Speaker Line Supervision
I'll be using STM32F4 to do this ...

This is a bit like &quot;I want to make a pavlova. I'll be using carrots . Which recipe should I use?&quot;
A much simpler solution would be to add 0.5 to 1.0 V DC on the line at the amplifier and monitor that at the far end.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Line monitoring circuit.

U1 injects some DC onto the speaker lines. This will cause a little heating in the speaker transformers.
C1 / R1 allow the audio to bypass the DC supply.
U2 will monitor the DC supply. C2 / R2 filter out the audio.

I have never worked on a design such as this but the principal is used on several systems.