5. What are the differences among the X5 3rd gen, X7 and X5 2nd gen?

A: The main differences are as follows:

1. Operating system
The X5 3rd gen is running with a deeply customized Android 5.1.1;
The old firmware of the X7 is based on Android 4.4.4, but the latest version is based on Android 5.1.1, same as the X5 3rd gen;
The X5 2nd gen is running a non-Android system developed indepently;

2. Amplifier module
The X7 has two parts--the body and amplifier module. We now have a series of amplifier modules (AM1, AM2, AM3, AM5, etc.) available for different use. The amplifer module of the X5 3rd gen and 2nd gen cannot be replaced, it's integrated to the PCB board.

3. Buttons, ports and other interfaces
a. Balanced output: The X5 3rd gen has a 2.5mm balanced output port at its bottom side. And the X7 will support Balanced out when paired with the AM3 amp module. But the X5 2nd gen does not support balanced output.
b. Micro SD card slot: The X5 3rd gen has two concealed micro SD card slots with card tray, while the X5 2nd gen has two open micro SD card slots and the X7 only has one.
c. Volume adjustment: On the hardware level, the X5 3rd gen utilzies an independant volume scroll wheel for volume adjustment, while the X7 and X5 2nd gen utilizes the volume buttons. On the software level, the X5 3rd gen and X7 support volume control by sliding on the touch screen.

4. Chip solutions
X5 3rd gen: SoC: RK3188, DAC: AK4490x2,  LPF: OPA1642x2, OP: customized OPA426x2
X7: SoC: RK3188, DAC: ES9018S, LPF: OPA1612 , OP: (it may change with different amp modules)
X5 2nd gen: SoC: JZ4760B, DAC: PCM1792A, LPF: OPA1612, OP: OPA1612+BUF634

5. Charging
X5 3rd gen supports part of the mainstream quick charge protocols (such as BC1.2, MaxCharge, etc.) and can be charged with the DC 12V/1.5A , 9V/2A quick charger or normal DC 5V/2A charger, while the X7 and X5 2nd gen only support charging with the normal DC 5V/2A charger.