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1.8KHz low pass filter

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neazoi

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1.8khz audio filter

Hello I need to make a 1.8KHz audio low pass filter preferably using an opamp like NE5514 or similar.
Any schematics available?
 

neazoi

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1 khz low pass filter

I just found a variable low pass filter attatched.
I believe this varies the cutt off frequency is that right?
What order do you believe is it?

I am a bit confused with this "2 P1" in the equation.
what P1 stands for?
 

flatulent

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2khz bpf

There was a typo. 2 p1 should be 1pi or roughtly 6.28.

Companies that manufacture op amps frequently give away programs for designing such filters. The one in the diagram is two poles.
 

neazoi

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filterlab spic

Thank you,
just a quick question, If I connect two identical filters in series, does the whole filter become sharper in the cut off region? (greater order?)
 

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lpf 2nd order filter topology

Yes. The attenuation at higher frequences is the sum of dB attenuation of each section. However, it is bet to use some form of classical filter poles, such as butterworth or chebychev in selecting the pole locations of both sections. That way you will have a flatter (smaller variation of attenuation) passband. It all depends upon the use of your filter. If you signal is just one frequency, the flattness is not that important.
 

LvW

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19 khz filter

neazoi said:
Thank you,
just a quick question, If I connect two identical filters in series, does the whole filter become sharper in the cut off region? (greater order?)
Here is a general recommendation:

1.) As a first step, clarify what purpose the filter should serve. Why do you or somebody else need a filter, which means: Which frequencies have to be eliminated resp. attenuated ? And by how many dB ? Otherwise you cannot fix the order necessary for your needs.
2.) A remark to the cascade of two second order stages (as mentioned by you):
You should be aware that a 1kHz cutoff filter in series with another 1 kHz filter will NOT have a common cutoff at 1 kHz. It is a simple matter to see, why !
 

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LvW

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1khz low pass filter opamp

neazoi said:
Right,
I found filterlab from microchip which I think is very nice and easy to use.
I have produced a 1.8khz lowpass filter using opamps which is about -20db at 2KHz.
I think it should be pretty good to my needs (as sharper cutt off as possible). here it is http://www.diyaudio.com/forums/show...d0d5ecdba10113b1daa5061&p=1906085#post1906085
The program offers to you two different filter topologies (Sallen-Key and MFB).
What did you choose ? I propose MFB !!!
 

neazoi

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how do i make a 9 khz bandpass filter

No I have used Sallen-Key.
Why do you propose MFB?
 

LvW

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38 khz bandpass filter opamp

The attenuation for higher frequencies is bad.
Above the corner frequencies the attenuation rises - OK, very good.
However, it reaches a maximum of only app. 30 db - and then it goes down again (which means, the output voltage goes higher again).
The reason: For increasing frequencies there is a parasitic way through the feedback capacitor which produces an output voltage at the opamp output impedance. This impedance goes high when the gain of the opamp stage decreases.
If you have another opamp available, put it as a buffer into the feedback path.
Or decide to use the other totpolgy: MFB.
You can see this effect clearly if you simulate the filter circuit (with a realistic opamp model !).
 

neazoi

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lowpass filter

Thank you for this important point.
I have used the MFB also with the same parameters and filterlab gives me the same attenuation at 2KHz, -20db.
So I think after your proposal I will go for the MFB, as long as the attenuation is the same at that point...
Is there any other point tht I should consider?

I was thinking of using the NE5514 quad opamp as it will save me chips for an 8-pole, but this is quite expensive on ebay! maybe someone else wants to sell me two of these chips?
 

LvW

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low pass filter

neazoi said:
Thank you for this important point.
I have used the MFB also with the same parameters and filterlab gives me the same attenuation at 2KHz, -20db.
...............
Yes, of course ! Since filterlab does show you always the ideal response. And this is independent on the topology choice. Filterlab does NOT simulate but calculate.
Therefore, I stated that only SPICE simulation with REAL opamp models can reveal the differences between the various topologies.
 

neazoi

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middle pass filter opamp

Thank you!
Since I do not have a simulator I will go for MFB based on your experience.
Thanks a lot.
 

LvW

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neazoi said:
Thank you!
Since I do not have a simulator I will go for MFB based on your experience.
Thanks a lot.
There is another advantage of the MFB topology: It is less sensitive to component variations (tolerances of R and C) as the S+K- topology. So, I think you´ve made a good choice now.
 

neazoi

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Thanks very much for your help!
 

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