How to get stereo and mono signals simultaneously?

Question

I have PAM8403 circuit module to drive my two 3W speakers. I want to drive a subwoofer with a different amplifier. I have a 3 inch subwoofer, and I'll get an amplifier for that. My question is how do I get stereo and mono (both) from my source simultaneously? Because if I short the two signals (L and R), or use a resistor and get mono signal, I lose the stereo signal, and signal from the wires becomes mono. I have tried using just L or R after merging the signals, but they do not stay stereo as they were before, so output from the stereo amplifier sounds mono. So what do I do to get separate mono signal from my stereo signals?

I am a beginner at electronics and speakers, so I request you to please construct your answers accordingly so I can understand better.

Answer 1

Instead of just shorting the channels, you should be mixing them with two high-value resistors, one in series of each channel

L ----+-------> to stereo amp
      |
     | | high-value resistor
      |
      + ----> to mono amp
      |
     | | high-value resistor
      |
R ----+-------> to stereo amp

The idea is super simple: Amplifiers have an input impedance that is typically much higher than what the sound source drives, so that it really doesn't matter to them that there's series resistors in the way. However, these high-value resistors will significantly reduce the amount of cross-talk, unlike shorting the channels.

If the cross-talk is still too high, you'd just place opamp voltage followers between the resistors and the stereo channels. These basically have infinite isolation.