please help me modify this bandpass filter

Discussion in 'Automotive Electronics' started by RHDE, Nov 12, 2015.

  1. RHDE

    Thread Starter New Member

    Nov 12, 2015
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    Ok i found this circuit and it looks like it will do what i need but my application isnt strictly audio related, i want to clean up the signal from an automotive knock sensor. This circuit claims to have a low pass range of 2.5 - 20kHz which should be fine but the high pass side is far too low i need it to be of similar range as the low pass or from say 5 - 16kHz

    also can someone please tell me if the gang pot's connected parallel or opposing?

    thank you!


    [​IMG]
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Do you need independently tuneable low and high cutoff?

    If you know the desired cutoff frequencies apriori, the circuit would be much simpler.

    Do not use the 741s; there are much better single-supply opamps available...
     
  3. RHDE

    Thread Starter New Member

    Nov 12, 2015
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    well i need to be able to isolate a narrow frequency but lots of cars have different sensor types and different detonation harmonics etc so i need to adjust the upper and lower cut of until i get the clearest signal with minimal background noise. Its claimed that engine knock mostly occurs around 6-8kHz but sometimes you might need to look at the second or third resonant if there's still too much engine noise. So a nice wide range is ideal.

    i really dont mind what components i use if you have better ideas? as long as it works and i can get the bits easy, i know very little about all that :)

    for the final application i will use a little pre-amp connected directly to the sensor and then use headphones to listen for the tinkle sounds, I currently just use the pre amp and headphones directly but theres a lot of other noise (tuning ignition and boost tables for turbocharged engines)
     
    Last edited: Nov 12, 2015
  4. crutschow

    Expert

    Mar 14, 2008
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    There's not much difference between a 2.5 - 20kHz and 5 - 16kHz bandpass.
    It's unlikely you could hear the difference.
     
  5. RHDE

    Thread Starter New Member

    Nov 12, 2015
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    sorry i didnt mention clearly the above filter has a high pass range adjustment of only about 20Hz up to 700Hz or something very low

    i need the Hi Pass to have similar adjustment range as the Low Pass or at least 5kHz up to 16kHzbut

    i need to isolate a very narrow frequency of say 2-3kHz but i also need to adjust the width and position of that band from 4 or 5kHz up to about 18kHz

    so i can do this manually with the 2 gang Pot's by setting the upper limit then setting the lower limit and i can move both these limits up or down
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Below is the LTspice simulation of your HP Sallen-Key filter with modified values.
    It's -3dB point can be adjusted between about 4kHz to 21kHz by adjusting the ganged pot U2-U3 (both pots increase or decrease resistance in unison).

    The three curves plotted are for pot wiper positions of 0%, 50% and 100%.
    That's for a linear pot.
    Log pots would place the frequency for the 50% setting closer to the midpoint of the two limit values.

    Adj HP Filter.PNG
     
    Last edited: Nov 13, 2015
  7. RHDE

    Thread Starter New Member

    Nov 12, 2015
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    Thank you sir!
    i was hoping it would be as easy as that! so the rest of the circuit can stay exactly as it is above, from C3 on??

    someone mentioned above that 741's are not ideal what should i use instead?? and would that then require any other changes?

    thanks
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Yes everything is the same from C3 on.

    A 741 likely will work satisfactorily for your application but there certainly are better op amps.
    Where do you normally buy your electronic parts?
     
  9. RHDE

    Thread Starter New Member

    Nov 12, 2015
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    Ok thank you

    im staying in Taiwan at the moment and theres an area not far from where we live in Taipei that has few city blocks of nothing but electronics shops! Its great, everything under the sun can be found there so i just take a list and show someone... i dont speak any chines :)
     
  10. tindel

    Active Member

    Sep 16, 2012
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    I ran across this at work yesterday and thought of this thread. I'm actually in the process of making my own bandpass filter so this has been some good review for me. Unfortunately this particular section of the app-note is only good for low-pass filters... simply changing the location of the cap/resistor pairs does not give you a high-pass equivalent.

    Check out page AN67-22.
    http://cds.linear.com/docs/en/application-note/an67f.pdf
     
    Last edited: Nov 14, 2015
  11. crutschow

    Expert

    Mar 14, 2008
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    MC33171 single, MC33172 dual and MC33174 quad as well as the MC34071 single, MC34072 dual, and the MC34074 quad op amps are all inexpensive choices to replace the 741 with much better specs for audio.
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    Are you going to the Guanghua Market area to buy the ee parts?
     
  13. RHDE

    Thread Starter New Member

    Nov 12, 2015
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    haha... yes i think it is Guanghua! theres a big hi rise department store full of mostly computer stuff then there's lots of little surrounding shops and underground plaza's etc too
     
  14. RHDE

    Thread Starter New Member

    Nov 12, 2015
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    one last question if anyone is still able to help?

    i plan to use this with a little 2 transistor pre-amp, would i put it before the filter or after??

    or would i even need the pre-amp now??

    cheers
     
  15. crutschow

    Expert

    Mar 14, 2008
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    Whether you need a preamp or not depends upon whether you need gain to get the signal to the desired level.

    Generally preamps are put before other circuitry (hence the name), but in this case it likely would make little difference.
     
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  16. RHDE

    Thread Starter New Member

    Nov 12, 2015
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    ok thank you, i will build it all into one circuit with pot to split the input signal from the sensor then i can adjust to what ever level is ideal :)

    on that note what resistance would about right for the input volume pot?... or would it be better placed between the amp and filter?
     
  17. crutschow

    Expert

    Mar 14, 2008
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    Typically it's placed after the preamp.
    10kΩ or so should work for a pot value depending on what the preamp can drive.
     
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