🚀 go-pugleaf

I noticed something interesting (and annoying). Many appliances in the
US (such as radios, cordless telephones and answering machines) run on
low-voltage (3-12V) DC current. They are usually sold together with an
AC/DC converter from 120V AC to DC of the appropriate voltage. 

When I bring them to Europe, there are two solutions (at least) to get
them working:

1) Buy an adaptor (autotransformer or else) to lower the AC from 220
to 120, and connect the AC/DC converter after the adaptor.

2) Buy another AC/DC converter, with the same DC voltage output (and a
reasonable current rating) and 220 instead of 120 V AC input.

Solution 1 works, even if the autotransformer gets a bit hot if you
leave it always plugged in (as you have to for answering machines, for
example). Since solution 2 is more compact (only the converter,
without adaptor), and the price of a converter is similar to the one
of an adaptor, I have often tried it. However, this solution does not
seem to work. In the radio/cordless telephone I always hear a
noticeable hum, probably at 50Hz (and due to the line frequency). 

Why? I have the following explanations:

a) The AC/DC converters you can buy separately from appliances are of
much lower quality of those you get together with them. 
But I opened both types, and the electronics seems to be the same,
more or less (I checked circuit configuration & component type).

b) The AC/DC converters sold with the appliances are in some way
specialized to filter out the noise components that give the most
inconvenience to the appliance.
But, again, their circuitry seems to be fairly simple and standard.

Does anybody know why solution 1 always works, while solution 2 seldom
does?

				Luca