Imperf3kt wrote: ↑
Wed Oct 17, 2018 10:49 pm
... It runs off 12v AC.
The preamp is not grounded, it runs from Live and Neutral only. Now the Neutral is connected directly to the ground plane of the amplifier, meaning any connected device (such as my Pi) will also be connected to AC.
Where does the 12v AC come from?
If it's a mains transformer - 240v AC in, 12v AC out - then that provides the isolation you require.
In fact, on the 12v side, there's no "live" or "neutral", they're just the two ends of the secondary winding.
Somehow, that AC is being converted to DC, and some part of the DC is connected to the metal-work and is labelled "Ground" for convenience.
Whether one end of the secondary winding is connected directly to that or not isn't important. If, for example, there's a bridge-rectifier in the way, then half the time one end will be connected through a diode to "Ground" with the other end at a varying voltage above it, and the rest of the time they'll swap round.
Ground, or Earth, is a somewhat misunderstood concept.
Mains gets generated at some large facility out in the wilds, and goes through several transformers before it gets to your house. At each stage, some part is connected to a stake shoved into the soil close by.
Mostly, they're three-phase, meaning three thick wires carrying the power, with a thin one connected such that its voltage is about average compared to the others, and called "Ground". It doesn't need to be thick, because the loads should be balanced so that almost no current should pass through it.
It's probable that only two wires arrive at your house from the substation - a transformer feeding several houses - with one of them coloured black or blue.
That one will be connected to the "Neutral" feeding your house wiring, and also to the green one (or green/yellow) for "Earth", and to the stuff you stand on when you're gardening.
Hopefully, the same one was chosen for all the houses round you, and since the substation possibly provides three phases, that one is the centre-tap of the three-phase transformer.
Hopefully, also, the loads on the three phases will be approximately balanced, so that there won't be too much current in the centre-tap. But if enough people on one phase go on holiday while enough of the rest on another phase run their aircon at full tilt, that centre-tap can carry the full output of the system. Then the ground point at your house could be at a wildly different voltage to the centre-tap of the substation.