I got a second-hand Neewer NW-3000 closed headphone which sounds like there is phase cancellation and no stereo effect. So I tested the headphone with a mixer and distinct tones on each channel; I figured out that each output channel is:
output_channel_right = output_channel_left = input_channel_right - input_channel_left
Then I can listen to my music, but only one of the input channel to the 2 headphone channels (i.e. the same for each ear), providing on balancing everything to the right (or the left) in the audio player...
The headphone speakers sounds good and there is no faulty contact at the plug. So what is wrong? Is it a manufacturing fault? Is the manufacturing fault probably from the jack plug? In another words, could I save these headphones by cutting the cable and change the jack plug only? Is it possible that a wrong connection caused such input_channel_right - input_channel_left result?