Repairing Thomann Box – bad hum
I have a Thomann the box 350w powered cabinet https://www.thomann.de/gb/the_box_pa502a_aktives_fullrangesystem.htm that has recently developed a very nasty hum (after having returned from a repair shop that cannot fix it).
The hum is so loud that if I turn the volume to even the first notch it feels like the speaker might blow!
If no XLR cable is plugged in then the hum disappears, and if I plug-in an XLR to minijack cable (which probably has no grounding), there is also no hum. But if I use other XLR cables, the hum returns immediately, even if they are not plugged in.
Any ideas what this is and how to fix it?