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Old 10th September 2012, 07:10 PM   #1
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Default RC Filters Backward?

Hi, I was wondering if someone has worked with this design before and could possibly shed some light on some confusion I have.
So I am working on constructing the PhonoDude Preamp (PhonoDude PCB Version) and I ran into some continuity problems:

In the schematic, C3 is followed by R7 then ground. C4 is followed by R10 then ground.

In the PCB diagram Im pretty darn sure it is the other way around where R7 precedes c3 and R10 precedes C4.

Can someone verify that I am missing something? The last time I checked, what defines a filter is what comes first, the capacitor or the resistor.

Thanks!

(Here are some supporting diagrams)

http://i.imgur.com/NZWGt.jpg

http://i.imgur.com/kn6Un.jpg
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Old 10th September 2012, 07:27 PM   #2
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The order of C3/R7 and C4/R10 is not important as they are in series. They are not a filter in themselves, but form part of a low-pass filter consisting of R8 and R9 too.
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Old 10th September 2012, 07:52 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by woodrough View Post

In the schematic, C3 is followed by R7 then ground. C4 is followed by R10 then ground.

In the PCB diagram Im pretty darn sure it is the other way around where R7 precedes c3 and R10 precedes C4.

Can someone verify that I am missing something? The last time I checked, what defines a filter is what comes first, the capacitor or the resistor.
Both implementations are strictly equivalent from a theoretical perspective.
However for noise immunity considerations, the version with the cap to the ground will normally be favored.
....but in an ideal world, it should change nothing....
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Old 10th September 2012, 08:16 PM   #4
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Ah! That makes plenty of sense... being the series..
Interesting point on noise considerations. Grounding the capacitor because there is more possibility of distortion through the capacitor than the resistor?
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Old 10th September 2012, 08:40 PM   #5
Enzo is offline Enzo  United States
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No, because the outer foil of the cap might provide some small amount of "shielding." Small, being the operative word there. Inner/outer foil is one of those discussions of capacitors that crops up now and then and it can go on for pages with nothing really being decided.

Any signal going through a series circuit goes through both parts, so whatever either part is going to do to it, it will happen.


A series RC circuit doesn't care which order it is in. You may be confusing something like power supply filters where it matters for examp-le whether the rectifier sees a cap first or an inductor first. But that is totally different issue.
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