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!

 

If your only interest is a simple design and suggestions of op-amps to use, I suggest either of these applications:

https://www.analog.com/designtools/en/filterwizard/
http://www.ti.com/design-tools/sign...en&HQS=sva-web-filter-filterdesignervanity-en

I tend to use the Analog Devices tool. TI has some nice short cuts and step by step procedures you can use to design a filter by yourself or test results from Analog Devices. You won't get identical answers, but you will get some ideas.

 

The 741 is an old, rather noisy op amp.
For audio applications it's better to use a low-noise device such as an NE5532, TL071, or MC34071.

 

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)

 

You can do a 3-pole active filter as shown here, which is generally the maximum practical with one op amp.

 

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.

 

One pole is 6db/octave, two poles is 12dB/octave and three poles is 18dB/octave.

 

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