Virtually every musician and event technician has been suffering from this distressful experience - this diffuse amp humming, occurring in the rehearsal room or (even more often and even worse) on stage. You often try to be okay with it, but that doesn't necessarily mean that you’ll be particularly happy with it in the long run, especially during the quieter moments of your performance. Then again, you can try to understand the cause and to eliminate the source of error.
Okay, let’s try to get to the bottom of this phenomenon: this humming from the amplifier or the mixer is, so to speak, current you hear. On the one hand, you have lots of different power sources, on the other, numerous different devices, which are all earthed and connected to sockets (different circuits/potentials). And, in between, instrument cables, practically becoming a kind of bridging between 2 earthed sockets - and you will hear that, because the so-called equalizing current flows through the shielding (ground) of the cable. This then will be perceived as a low-frequency interference signal. Experts call this phenomenon a ground loop.
Then, how do you get rid of this annoying noise? It’s probably not possible or feasible to connect all devices to one socket, right? A simple, very functional solution is the use of DI boxes (e.g. CORDIAL’s CES 01 or CES 02). The signal then flows via the DI box and into the PA. For each channel you need a DI box or a stereo DI box for stereo devices. In addition, you can use line transformers (e.g. CLT 1 in Neutrik housing), which help to eliminate the equalizing currents (direct current) between the devices by interrupting the ground connection of the signal conductors. Warning: Danger to Life! If you value your life, never ever interrupt the protective earth conductor yourself!
Unwanted magnetic fields, so-called electrosmog, can also be a cause. In addition to using high-quality material (well shielded cables, high-quality power supply units), it surely helps to separate the signal lines (power, light, audio) spatially as much as possible, not to let them run in parallel, but ideally star-shaped (galvanic separation) and to use balanced audio cables if possible.
So, if you want ground loops to be a thing of the past: Acquire and make use of the relevant know-how and employ proper and appropriate tools.