ac filtering - across the line and line to ground

makepeace

hi guys.

my dad just gave me a broken old yammie natural sound integrated stereo hi-fi amp. turns out a capacitor that runs across the mains lines blew up (spectacularly at that!). apparently its a common occurrence in these amps. a replacement is easy to find and it should be a winner if i can get it going.

anyway, this led me into doing some research on simple ac line filtering. there are two types: across the line and line to ground. across the line consists of an X rated capacitor (suitable for high intensity AC abuse) that is put across the mains lines. line to ground consists of two Y rated capacitors (also high AC, but have an added safety factor when they blow, they don't short the mains to ground) running from each line to ground.

my question is: do any guitar amps (tube) make use of this type of filtering? and if not, why not? in the few dozen circuits i've checked out, there is no such thing. why not use this in conjunction with your normal DC to ground filters? does it have negative connotations?

more info here:
http://www.justradios.com/safetytips.html

Gearhead

Afaik these filters try to isolate your amp from anything non-sinus in the net. Although it might be very useful to have, I reckon the filtering in your average DC power supply will take care of most of the garbage. Obviously your amp will be cleaner with such a cap, but the law of diminishing returns might argue that the money is better spent somewhere else. In pro rigs, this function is done (and done better) by a power conditioner.

makepeace

cool thanks for the insight.

have read a bit more and it seems that line to line filters symmetric interference and line to ground - asymmetric. these ac filters also help to prevent interference from your appliance going into the net, and are required in some parts of the world.

i see how the cost factor is important, but. an average suitable line to line cap is R10, and 2x line to ground caps cost R6.

odd how its not more conventional in our amps..