The DFplayer has a bridged output for a speaker but I need more volume. Can I connect a mono amplifier input to one of the DFplayer outputs or should I use an op-amp differential to single-ended circuit in between?

 

Can you not tap into the Input of the bridge amp, and put a better amplifier on it? Otherwise you're going to have to Sum the output and re amplify it.

 

Hello,

Did you see this cheat page for the DFplayer:
https://reprage.com/post/dfplayer-mini-cheat-sheet

An other page is mentioning pin 4 (DAC_R) and pin 5 (DAC_L) for ouputs:
https://www.electronicslovers.com/2018/01/interface-dfplayer-mini-mp3-module-with-arduino.html

Bertus