Sound Quality bad when using Fiio BTR5 2021

[Som09mer]

Plan

Premium

Device

iPad mini (6. Gen)

Operating System

iOs 15.5

My Question or Issue

When using the Fiio BTR5 on the iPad, the sound has some kind of echo and sounds distant. Plus it sounds a bit distorted.  Youtube or other sound sources sound normal. On my iPhone SE 1.Gen or MacBook it works without problem.

Equalizer and any other sound altering options are turned off (like on the other devices).

Already tried clearing the cache, restarting the iPad and re-installing the app.

If anyone has any other ideas, I would be really thankful.

[Som09mer · Accepted solution]

Was able to find the problem. 

The iPad was set to "Mono Audio only". 

For some reason, this caused the problem, but only with Spotify. Every other App was working fine.