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I have a PAM8403 3W dual channel audio amplifier board bought from Amazon and I would like to connect one of it's output signals to LM3915 dot/bar display driver. The problem is that PAM8403 outputs differential signals and I can't use that as an input to LM3915.

schematic

simulate this circuit – Schematic created using CircuitLab

I tried to couple R+ signal to the led driver using a 100uF electrolytic capacitor but that didn't work for some reason. How do I source the signal to the LM3915?

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That PAM8403 is a class D amplifier so its output signal will be PWM (switched). So you need to filter the signal. Also, on average it will be half the supply voltage (as it switches between Vdd and ground) so you need to AC couple the signal as well as the LM3915 expects a ground referenced signal.

That the outputs af the amplifier are differential does not matter much, you can just use only one output. The other output carries the same signal but inverted (upside down). If you'd connect a speaker with a series capacitor between any of the outputs and ground, you'd hear the same signal (although inverted but your ear cannot detect that, only relative to another signal).

You could try this:

schematic

simulate this circuit – Schematic created using CircuitLab

C1 is the AC blocker cap

R1 and C2 are a low-pass filter with a cutoff frequency of around 10 kHz

This is just a suggestion and staring point. Don't blame me if it doesn't work as expected ;-)

You could also consider using the input signal of the PAM8403 as a signal for the LED driver. Optionally with a simple audio amplifier. A basic LM386 audio amplifier module will do the job and these are cheap.

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