Pre-story: there's an analogue speaker connected to some circuit (wireless doorbell really).
I want to figure when speaker becomes active (i.e. produces some sound). Basic measurements show there's +2.6V whenever speaker is active. I do not need to know the voltage or current on the speaker (so there's no need for ADC), just the fact the signal is there. 2.6V seems to be close enough to HIGH - but the tricky question is how to measure this.
Very schematically the circuit is drawn here:
The question is: how can I detect the speaker is active using GPIO, ideally without ADC?