Features: B4 is a vacuum tube dual-mono headphone amplifier, the sound feels warmer than transistor products. Top HIFI original DALE resistors; Precision rings resistance. It has built in bettery protection circuit.It powers off after fully charged automatically, the charger led turns to Green Used for Ipod- cellphone. MP3, connect to B4 input socket by 3.5 to 3.5 audio cable,suitable for most of balanced and standard headphones. When you are charging the battery, the B4 amplifier can not turn on, until you pull out charger interface,just make battery safe. Specifications: Item: Vacuum Tube Headphone Amplifier Gold Plated Input/Output Socket Chip: NE5532 OP (can be upgraded ) Capacitor: riginal nichicon electrolytic capacitor Battery：Lithium batteries voltage 12v 1000mA Charging Time: 2-2.5 hours to charging full the battery Working Time: 4-4.5hours Output Power: 450mW SNR：99dBm Suggested load impedance:16-400Ω Output jack: 2.5 balance jackx1 3.5mm jack x1 Size:approx 115x69x23MM/4.53x2.72x0.91in Weight: 240g Package Includes: 1PC Headphone Amplifier 1PC POwer Supply 1PC 3.5 Audio Cable note: Due to the problems of lighting effects and shooting angles, there will be certain tolerances in the product, please understand. Please allow 0-1cm error due to the manual measurment. when using B4 with your cellphone, if you heard some "zhi-zhi" noise in your headphone. Probably it is the inteference from your cellphone such like WIFI or 4G signal, which intefers the tube and cause noise. That is normal. Because there are microphone effect with all tubes-. To avoid it, please keep your cellphone far away from B4