Headphone Impedance Demystified: Do I Need a Headphone Amp?
Archimago's Musings: MEASUREMENTS / MUSINGS: Headphone impedance, sensitivity, efficiency, and amplifier output impedance. (And a quick thought on Darko's All You Need Is)
HiFi Amplfiers Headphone Earphone Amplifier Portable Aux In Port For Phone Android Music Player AMP With 3.5 mm Jack Cable - AliExpress
Help Guide Listening to music with headphones via balanced connection
LiNKFOR USB Audio DAC Headphone Amplifier Digital to Analog Converter 6.35mm to 3.5mm Adaptor Sound Card USB to Coaxial S/PDIF
What impedance should I choose?
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Headphone Amplifier, Portable HiFi Headphone Amp 3.5mm Stereo Audio Gain Switch Support Impedance 16-300Ω Headset for MP3/MP4 /Phones/Digital Players/Computers/Xbox one,PS4,PS5 Controller : Electronics
Headphone Jacks and Plugs: Everything You Need to Know
What do the green, blue and red wires do in an earphone? - Quora
DIY Hi-Fi: Class a Hybrid Headphone Amp : 6 Steps (with Pictures) - Instructables