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High Pass Filter Design?

Discussion in 'General Electronics Discussion' started by Arnak, Apr 1, 2011.

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  1. Arnak

    Arnak

    22
    0
    Apr 1, 2011
    Hi Folks,

    I intend to add a high pass filter to remove 200hz and below to a directional
    dish that I am constructing for bird song recording.

    I will be using 2 x panasonic WM61A mics.

    I can see that an RC combination would do that but how do you calculate the
    optimum values for the resistor / capacitor combination?

    Making the assumption that you would want to reduce signal loss as much as
    possible how do you work out the best values please?

    Martin
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Noswaith dda
    The rolloff frequency will be R.C., say 500k and 100nF. The actual values will depend on the output impedace of the microphones and the input impedance of the amplifiers. The rolloff will be very gentle with just a RC filter and it may be better to make a pair of active high pass filters using op-amps, the more complex, the flatter response can be and the steeper cutoff.
     
  3. Arnak

    Arnak

    22
    0
    Apr 1, 2011
    High pass filter

    Hi Duke,

    Thanks for that info that will be enough to get me started.:D

    I will consider an op amp filter as the next stage.

    Just out of interest, what formula do you use to determine the component values?

    Martin
     
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Hi Martin,
    I'm no expert but I have a book "Electronic Filter Design" by Aurther B. Williams. This gives normalised values for various filters, the most tolerant would be the Butterworth filter and I could calculate the values for, say, a three pole 200Hz high pass active filter (no inductors) if you wish. It will take a little while to get my brain into gear!
     
    Last edited: Apr 2, 2011
  5. Arnak

    Arnak

    22
    0
    Apr 1, 2011
    High Pass Filter

    Hi Duke,

    No that's OK thanks, I was just interested in how the values were arrived at.

    I have checked the net and found plenty of info on filters but not on how the results are calculated for a given application.

    A book on filters seems to be the way to go.

    Thanks,

    Martin
     
  6. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    I have looked at a 3 pole Butterworth filter. I have used all capacitors at 100n and selected resistors to the nearest preferred value. Will try to attach the diagram. I have 'tried' it with 5SPICE.
     

    Attached Files:

  7. Arnak

    Arnak

    22
    0
    Apr 1, 2011
    High Pass Filter.

    Hi Duke,

    Brilliant, thanks very much for that.:D

    What op amp do you recommend with that circuit?

    Martin
     
  8. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Hi Martin,
    I don't know the best op-amp, I am still working in the age of the 741. I expect there are better ones around now.
     
  9. Arnak

    Arnak

    22
    0
    Apr 1, 2011
    High Pass Filter.

    Hi Duke,

    741's will do fine for me thanks.:)

    Martin
     
  10. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
    Hi Arnak
    For an alternative approach, what about recording the entire bandwidth of the microphone and subsequently filtering the sound using Audacity? The advantages of this approach are that the system is simpler and thus more reliable, and that the microphone does not get noised up by the filter electronics.
     
  11. Arnak

    Arnak

    22
    0
    Apr 1, 2011
    High Pass Filter.

    Hi Poor Mystic,

    That is certainly one way to approach the problem, I don't have enough technical knowledge to say whether filtering before or after is the best way forward.:confused:

    I'll have to leave that to the experts although I agree it it must be a valid suggestion thanks:)

    Arnak
     
  12. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
    Hi Arnak :)
    I'll give my reasons for my opinion:

    "I think that the quietest affordable professional mic pre-amp should be set to provide a suitable level to a digital recording system. e.g. the soundcard on your laptop.
    The signal can be examined and conditioned as required at a later time."

    This method has the following advantages over analogue filtering:
    1) The unfiltered version of the signal can be archived.
    2) Much better filter characteristics can be achieved with software that costs nothing (like Scilab or Audacity) than can be achieved with hardware.
    3) Hardware filters are inevitably noisy because of the contributions of shot noise from the transistors, thermal noise from the passive components, power supply noise, and environmental noise. These noise sources have no counterpart in digital filters.
    4) The effort involved in building an analogue filter is fairly large.

    Iit seems clear to me than digital filtering of the cleanest possible signal must be the better answer.
     
    Last edited: Apr 9, 2011
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