 19 '20 at 17:51
  • Then you are definitely missing a resistor somewhere. – Andy aka Dec 19 '20 at 17:53
  • @Andyaka Thank you so much for the patience. I added two more components which were located far away from the rest of the circuit and I thought were irrelevant. It is a double sided board with SMD components, and it's very hard to follow the paths when they go underneath other components. Also, I think I drew the input power to the OP-Amps correctly? –  Dec 19 '20 at 18:31
  • Regarding third question, I'd say the OPA's are driving the two outputs to be opposite from each other, plus they reduce the effect of the class B outputs. – Pete W Dec 19 '20 at 19:30
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    @PeteW Seems like you are correct. Thank you for the comment! It was a huge help. –  Dec 22 '20 at 02:06
  • Glat to hear it. Just curious, what product was this circuit come out of? – Pete W Dec 22 '20 at 12:29
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    @PeteW It is an iPod speaker system. Brand: Speler, Model: BIM-169. Looks very similar to this. –  Dec 25 '20 at 07:49

1 Answers1

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A more conventional/understandable way of drawing the circuit diagram.

Amplifier circuit diagram