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Schematic

I found this circuit on a YouTube video and tried copying it down into a circuit for testing, but the audio that comes out of it is really low quality and distorted. I don't expect sterling quality out of a quick hobby circuit like this, but I do expect something better than what I'm getting. This copying comes with the caveat that I don't really know where to find the video anymore. The schematic, however, is a faithful reproduction of what I have built, complete with all alterations from spec.

I had to fudge some of the capacitor values, given what I have on hand. I added in separately the part where the two channels get mixed with 1K resistors, as I just have the one speaker here and this seems an acceptable way to mix two channels. The audio source has just been my phone, as it's the most convenient thing on hand with the jack in question. The physical circuit is built on a mini solderable bread-board without power rails on it. I know that I should have kept it on a solderless one until I had it thoroughly debugged, but I got excited and locked myself into harder changes.

My questions are:

  1. Is there anything about this circuit that suggests excess distortion?
  2. I think I accidentally swapped the order of R3 and C4. Most other circuits seem to have the resistor closer to ground. Is this a problem?
  3. I had to fudge the numbers on a bunch of the capacitors, making them somewhat bigger than was called for. In general with circuits like this, what range of sizes should the capacitors be and should I err on the side of bigger or smaller when I don't have an exact match?
  4. Just what is the purpose of C3? I see it in some example circuits but not in others. The datasheet was not enlightening for me.
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  • \$\begingroup\$ What is powering it, a 9V battery, or something more powerful? \$\endgroup\$ – Justme Aug 7 '20 at 5:08
  • \$\begingroup\$ It's powered by a 9v battery. \$\endgroup\$ – Michael Cordingley Aug 7 '20 at 5:10
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    \$\begingroup\$ Is the output still distorted even at low volumes (i.e. adjusted from your phone)? \$\endgroup\$ – Rohat Kılıç Aug 7 '20 at 5:10
  • \$\begingroup\$ I get progressively less distortion at lower volumes. It eventually cleans up at extremely low volumes. \$\endgroup\$ – Michael Cordingley Aug 7 '20 at 5:11
  • \$\begingroup\$ @VTNCaGNtdDVNalUy Av=20 when there's no gain resistor. \$\endgroup\$ – Rohat Kılıç Aug 7 '20 at 5:15
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  1. Input signal voltage can be too high. Max input voltage for the chip is +/- 0.4V before it is damaged and phone output could easily exceed that, so you should use volume control pot at input. Power supply can be too weak, especially if it is a 9V battery, so it does not provide enough current.
  2. No, it does not make any difference
  3. The values look fine. Datasheets usually tells example circuits and why each part is necessary and what it does and effects of changing values are sometimes documented. This is an audio amplifier, changing values by a reasonable amount will have little performance effect, and frankly the part is so old that it specifies capacitor values that are hard to find these days.
  4. C3 at the bypass pin is a bypass capacitor to filter noise and ripple from the internal bias node it connects to. Which also reads in the datasheet.
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  • \$\begingroup\$ @VTNCaGNtdDVNalUy I get that there are issues with the quality of the circuit. It's why I'm here. \$\endgroup\$ – Michael Cordingley Aug 7 '20 at 5:28
  • \$\begingroup\$ @VTNCaGNtdDVNalUy Look at the schematic on the datasheet. There's a feedback network built into the device. It's not an opamp in the traditional sense. It's intended as a power audio amplifier. And, of course, the makers provide a "minimum design" without any external feedback as an Av=20 system example in the datasheet. I discuss its cousin, the LM380, here. \$\endgroup\$ – jonk Aug 7 '20 at 5:28
  • \$\begingroup\$ I think Justme has it right in paragraph 1. \$\endgroup\$ – user105652 Aug 7 '20 at 5:34
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    \$\begingroup\$ No, it does not need more resistance, but a voltage divider to divide the voltage down for the input. Which actually is easy as you have the 1kohm mixing resistors in series already, so you only need single resistor to ground. Or a potentiometer or a trimmer. A weak supply like a 9V battery cannot keep voltage at 9V when it has to provide current. The output signal then has no way to keep its original waveform if the supply voltage drops too much, and a clipped waveform results into distortion. \$\endgroup\$ – Justme Aug 7 '20 at 5:50
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    \$\begingroup\$ Also do note that you have an 8 ohm speaker. Putting a 1V signal over it requires 125mA of current at 1V peaks. That is only 0.125W of power. And 9V batteries won't last long giving 125mA peaks. It is more likely that you get better sound by connecting the speaker directly to the TRS jack. \$\endgroup\$ – Justme Aug 7 '20 at 5:54
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C3 should be somewhere close to C5 (same or double the value) since its job is to supply the instantaneous power that will be delivered to the load via C5. You can reduce it if you have a very well regulated supply, but a 9V battery doesn't count.

You should also check that the DC voltage on U1 output is half the battery voltage (give or take). If not, the output swing before clipping will be reduced. That would suggest some DC leakage somewhere, faulty C1 or C2.

Apart from that it sounds as if it's doing what an LM386 does. You need an oscilloscope to be sure, but I expect you're just overloading the puir wee thing since it's clean at low volumes.

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If you have a scope and signal generator, then check out the distortion at BASS tones versus TREBLE tones.

The bypass cap will not help on the BASS tones.

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