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KVR Forum » DIY: Build it and they will come
Compyfox
KVRAF
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 Posted: Sun Jul 29, 2012 5:27 am reply with quote
I'd love to create a passive HPF (or lowcut) circuit for a project I'm working on. I just need confirmation if I'm on the right track or not.

My planned cutoff frequency is 40-50Hz, ideally 40Hz, but I don't know if that resistor value is existing. Also planned is at least a second order (12dB/Oct) filter, ideally fourth order (24dB/Oct).

I know that in order to get a steeper cutoff, I need to put the modules in series (up to 4 times). Maximum voltage used: a standard CD signal or guitar signal.

THIS IS FOR TESTING PURPOSES ONLY - I do not plan anything greater with it.

These are the values I've calculated:

R= resistor, C = capacitor

40Hz cutoff:
R = 390 Ohm

50Hz cutoff
R = 330 Ohm
C = 10uF

Anyone can confirm that?
And if so, I hope the resistor values exist in real life. Though I'm sure I have some leftover 330 Ohms at my disposal.

^ Joined: 18 Oct 2003  Member: #9761  Location: Berlin, Germany
skitchy
KVRist
 Posted: Sun Jul 29, 2012 6:09 pm reply with quote
If you dont have the right resistor values or they dont exist, you can make them up yourself.
Use multiple resistors in series if you want to add the values ie R!+R2=resistance. So a 1k and a 2k in series would be the same as a 3k by itself.
Using 2 resistors in parallel gives you (R1+R2)/2. So using the same example (1k+2k)/2=1.5k
Using more than 2 resistors in parallel also works, but the formula gets more complicated at this point.
Search for RC filter calculator, and parallel resistor calculator - there are loads of web pages where you just fill in the values and the calculations are done for you.
Hope that helps. Let us know how your project goes - we need more DIYers on KVR
^ Joined: 19 Jul 2010  Member: #235948
Compyfox
KVRAF
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 Posted: Sun Jul 29, 2012 11:14 pm reply with quote
Hm... I'm lucky in this case. Looks like there are 330 and 390 ohm ones. Apparently the math tool I used spit out these values - which seem to be normal available ones.

The question still remains... Are these values correct?
Or in other words: did someone build passive LowCuts/HPF at a similar frequency already?
^ Joined: 18 Oct 2003  Member: #9761  Location: Berlin, Germany
xtp
KVRian
 Posted: Thu Aug 02, 2012 4:41 pm reply with quote
Hi, heres a quick easy tool to experiment with - shareware.

Its only crippled by the size of the work area, so if you have a largish circuit you have to get creative or use another spice program like Kicad.

http://www.5spice.com/

you can build your passive filter in a few minutes and have it run a test which you can print out if you want.

-----------------------------------------------

i just drew it and ran it, 40Hz and 47Hz was the output. 50Hz was at -2.8db or thereabouts.
^ Joined: 03 Jun 2006  Member: #109416  Location: The West Coast of New Zealand
mahaya
KVRist
 Posted: Fri Aug 03, 2012 1:40 pm reply with quote
You should be aware that if you cascade two or four filters you'll get a total of -6 or -12 dB at the -3 db frequency of a single stage. To keep the -3 dB freuquency you need to move your filter frequecy to sqrt(2^0.5 - 1) or sqrt(2^0.25 -1) of your target frequency. Also you will have a very slow roll off.

So although you'll get the 24 dB/octave by cascading four stages the cut off will take place at a higher frequency and the filter will also affect the pass band.

Another thing is that each following stage loads the previous stage.

Chris
^ Joined: 20 Nov 2009  Member: #219955
earlevel
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 Posted: Fri Aug 03, 2012 2:54 pm reply with quote
Compyfox wrote:
THIS IS FOR TESTING PURPOSES ONLY - I do not plan anything greater with it.

Then build it and test it!

But pay attention to what Chris said. I don't know what you're using it for, but loading and corner frequencies characteristics are among the reasons people use active filters for high orders...Don't forget component tolerance is a factor as well...
----
My audio DSP blog: earlevel.com
^ Joined: 04 Apr 2010  Member: #229147
xtp
KVRian
 Posted: Fri Aug 03, 2012 5:34 pm reply with quote
earlevel wrote:
Then build it and test it!

I think this really sums it up. When it comes to audio in the end the ear wins and quite often what looks good on a mock up does not always sound good.

With filters I have found it useful to replace the resistor with a pot and then put it on the test board a few weeks.

Once I find I am using a single frequency I measure the resistance and replace the pot with a resistor.

I find it easier than pulling resistors in and out and easier to twiddle while playing, and less messy than a rotary switch.
^ Joined: 03 Jun 2006  Member: #109416  Location: The West Coast of New Zealand
felix80
KVRer
 Posted: Tue Apr 02, 2013 1:47 pm reply with quote
As per xtp suggestion, use variable resistor i.e. potentiometer or preset rather than making use of fixed valued resistor. Check the output varying resistor value, if it is working fine at the particular value than measure that value and replace with fixed one.

printed circuit board assembly
^ Joined: 15 Oct 2012  Member: #289967
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