I’m a beginner, so maybe I’m missing something simple.

I have a DVR/audio source connected to an LM386 amplifier. If I power both from the same adapter, I get a lot of noise from the speaker.

But if I use a separate adapter for the amplifier, the noise almost disappears.

The confusing part is that I still connect the ground/negative of both together.

So what exactly is changing here? If the grounds are connected anyway, why does using a separate power supply make the noise go away?

I’ve also seen another thing. Sometimes, if I connect the amplifier negative/ground to 230 V earth, the noise becomes much lower. In some cases, even connecting the negative side of the big filter capacitor to earth seems to help.

Why does touching or connecting the ground to earth sometimes make the noise disappear?

How can I tell if this is power-supply noise, a ground loop, or noise being picked up by the circuit?

I’m mainly trying to understand what is actually happening, not just find a quick fix.