X2 Line capacitors

[frank1]

Originally Posted by DF96

An X2 cap across the mains transformer primary can suppress arcing at the switch so lengthen its life and reduce switch-off noises. It will make very little difference to sound under normal operation.

If you are worried about this then it would be better placed across the switch contacts in series with 100R to form a snubber.
Frank

[mrVetz]

Putting a X2 type cap across the transformer primary or across the switch does help with the noise from turning the switch on & off, especialy if the switch is low quality (this works only if grounding is done properly). Adding a full mains filter with X2 and Y2 capacitors, a resistor and a choke will help if you have noisy equipment around the house. I can't say that I heard a sonic improvement, but the stray noises from the mains line seem to be less audible.

I use the volume control on my CD player (marantz cd 17) so I can hookup my lm3875 power amp directly to the CD player. This gives me the best overall sound but makes noise an issue. The filter comes in handy to minimize the mains noise.

[DF96]

Putting the cap across the switch means the device is never completely switched off, unless the cap is very small.

[mrVetz]

If it's placed after the switch parallel between the two AC lines, no

[FoMoCo]

Originally Posted by DF96

... With the capacitor in place across the transformer primary the frequency components in the switch-off transient are much lower than they would be, so distance of connection does not matter.

This also makes a nice resonant tank. Unless you get really unlucky with component selection, I'd expect it to be of no real consequence.

Originally Posted by DF96

Putting the cap across the switch means the device is never completely switched off, unless the cap is very small.

True. But, the capacitor value should be extremely small. Realistically, this isn't a concern. Just don't go throw a huge cap across there. 0.1uF or less is a good starting point.

Originally Posted by frank1

If you are worried about this then it would be better placed across the switch contacts in series with 100R to form a snubber.

+1 100R and 0.01uF, if you're concerned about the switch contacts arcing. Otherwise, there's no purpose.

[DF96]

The transformer will probably have sufficient resistance to damp the oscillation on switch-off. Those seeking perfection may, of course, calculate the right values of snubber resistance and capacitance to ensure critical damping. The rest of us will just put 10-47nF across the transformer primary and be happy.

There is another way of looking at it. X2 capacitors can gradually lose their value, due to internal spark erosion. If the equipment spends more time off than on, then put the cap across the transformer. If more on than off, put the cap across the switch. This will maximise the life of the cap.

[tsiros]

I fear sticking devices in mains switches/outlets is a not-so-good idea as far as safety is concerned.

[DF96]

Nothing is being put in a mains outlet. This is all inside the equipment case.

[AndrewT]

The primary purpose of an X2 capacitor across the Live and Neutral of the Mains supply is to attenuate the interference that comes in with the required 50/60Hz.

This attenuation is achieved by "filtering" the high frequencies that compose the interference.
The cables feeding the property have inductance. Adding the capacitor creates the LC filter with a 2pole roll off characteristic. This will be fairly effective from the 10s of kHz to the low MHz.

John Curl suggested Live to Neutral capacitor value of up to 10uF.

[DF96]

Different issue. I don't think I would want to add 0.74A of reactive current to my mains supply. For a start, it could increase induction into nearby signal cables.