I have an old synth that has single headphones output (but user manual states that it can be connected with line-level inputs). I also have an audio interface that has one combo Mic/Line mono input and one Hi-Z instrumental mono input.
Suppose that there is a Y-cable with single stereo jack on one side and two separate mono jacks on the other side. If I plug stereo end of the cable to the headphones output and plug one of the mono ends to line input of the audio interface while keeping the other branch of Y-cable unplugged, is it safe? By safety, I mean absence of damage to both the synth and the audio interface. For example, I found an information that using stereo cable for connecting stereo output to mono input may result in back-feeding and destroy of stereo output. And what about signal quality? Can a signal be distorted or corrupted with noise by this way of connecting things?
As an alternative, I also can plug the other branch of Y-cable to instrumental input. I guess that gain of this input must be set to very small level. If doing so, is it safe? Is it better or not in comparison with the previous idea?