I've built the M0AYF active loop antenna for my RTL-SDR, using BC847C for the amplifier transistors.
There are a couple very strong AM stations here, usually playing music. What I'm noticing is that whenever I tune to something strong, like another station with spoken word, I hear the music in the background. I can even hear it in obvious QRM signals (buzzing is modulated to the music).
this didn't happen with passive antennas, so I suspect the amplifier circuit.
What is this effect properly called, what could be causing it and how to fix my receiver? Is the amplifier gain too strong? (How would I lower it - reduce the supply voltage? Add a voltage divider in the signal path?)
Edit - the solution: Following the answer and comments, I built a 30MHz low-pass filter using the schematic from here: https://www.hfsignals.com/index.php/ubitx-v6/
The filter is connected directly to the antenna, before going into the amplifier. Neither side is grounded, the "GND" in the filter diagram is actually one leg of the loop.
This largely removed TETRA interference and FM stations and the overloading problem is completely solved. There is still some QRM left, but that's probably unavoidable when receiving in urban areas.