1. Mini size and unique design: built-in tube makes you have a tube amplifier which can be held by one hand, convenient to carry and can enjoy HiFi music everywhere.  2. With built-in battery protection circuit: battery lets your listening feast without time limitation. It can't be turned on during charging and will power off automatically after fully charged, and charger LED will turn to green.  3. Full, delicate and soft sound: warmer than transistors, independent op amp and power supply inhance the SNR and degree of separation, less interplay and higher sound quality.  4. High-end making material and craft: using gold-plated socket, electrolytic capacitor, and precision ring resistance.    The difference between B4 and B4-X  1. B4-X is a dual-mono balanced version. Its left and right channel are amplified by independent components, and power supply, tubes, and op amp are all independent and symmetric. So the degree of sound separation is higher than B4.  2. B4-X adopts 2 groups of independent power supply(+12V and -12V) for 2 op amps, less mutual effects and can drive 300Ω headphones.  3. The circuit adopts dual op amp parallel output, with higher output power.  4. B4-X has 3.5mm and 2.5mm 2 kinds of output interface, suitable for most of balanced and standard headphones.  Tube: RAYTHEON JAN5899 x2  Lithium Battery: 12V, 1000mA  Charging time: 2-2.5 hours  Working time: 4-4.5 hours  Output power: 450mW  Voltage: AC100-240V  SNR: 105dB  Colour: Black  material: metal  Suggest load impedance: 16-300Ω  Output: 2.5 balanced Jack x 1, 3.5mm Jack x 1  Size: 115 x 69 x 23 mm (LxWxD) (does not include the knob)  Plug Type:EU Plug,it applies to Indonesia, Germany, France, Spain, Italy and so on    Package Contents:  1x headphone amplifier  1x power supply  1x 3.5mm o cable(The style and color of the o cable are randomly issued Do not accept designation)  2x coils (coil color random Do not accept designation)