Usage Tips:[Mini HiFi Headphone Amplifier] - This headphone amplifier is designed to allow you to enjoy music anytime, anywhere, and the sturdy aluminum alloy shell allows for long-term use. [3.5mm AUX Input and Output]- Supports most high-impedance 16-300Ω headphones, 3.5mm AUX input and output, signal-to-noise ratio Greater than 100Db, THD+N=0.0003%, Make it Possibility to Output not just Louder Sound, but a Sharper and More Real Stereo Sound without Distortion. [Two-Stage Gain Switch]- This headphone amplifier enhances the signal input to the amplifier circuit for optimal performance from your headphones. Not only can it output louder sound, but it can also output clearer and more realistic stereo sound without distortion. [Rechargeable Lithium Battery] - Using 1000mAH rechargeable lithium battery, the headphone amplifier can work for more than 8 hours. In addition, there is a volume control knob for adjusting the volume [Wide Compatibility] - This amplifier is very compatible with various 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: 3.5 mm jack Audio output: 3.5 mm jack Battery capacity: 1000mAH Working voltage: DC3.7-4.2V Charging voltage: DC5V Material: Aluminum alloy colour: black Package Contents: 1 x Earphone 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 measurement allowed error is +/- 1-3cm.