I found a good post by someone techier at soundonsound.
”Ah yes, the classic case of an 'unbalanced stereo' output feeding a balanced mono input!
The key to understanding the problem here is to remember that:
1. a 'balanced input' always employs a differential receiver (whether constructed from a passive transformer or active op-amps).
2. a differential receiver is only interested in the difference in signal voltages between its two input terminals (hot and cold, or plus and minus).
3. anything that presents the same voltage to the two terminals (the so-called common-mode condition) will be ignored, which is where the 'interference rejection' property comes from.
So, in this case, you end up presenting the left channel to the differential receiver's hot terminal, and the right channel to its cold terminal. The output will be the difference between the two, and anything that is the same on both will be ignored (and removed).
Consequently, anything panned centrally -- kick drum, bass, vocals etc -- all disappear, and all you're left with is any instruments panned widely, and the stereo reverb! Or, if the source was 'dual mono' -- such as a talking book where the narrator us panned centrally -- very little at all!
There are two solutions. Mike mentioned using a special adapter that mixes the left and right channels together into a mono unbalanced signal and presents that mixed signal to the hot terminal, while the ground reference of the unbalanced source is wired to the cold terminal.
The other solution is a Y-split cable that presents the left channel to one mono unbalanced TS plug, and the right channel to a second unbalanced TS plug -- these would then be plugged into two separate mono channels, or the left and right inputs of a stereo channel, depending on what was available.”
https://www.soundonsound.com/forum/viewtopic.php?f=15&t=57712