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Old 16th February 2010, 01:20 PM   #1
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Default Capacitor Across Power Switch to Control Switch On Noise?

I have a jfet based preamp that really makes a racket on turn on. It runs off 24vdc. Would putting a small value .22uf/630v capacitor across the power on switch be a way to control this noise?
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Old 16th February 2010, 01:44 PM   #2
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It depends on what makes the turn on noise. The power switch or your jfet preamp. But try the cap. It may solve the problem
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Old 16th February 2010, 02:16 PM   #3
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0.22uF is quite large. Use a 10nF /240Vac Class X or X2 Suppressor Cap across the power switch contacts. Silent ON/OFF.

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Old 16th February 2010, 05:05 PM   #4
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I'll give it a try. Can same approach be used across a 110v switch?
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Old 16th February 2010, 05:11 PM   #5
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Quote:
Can same approach be used across a 110v switch?
Yes. Only difference is the Voltage rating of the cap.
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Old 16th February 2010, 05:15 PM   #6
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No problem with 110V switch. What is important is to use CLASS X or X2 caps. These are specially manufactured (approved) for Mains use.
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Old 17th February 2010, 12:17 AM   #7
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Originally Posted by Michael Chua View Post
No problem with 110V switch. What is important is to use CLASS X or X2 caps. These are specially manufactured (approved) for Mains use.
Because (and this is especially true with bigger power amplifiers) if you switch off at the wrong time in the power line cycle (when the transformer field is at maximum) The transformer field collapses, but the current has no-where to go (the switch is now open) so the primary voltage heads towards infinity! Only the circuit leakages limit this voltage, but it's all across the capacitor.

Now in real life things aren't this bad, but it does stress the capacitor.
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Old 17th February 2010, 12:26 AM   #8
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No problem with 110V switch. What is important is to use CLASS X or X2 caps. These are specially manufactured (approved) for Mains use.
And the other reason is because the capacitor is always across line voltage when the unit is switched off.
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Old 18th February 2010, 11:15 AM   #9
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Quote:
Originally Posted by DreadPirate View Post
I have a jfet based preamp that really makes a racket on turn on. It runs off 24vdc. Would putting a small value .22uf/630v capacitor across the power on switch be a way to control this noise?
Have you tried a slow start (power transistor/resistor and cap) as you say it runs off 24 Vdc, to get the rail to rise slowly.

Or connect a normally closed relay across the output shorting to ground, and again arrange for small delay before it opens.

Is the circuit "oscillating" as the DC conditions stabilise ?
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Old 18th February 2010, 11:52 AM   #10
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I'm going to try the capacitor trick, if not, where is a power trans/resistor/cap circuit I can try? Would it power off the 24vdc, I don't want to provide another power source just for it. Using the 555 and relay is a bit more complicated than can be justified for this project.
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