I have a smartphone with a 3.5mm jack that works with a pair of stereo headphones, or works with a handsfree earpiece. The jack is 4 conductor: GND, audio left, audio right, and mic. I modified an existing handsfree by replacing the 2.5mm 3 conductor plug on it with a 4 conductor 3.5mm. It works, but the problem is the volume through the speaker on the handsfree.
Turning the volume all the way up should be too loud, I say. In this case, it's about right for quiet environments. The handsfree earpiece works fine on old style cell phones that have no concept of left and right audio outputs. I'm guessing what is going on here is that the total audio output power is being split between each channel, so I wanted to try combining them in an external 3.5mm to 2.5mm adapter. What would be the best way to do this? Use a resistor from each output channel to the handsfree earpiece speaker? Or maybe, the speaker's impedance isn't correct. Here is the pinout of the phone if you're curious:
Thanks in advance.