Please connect the input devices, like computers, mobile phones, to in port, and then plug the headphones into out port Correctly . Increase the volume of the input device and increase the master volume of the product It is not recommended to use while charging, line noise may be generated. This is why the product is powered by lithium batteries, and battery power supply can avoid line noise No, the USB interface of this product only has the charging function When an unshielded signal line is empty, an interference signal enter the amplifier and be amplified. If you dislike the sound, we suggest you replace the shielded signal line or unplug the signal line<br><br>Hign-concerned Chemical:None Type:Mini Amplifier Recommended Headphone Maximum Impedance:101-150 Ohm Color:As Shown Material:Aluminum Alloy<br><br>Package Contents: 1 * Audio Amp<br><br>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.