> For line inputs, as they're usually AC-coupled Headphone amplifiers often have DC-coupled 10k potentiometer from input to ground with washer feeding a DC- or

> For line inputs, as they're usually AC-coupled Headphone amplifiers often have DC-coupled 10k potentiometer from input to ground with washer feeding a DC- or AC-coupled gain stage. So much for AC/DC detection. I think impedance measurement is used in practice. I can hear a quiet beep when I connect headphones to one of my laptops, for example.