I have a smartphone that uses a 3.5mm 4 conductor jack that, as you may know, contains both left and right outputs, and mic in. I have existing earpieces that use a 2.5mm 3 conductor plug, which I'd like to use on the phone.
I could get good volume with my very old Kyocera phone from 2003 which natively has a 2.5mm mono jack. However, I made an adapter for the smartphone which converts the 3.5mm plug to 2.5mm jack. Since the earpiece is mono, I connected its speaker to just one channel on the smartphone (only left or right). It works but the phone's volume has to be turned up almost all the way to be loud enough. I don't like this as there isn't any extra volume when it's needed.
My wife's smartphone has this same problem. It's a different brand but has the same type of jack. This is very simple but annoying problem. The old low tech phones were better at being a phone!
I thought maybe I could combine the left and right out into the mono earpiece with two 10 or 20 ohm resistors, but it doesn't seem to do much. I'm sure a decent amount of power is being lost in the resistors. Could it be an impedance problem? The earpieces work great with older phones. We don't want to use stereo handsfree earphones for phone calls.
We did get a mono earpiece from a store that was 2.5mm with a 4 conductor 3.5mm adapter, but that sound volume from that one was worse than any.
I have not been able to find info on this at all. It's frustrating.