I'd like to capture sound from a speaker output.
The system seems to be running with a small DC current (bias, so voltage across speaker is about 3 V), so the audio AC signal could be (de)coupled from a load resistor via a small capacitor.
The 10 kOhms resistor drops the 6.5 VDC (open circuit) to 5 VDC. The audio signal is in the order of 10 mVpp.
I would then need to amplify this signal and achieve some sort of galvanic isolation. Probably an opamp and a low frequency audio transformer (<1Vpp according to https://en.wikipedia.org/wiki/Line_level). However, this requires a power supply for the opamp which would be tricky to derive from the system, so I'd need an isolated dc-dc converter.
Audio quality is not a priority - a simple circuit is (one-off build).
If the "primary" circuit could be eliminated using a high impedance transformer directly, it would be even better. The secondary signal then could be brought to line level with a transimpedance amp.
Maybe a mains voltage relay coil with >10 kOhms could be used creatively (but: core? secondary?)? Is a high sensitivity hall-effect sensor a feasability?
Any ideas appreciated!