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Old 11th December 2006, 10:09 AM   #1
klitgt is offline klitgt  Denmark
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Default cap across mains switch?

Hi

In my DIY preamp the transformer is a 2x24V 30VA thoroidal that I switch on and off the mains with a simple small on-off switch.

If I switch off the mains while the power amp is still on there is a "bang" in the speakers .
In most factory made preamps etc there is normally a component across the mains switch.

Can anybody tell me what this component can be and whether this is preventing noise when switching off the amp.

I want to have the same thing in my preamp, thank you for advice
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Old 11th December 2006, 10:45 AM   #2
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A cap across the mains switch is to extend it's life by absorbing the arc when breaking the connection. It will do absolutely nothing for your pop problem which comes from the fact that as the power supply builds up or comes down the op-amps etc react.
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Old 11th December 2006, 11:01 AM   #3
klitgt is offline klitgt  Denmark
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Quote:
Originally posted by richie00boy
A cap across the mains switch is to extend it's life by absorbing the arc when breaking the connection. It will do absolutely nothing for your pop problem which comes from the fact that as the power supply builds up or comes down the op-amps etc react.

Thank's for reply, but it is not the building up or coming down that causes the pop. The output is shorted to ground at powering off and the short is coming off with a 20 seconds delay at power-on.

The pop comes exactly the moment where I power off and is not coming when powering on.
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Old 11th December 2006, 11:59 AM   #4
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Maybe then the issue is the cap at the amp input is being grounded by the shorting transistor, causing it to charge a bit hence generate a pop.
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Old 11th December 2006, 08:27 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
there should be enough reserve in the smoothing caps to keep the pre-amp alive for at least a few hundred mS.
If you hear a bang at the power amp outlet then something is kicking in before the mute becomes effective.

Is there DC on the output of the pre-amp?
If you kill the DC and output signal with a mute, you send a spike to the power amp. A VERY BIG spike.
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