Why do headphones work on Bela Mini line output?

count_the_rings

Apologies if this is covered somewhere else! I have a Bela Mini board, and I understand that the headphone output is amplified by the TLV320AIC3104 codec for driving headphones. But I'm a bit confused that if I connect headphones to the line outputs, they seem to work just fine - no noticeable difference in volume or audio quality..? I've tried a few different sets of headphones with different impedances and this always seems to be the case.

Obviously I will continue to use the headphone outputs for headphones as intended - I'm just trying to understand why this is - e.g. if I connect my headphones to the line level output of my digital piano, there is no sound - so is the line output of the Bela Mini/TLV320AIC3104 codec unusually "hot"? Or is there something else in terms of impedance etc that I'm failing to understand..? Thanks!

giuliomoro

this is the relevant page from the datasheet:

You see the line out is spec'ed with a Rload=10k, while the headphone out has a Rload=16ohm. They have the same nominal voltage outputs (which explains why connecting either to something else's line input doesn't make much of a differencec), however it doesn't really specify what happens when Rload on the line out goes very low (30 ohm or so). What you can expect is that with high power and low impedance outputs, the noise performance would get worse and also the low-frequency response; if you play some bass-heavy music and crank the volume all the way up to 0dB (or even +2 or +3) on both, you should be able to clearly tell the difference.