RC Filters Backward?

woodrough · 10th September 2012, 07:10 PM

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

richie00boy · 10th September 2012, 07:27 PM

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.

Elvee · 10th September 2012, 07:52 PM

Quote:

Originally Posted by woodrough

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....

woodrough · 10th September 2012, 08:16 PM

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?

Enzo · 10th September 2012, 08:40 PM

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.

10th September 2012, 07:10 PM	#1
woodrough diyAudio Member Join Date: Aug 2012	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

10th September 2012, 07:27 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	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. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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10th September 2012, 08:16 PM	#4
woodrough diyAudio Member Join Date: Aug 2012	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?

10th September 2012, 08:40 PM	#5
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	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.