Sony Portable DAC/Headphone Amplifier Silver PHA1A - Best Buy
Amazon.com: TNP Portable Headphones Amplifier Stereo Headphone Earphone Amp Volume Control Audio Booster with RCA Input 3.5mm 6.3mm Output Jack & Power Switch : Electronics
Amazon.com: eSynic Professional 16-300Ω Headphone Amplifier Portable Headphone Volume Amplifier Rechargeble Headphone Amp Desktop Amp With 3.5mm Jack & A Two-stage Gain Switch for MP3 MP4 Phones and Computers etc : Electronics
Z10e Electrostatic Headphone Amp / Integrated Amp - Vacuum Tube Electrostatic Headphone Amp by Linear Tube Audio