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In the following schematic, I would like to decode PCM audio from my Raspberry Pi and output the signal to a 3W speaker. The PAM8302 datasheet is quite light. I don't understand what the input levels are. On the other hand, the PCM5100A PCM decoder offers a 2.1 Vrms stereo output with a 1kOhm load impedance.

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I would like to properly connect the decoder to the amplifier without a potentiometer. Any idea?

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    \$\begingroup\$ Where did you get that diagram? R45 looks like a really bad idea. A potentiometer is not (in general) a bad idea, it is just that R45 is connected rather strangely. \$\endgroup\$
    – JRE
    Aug 5, 2021 at 7:53
  • \$\begingroup\$ I got this from a random internet forum. What do you propose instead of R45? Putting IN- to GND and IN+ directly to OUTL and OUT R? \$\endgroup\$
    – nowox
    Aug 5, 2021 at 8:04
  • \$\begingroup\$ As U21 seems to have internal biasing to its mid-supply, one way would be to remove R45, feed into IN+ via C61 and leave IN- open. A better way would be to invert OUTR (digitally or with opamp) and feed that separately into IN- via C62. This will double to mono output amplitude. \$\endgroup\$
    – tobalt
    Aug 5, 2021 at 9:56

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The datasheet clearly states the closed-loop gain :

Maximum Gain : As shown in block diagram (Page 2), the PAM8302A differential gain for the IC is : A = 20log (RF/RI). The PAM8302A sets maximum RF = 150kΩ, minimum RI = 10kΩ, so the maximum closed-gain around 23.5dB. If need gain adjustment, you can put external RIN at the input pin, and gain calculate as 20log (150K/ (10K+RIN)).

So you can reduce gain to adjust best dynamic range if required by adding x2 Rin on each + and - inputs. Otherwise if you require more gain you would need extra amplifiers to do that, but I presume the desired purpose of the pot is to reduce the gain so that max input occurs at max output with minimum distortion etc.

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The connection of R45 looks very odd. I would expect the bottom end of the potentiometer to be connected to ground (and still connected to IN-).

If you don't want a potentiometer, replace it with two fixed resistors. If you aren't sure how to do the gain calculations, the easiest solution is to temporarily use a potentiometer. Adjust the volume until it sounds right, then remove the potentiometer without turning the shaft on it, and measure the resistances from the centre to the two ends with a multimeter. that gives you the two fixed resistors to use.

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