Capacitor from 120VAC to ground - diyAudio
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Old 3rd October 2012, 01:44 PM   #1
diyAudio Member
Join Date: Feb 2012
Default Capacitor from 120VAC to ground

Hi, I've noticed a lot of old amps have a .05-600V or so capacitor going from the hot side of the 120V to ground. I was under the impression this was meant to filter out noise but I rarely see builders do this anymore...any reason why? Am I missing something?

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Old 3rd October 2012, 01:58 PM   #2
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
The purpose of this capacitor was to ensure that the switch outlived the guitarist. At first it would reduce arcing so prolonging switch life, but then it would fail and electrocute the player.

The correct way to minimise switch arcing is to put a lowish value (10nF?) X-rated cap across the mains transformer primary (i.e. after the switch). The cap reduces the voltage developed across the opening contacts, and then the transformer resistance dissipates the stored energy as heat. A snubber can also be used.

Any cap from mains to chassis must be Y-rated and even lower in value; in practice it may be better to buy a packaged mains filter.
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Old 3rd October 2012, 02:09 PM   #3
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Thank you.
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