Amazon.com: Headphone Amplifier Portable 3.5 mm Audio Amp with Lithium Battery for MP3, MP4, Tablets, Smart Phone, Digital Player, Computer PC : Electronics
mixer - Headphone amplifier for guitar with stereo MP3 input - mixing - Electrical Engineering Stack Exchange
HiFi Audio Headphone Amplifier Earphone AMP MP3 PC Phone Stereo Music Improver 16-300Ω | Wish
Portable 3.5mm AUX HIFI Headphone Amplifier Stereo Audio 16-300Ω for MP3/MP4/PS4 | eBay