Now with a volume control so you can connect directly to the line out of a digital audio player and control the volume directly!This Portable Headphone Amplifier has a higher power output, better signal to noise ratio and lower distortion! And with the new TI's headphone amp solution. Small aluminum shell, on the back with screw-free tool for removal can be easily twisted under the screws to open the machine to replace the battery. Driver Goethe, Alice, Iron Triangle and other low-impedance headphones outstanding performance. 2 pcs 9V batteries need to be used for this Earphone Amplifier in RA1.Time to try this Headphone Amplifier!