Very good point about the float voltage. ?I took another look at the three examples I'd posted earlier, simply probing both sides of the output with no ground path other than the two 10x probes themselves, then adding a resistor to ground on the negative side. ?Floating, both switchers show a 160 V p-p 60 Hz wave superimposed on both outputs, essentially full line voltage; introducing a 100k ground resistor reduces this to about 30 V, indicating a 400k impedance from line to output. ?The transformer-based supply starts out with 8 V of 60 Hz line coupling (probably capacitively coupled across the transformer) with only the probe loading, reduced to 2 V by adding a 1M ground resistor. ?I added photos of these measurements to the album.
I guess best practice if using an ungrounded wall wart would be to make up all connections first, with the radio definitively grounded, then plug the wall wart into the mains. ?Doing the barrel connector last there'd be a danger of contacting the positive terminal first, discharging the float charge through the radio.
