hi I wanted to do the same thing.
HEre is how I think it could be done:
Get a hifiberry DAC 3.5mm headphone jack version that is super compact
https://www.hifiberry.com/shop/boards/h ... lus-phone/ with a really great solid state chip, from a company in Switzerland. I do think it works with some audio linux server distros like pi player or volumio (please check before starting out on a project!). THere is an even smaller version for the pi zero with RCA jacks (
https://www.hifiberry.com/shop/boards/h ... -dac-zero/) but if you look closely it has a footprint for 3.5 mm headphone as well, just mabye some components missing that need to be soldered or ask nicely if they have a phone jack version for the zero if you prefer ultrasmall size.
The burr brown chippies used for the DAC (
http://www.ti.com/product/PCM5102A or
http://www.ti.com/lit/ds/symlink/pcm5122.pdf) on top should provide sufficient juice (i.e. >2V RMS) for earphones and maybe headphones (depending on impedance maybe...)
You could use a rechargeable 5V battery pack for usb charging as starters to check if it works. Key will be to determine how much capacity is needed for a player with decent running time (like 5h or so). LiPos would be neat but they need another board to bring up the voltage from 3.7v to 5v.
you could try to experiment with a rPi v3+ or pi 4 (dont know how warm they get when playing loss less etc using music server software.) with WLAN and BT.
As streaming is not environmentally friendly, pack your music onto the uSD card of the system and use your mobile via wlan or bluetooth to control.
Might be good to add a power button:
https://www.makeuseof.com/tag/add-power ... pberry-pi/
I havent tried this yet so you should check out the software distros before buying stuff.