2KHz Active Low-Pass Filter needed

Thread Starter

georgedb

Joined Jul 29, 2017
49
Hi,

I don't know much about filters, read some things and that was more or less clear on a high level. But calculating such a filter would not be possible for me, based on a huge knowledge gap. I have an interest in this very old schematic, but it mentions an OKI ALP/2 and ALP/4 filter of which I could not find ANY information on the Internet.

I found this active low-pass filter calculator, but what OpAmp to use for my purpose (the less pin count the better, could a 741 be used?) and what Q-factor to set (I tried to understand some explanations about it) for the calculation with heading "Calculate the R and C values for the Sallen-Key filter at a given frequency and Q factor"? Or do I need to use that Damping Factor or maybe a totally different calculator. My goal is to have a simple schematic and BOM ;-)

I think the OKI ALP/2 is a 2KHz filter, but the text mentions a cut-off frequency of 1.7KHz... Do I need to set 1700Hz?

Thnx for your help!
 

jpanhalt

Joined Jan 18, 2008
11,088

LvW

Joined Jun 13, 2013
1,313
Lowpass filter specifications consist of
* Filter order required (determines the transition range between passband and stopband)
* Filter approximation (form of the passband: With or without "ripples")
* End of passband (cut-off frequency).

Please note that in many cases, higher filter orders (n>3) are realized by cascading active 2-nd-order stages.
For a second order stage, the Q-factor (Q=1/2d with d=damping factor) determines the "form" of the passband.
Examples (2nd-order):
* Q=0.7071 (Butterworth response, maximally flat without peaking)
* Q=0.577 (Bessel-Thomson response)
* Q=0.9565 (Chebyshev response with 1 dB peaking in the passband)
 

Thread Starter

georgedb

Joined Jul 29, 2017
49
You can do a 3-pole active filter as shown here, which is generally the maximum practical with one op amp.
It took a while, but that is what I used. Thanks all for the explanations / help!

I'll use this 3-pole, 6dB/octave Butterworth filter, based on a 1700Hz cut-off frequency, based on the calculations of that tool, with an NE5532 opamp that I apparently had in my toolbox.
 

PeteHL

Joined Dec 17, 2014
363
Designing a low-pass active filter is very simple if you follow a guide at linkwitzlab.com. See the equations on page 3 and a chart on page 4 for the Q value of each stage depending on the order of the filter. w0 (omega-zero) equals the cut-off frequency (2 kHz) times 6.28. The Q is according to one or more stages of filtering. Select a trial value of C2. Then solve for R and C1. Higher than first order filters are the type Linkwitz-Riley. Here is the link:

http://www.linkwitzlab.com/filters.htm

Regards,
Pete
 
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