In ear canal phones vs Around the Ear Speakers amp question

[haroldo_psf]

My motorcycle audio system (Autocom, fwiw...) comes with a set of around the ear speakers. In order to use in ear type earphones (like the Etymotics in ear canal and such), they sell an adapter that has, in series with both left and right channels a 100uF e-cap and a 66 ohm resistor.

1) Why is this needed?

2) I did an experiment where I bypassed this series resistor and cap, and used that with my in ear canal phones...No sound came out, but they got REALLY hot inside my ears really fast. I power everything down and luckily there was no damage to my Etymotics phones. Does anyone know what's going on? Why by removing the series resistance and capacitance, the in ear phones don't work?

[Steerpike]

The existing amplifier is putting DC through the earpiece transducers. That is generally a no-no for any transducer - but they must have some special reason / design for doing it.
If you apply DC to a 'normal' headphone or speaker, you'll cook it, as you found out!

The DC will also force the voice coil to push hard up against it's end-stop, preventing any vibration due to music signals.

The capacitor blocks DC. The resistor adjust the sensitivity.