Why iso transformers for AC/DC amps?

samgraysound said:

This one has been modded so the .05uf and 270k resistor between neutral and chassis are bypassed. So circuit ground is the chassis. Safety ground and neutral are both connected to chassis.  power switch is between hot and the rectifier. Also added cathode caps for the 1st and 3rd stages, and removed it from the 2nd. Changed various resistor and cap values.

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Tubetec said:

Thats good solid advice PRR , you simply cant rely on everything being correctly wired everywhere you go ,and its not worth taking a chance on a persons safety by 'hit or miss'  .
Fully agree.

On slightly different topic, I can't understand how uninsulated devices are allowed to use ungrounded switchmodes as half the mains voltage can appear on the ground line/0 v .

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But they are not "uninsulated", all offline SMPS include a transformer with galvanically isolated primaries vs. secondaries.

And not only "isolated"  but they must pass lots of HIPOT tests (think applying 1500V or 3000V and checking no current passes between primary and secondary), "creepage distance" requirements, meaning high voltage points must be separated by "so many mm" , etc.

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