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