Headphone Amplifier Hi-Fi Headphone Amplifier Buit-in Power Bank, It enhances the signal input into the amp circuit for optimal performance of the headphones. 3.5mm AUX input and output and SNR>100Db and THD+N 0.0003% make it possibility to output not just louder sound, but a sharper and more real stereo sound without distortion. Hi-Fi Headphone Amplifier Buit-in Power Bank, with 5F1A USB output. Play music time: 30Hours Working voltage: DC 3.7-4.2V Maximum power output: 100mWPower bank specification: 1,Battery capacity is 3000mAh(3v) Lithium-Battery. 2, Power bank output Max is 5V1ANOTEMake sure to turn the VOL knob to the minimum before putting on the headphones.USB of this product only has a charge function, does not have OTG decoding function. Q: It is turned to the maximum, but the volume is still not loud enough. What should I do? A: Our product is designed with a gain switch which can be set to H position if the sound is not loud enough. Q: My headphone impedance is 200?. It can work? A: According to our design theory, the product can support a headphone with 16-300 ? impedance. Therefore that is completely OK. Q: All cables are connected, but there is no audio output? A: 1. Please check the power indicator is on, if not, turn the otentiometer on. 2.If the power indicator lights, check that the VOL knob is in the smallest state, if it is, and it is normal to rotate clockwise in the minimum state. 3.If both of the above are normal, check whether the audio source playback device is paused, muted, or the volume is minimal.iphone to headphone amp Lightning to 3.5mm cable is not included, you need to buy it yourself