[Mini HiFi Headphone Amplifier] - This headphone amplifier is designed for you to enjoy music anytime, anywhere, and the robust aluminum alloy shell allows for long-term use. [3.5mm AUX Input and Output] - Supports most high-impedance headphones of 16-300Ω, 3.5mm AUX input and output, signal-to-noise ratio 100Db, +N=0.0003%, Ability to output not only louder sound, but a more real stereo sound without distortion. [Two-stage Gain Switch] - This headphone amplifier amplifies the signal input to the amplifier circuit for optimal performance from your headphones. It can not only output louder sound, but it can also output clearer and more realistic stereo sound without distortion. [Rechargeable Lithium Battery] - With a 1000mAH rechargeable lithium battery, the headphone amplifier can work for more than 8 hours. In addition, there is a volume control knob to adjust the volume [Wide Compatibility] - This amplifier is highly compatible with digital devices, such as MP3, MP4, computers, CD players, and personal media players. Specification: Plug and play, with volume adjustment knob. Load impedance: 16-300Ω Audio input: 3.5mm jack Audio output: 3.5mm jack Battery capacity: 1000mAH Working voltage: DC3.7-4.2V Charging voltage: DC5V Material: Aluminum alloy color: black Package contents: 1 x Headphone amplifier 1 x Audio cable 1 x USB power supply line 1 x English manual Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The allowed measurement error is +/- 1-3 cm.