Choosing values of caps and resistors in high pass filter

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Well-known member
Nov 28, 2008
Zagreb, Croatia
I know this is probably trivial question for most of you, but as a self thought enthusiast I couldn't find a place where this is explained, so I have to ask here for some explanation.

I want to put a simple passive high pass filter at the output of a unit I've built (CLX-VU to be exact), since I have some problems with sub frequencies it creates when processing low percussive material. The amount of energy below 20hz is too much and is triggering other dynamic units in chain  needlessly.
So I'll strap a RC filter at about 15hz at the both output pins.

And here's my question:

If I go and calculate this filter using passive HPF formula fc= 1 / (2*pi*R*C) I can get 15hz cutoff using 100nF cap and 100K resistor. But the same frequency can be obtained if I use 100uF cap and 100 ohm resistor!
What is the practical difference?
I know both will work.
My gut feeling gears me towards 100nF film capacitor rather than using bigger 100uF electrolytic, but I want to know more!
Or is there any other R/C combination that for some reason is more adequate in this situation?
Line output where I'm putting this filter is impedance balanced and is used mostly on unbalanced insert of my console (so going back to console's insert receiver) but sometimes it will be driving daisy-chained into other gear's input. If that is to any concern when choosing values.




Well-known member
Staff member
Nov 30, 2006
Hickory, MS
adding a simple passive filter in series with an normal I/O interface must also factor source impedance of the outlet (in series) and input impedance of the following input in parallel. This will work for a fixed combination of products but results may get a little weird if you mix and match gear.



Well-known member
Jan 30, 2010
Maine USA
> I can get 15hz cutoff using 100nF cap and 100K resistor. But the same frequency can be obtained if I use 100uF cap and 100 ohm resistor!

Can your source even drive 100 Ohms? (Most won't sound happy.)

Is it really "100K" if the box it feeds is 22K or 10K (very popular input impedances on modern boxes)?

Here's a dart-toss. Most gear will drive 1K, and most gear will show 10+K at its inputs. Take the Geometric Mean. (Multiply together, then take Square-Root.) 1,000*10,000= 10,000,000. Square-Root of that is about 3,000.

So 3,000nFd (what I might call 3uFd) and a 3.3K resistor (round-up, as you will see).

Key computation: 3uFd into 3.3K is 17.7Hz, acceptable.

With complications:
3.3K||10K = 2.48K, F3 becomes 21Hz
3.3K||22K = 2.87K, F3 becomes 18.5Hz

Since in-general, "nobody" has speakers that go below 50Hz, and the [email protected] case gives [email protected], and *all* rooms have +/-3dB or worse bass-bumps, the 18Hz-21Hz zone is not wrong.

In basic commercial caps, 3.3uFd is more common than 3.000uFd. So everything is 1.1 times lower(0.909). The "21" becomes 19, which is closer to your original (and perhaps arbitrary) spec.

OTOH, you could by a 10-pack of 1.0uFd, and try 2, 3, 4 etc in parallel until you find a happy balance of boom and freak-out.

As John says or implies: a single simple passive R-C is hardly a sharp tool, and highly affected by what you connect to.

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