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Old 12th June 2006, 03:24 AM   #1
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Default Noise Suppressor, just a cap?

I just got done building a voltage regulator for my LM386 based microphone amp so it will run off of 9v supply.

I will be making another LM386 so i am not able to test how much the engine noise has been reduced, the first one was lost.

I was directed in the path of buying a noise supressor for the engine noise problem(when the car is off, i hear the really loud humming through my sound system but not the alternator noise since its not running, i have the output of the mic amp running to my cd players AUX input)

Is a noise supressor basically just a capacitor?

If so is the polarized cap just wired in parallel with the device's power (+) line?

If not, how is it made?

im just needing one that will handle 1.5 amps max
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Old 12th June 2006, 03:38 AM   #2
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In circuits where power loss doesn't matter; resitors in series followed by capacitors in parallel are used to supress noise. This is called an RC filter

Now in the case a power circuit resitors are replaced by inductors or common-mode chokes (2 inductors wound on the same iron). This is popular choice in power circuits because you don't want to make heat (waste energy), or waste voltage in your power supply. Also, an inductor in series has a similar effect as a cap in parallel. So, an LC filter is twice as effective as an RC filter. A CLC filter... 3 times.

See your regulator post... I tried to draw a pi network for you there. If you place a cap across the input and ouptut of the picture below... you will have a common mode filter. If you lose the inductor on the bottom, you will have a CLC pi network.

The common mode type generally works better especially if you a noisy ground as well.
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File Type: gif common mode.gif (2.7 KB, 297 views)
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Old 12th June 2006, 04:32 AM   #3
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so it might be cheaper to just buy a 10amp filter to power my voltage regulator and then have the voltage regulator power my mic amp?

a 10amp is the least one i found on the net, but ill go to a local shop tomorrow
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Old 12th June 2006, 04:34 AM   #4
poobah is offline poobah  United States
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If you really want to know how to make one...

How much current do you think your load draws?
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Old 12th June 2006, 04:49 AM   #5
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id say .1 amps
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Old 12th June 2006, 05:22 AM   #6
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Click the image to open in full size.

is this at all what you were trying to draw out?
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Old 12th June 2006, 09:18 AM   #7
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That last pic looks like almost every "black box" filter that i've seen inline on the car-stereo power leads.Pretty basic little LC filter.
Give it a try.
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Old 12th June 2006, 02:58 PM   #8
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Yep,

Often they have a cap at the input too.

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Old 12th June 2006, 06:04 PM   #9
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what value should I use? im still trying to learn this type of electronics so im not sure what to use. if the smaller values, since im using a very small amount of current, would work or the larger values would work for the cap and the inductor
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Old 12th June 2006, 06:10 PM   #10
poobah is offline poobah  United States
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Just buy one... it'll have the input cap in it.

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