how to build a butterworth filter

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mariussica said:
is opa 695 an amplifier.
Google tells me that OPA695 is a high bandwidth current-feedback opamp. Is that suitable for what you need to do? What did Google tell you?

I read everything about filters
Very unlikely. I have been looking at electronics for nearly 50 years and I haven't yet read everything about filters.

You have not answered my questions. At the very least you need to answer them in order to get someone to do your work for you. For example, why do you want a Butterworth filter? Do you know what a Butterworth filter is? What is the passband? What is the stopband? How many poles or orders? Active or passive?
No I dont need somenone to do the work for me , I want an advice , maybe someone have done something like this . I need a butterworth filter because I have a project with a 2nd active butterworth filter low pass and I want to build a device to connects via bluetooth with my mobile phone . Yes I know what is a a butterwort filter . I read about passband , A stop band is a band of frequencies . PS: I want to learn
OK, we are beginning to make some progress. You want a second-order low pass Butterworth filter. You still haven't said what it is for: audio tone control, removing Class D switching noise, removing RF interference? I'm not sure you have told us whether you are sending audio to your phone or taking audio from your phone.

If you want to learn then you have to ask the right questions. So far you have given us very little information. In engineering context is everything.
mariussica said:
I want to send audio with headphones .
Headphones don't send audio, they receive audio. We still don't know what you are trying to achieve. If you can't give a clear description of purpose there is no chance whatsoever of anyone (including you) designing a filter to do the job.

I have a bright idea. Why don't you post here the exact question set by your Prof? That will avoid all these Chinese whispers as you leak out the requirements, drip by drip.
Build an 2nd filtre butterworth down pass with OPA695 , but I want to transform this filter ( if it^s possible ) to a into a multi device that connects via Bluetooth with my phone ( or data cable ) , the device will have headphones through which you put music on the phone .
if it^s possible . I am sorry for my english . Thanks
I don't think English is the problem. What you say makes reasonable sense as English sentences, but not much sense as technical statements.

So now we find that the actual requirement is to design a second-order low pass Butterworth active filter using an OPA695 opamp. You still haven't specified a corner frequency.

You still seem to be confused about what headphones do.

A simple analogue filter does not talk Bluetooth, or any other digital protocol.

You seem to be unable or unwilling (I suspect the former) to explain exactly what you are trying to achieve. Difficult to help under these circumstances. Maybe someone else can take over?
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