low noise supply

My objective is to get switching supplies as far away as possible.

Thanks, but I don't want to run the AC line into my box. Hence the preference for an external wart. The AC output warts usually have a single isolated secondary.

Companies are making post switcher LDOs now with very low noise. For example:

They are nice for ultra low noise synthesizers.

Yes, you're right, and primary resistance will be high so surge currents will be limited

Cheers

Klaus

As far as necessary, sure. As far as possible, it's NOT the big problem. Magnetic fields around a leaky line-frequency transformer fall off as 1/ R**3 and distance is cheaper than effective (mu-metal?) shielding.

One foot of distance gives three orders of magnitude improvement over one inch.

But around a 200 kHz switcher, you don't need mu-metal, the skin depth in aluminum is 183 um, so the same three orders of magnitude costs you 1.2 millimeters of aluminum.

Meanwhile, any OTHER item near your sensitive box, that develops a 60Hz field, ruins the pristine voltage source. I've seen a month go by while a high-intensity light's transformer was the dominant problem. A linear regulator is pretty good at rejecting 60 Hz, but it CAN'T do anything about pickup in the output side wires.

How are you going to make a single part automatically switch 115/220?

The pic that I posted at the start of the thread is the classic

Or we could just stock 120 and 240 volt warts and ship the appropriate one. Then the negative could be done at 50/60 Hz, like in the netlist I posted.

...by sticking a warning label on the widget?

RL