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 300 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 SNR: 105dB Suggest load impedance: 16-300 Output: 2.5 balanced Jack*1, 3.5mm Jack *1 Size: 115 x 69 x 23 mm (LxWxD) (does not include the knob) Plug Type: US Plug, it applies to Canada, India, Mexico, Thailand, Taiwan, the United States, the Philippines and so on. Material:Metal colour:Black Package Contents: 1 * headphone amplifier 1 * power supply 1* 3.5mm audio cable (the audio cable style and color are randomly sent, and the designation is not accepted) 2 * coils (coil colors are random, no designation is accepted) 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.