HUD-DX1 can be used by changing output gain according to the impedance of headphone.
Basic setting upon shipping from the factory is that all JP1 and JP2 jumpers are stuck. If all two jumper caps applicable to left and right channels are removed, headphone output gain increases by about 1.5 times.
Basic setting (about 16 ~ 300Ω): They are basically stuck into two pins upon shipping from the factory.
High impedance setting (about 300 ~ 600Ω): If high impedance headphone is used, remove all jumper caps for use.