Hi there,
this is not really MCU related but I want to have an additional speaker connected to the phone.
As I have some cheap PAM8403 board lying around I figured I want to use that for amplifying the handset speaker.
So using the two wire handset speaker as input to one channel + GND of the pam8403 I would get nowhere (almost silence / barely audible). Using just one of the leads as input to the PAM without connecting the PAM audio GND input I will get great audio as soon as the handset is off-hook but there will be significant buzz when the handset is on-hook.
I do get rid of the buzz when I hook up 270Ohm resistance in series with the output of the PAM to the speaker but that will also bring down the volume too far.
That phone is hooked up to the FXS port of an MTA which in turn is powered by a wall-wart.
The PAM8403 is powered by 5V from another wall-wart.
Any advice on how to filter that buzz or how to get rid of it?
Best,
Tillman