Why iso transformers for AC/DC amps?

[Tubetec]

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'  .

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 .

 

[JMFahey]

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.

DEADLY MOD ... who did that?  Why?

You just removed the only, feeble, user protection.

The reason of the 270k is that in case of wrong plugging, wrong outlet wiring, or the very common wrong extension cord, current through your heart is limited by the 270k resistor, "same as in a neon screwdriver".

The .05uF cap which MUST be UL rated or meet current safety ratings, is to bypass Audio but limit 60 Hz current through your heart.

Explain this to your Customer, tell him *only*  safe option is to properly install an ISO transformer and if he does not accept, make him leave your shop with the amp under his arm.

Being a Tech, you are way more responsible than him for whatever happens.

 

[JMFahey]

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 .

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.

 

[Tubetec]

I understand the use of the 270k and .05uf now ,thanks Jm.
I presume in the case of a 1:1 these components are no longer needed .

In the case of one particular  switchmode ,it was ungrounded at the plug but still had 1meg and x cap at the input which was connected to signal ground, in the absense of a proper ground on the unit ,half the mains voltage was visible on the 0 v line ,not good on a piece of test equipment .