It's hard to make a really low noise widget that's powered from a switching wall wart. The warts are noisy, both diff and common-mode. I guess a really radical multi-section common+diff mode passive filter would help some.
I was thinking that an AC-AC wart, just a transformer, would work. If I got a 240 volt input version, it could work at 120 or 240 with some secondary side switching.
The classic 120/240 circuit is
which I'd use at some low secondary voltage.
The switch could be an SSR that would automatically switch, based on the output voltage. If I wanted, say, 12 volts DC, a comparator with hystersis could do it.
If I want positive and negative rails, something needs to be added. An internal autotransformer would work, but 60 Hz magnetics tend to be big.