Anyone use an IEC line filter plug??


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If there is a lot of RF interference from external sources such as radio stations, or if you have a lot of cheap Switch Mode Power supplies the power line can be carry ultra high frequency noise into you amp. This might be audible or not depending on the circuit and the build, it will just sound harsh and fatiguing. It will shine a spot light on any shoddy build practices by forcing them into instability and oscillation. These IEC filters are designed to isolate the circuit from the line and prevent these issues.
I use them when I have ones to salvage, but I have never gone to the trouble of actually buying one. They are much more important in digital circuits where the noise will actually introduce gross distortion into the data carrying signals.

When you get them cut open, do you usually find metal film caps or ceramic "Y"caps? Metal film types are more likely to fail open.

They are always ceramic. I remove them because they add garbage to the chassis. Such filters are not for protecting the amp from the garbage in the power line. They are for protecting the power line from the garbage caused by a SMPS of the device where they are installed. Try for yourself such a filter in a preamp with unbalanced inputs. Connect a notebook as a source, and listen to the noise generated by a battery charger.
I use Schaffner FN9262B-6-06 medical iec units bought from digikey. Very good quality and tight comnection. Other iec inlets from oter european countries and china have had issues. Plug would pop out for example. Also use RIFA 840 polycaps X2 10nf over the switch and 100nf over the secondary before rectifier.
Been using also Wima X2 mkp caps before and worked perfectly for over 10 yrs.