Audio amplifier - boost a narrow frequency band 3db?

Discussion in 'General Electronics Chat' started by peskywinnets, Apr 24, 2012.

  1. peskywinnets

    Thread Starter Active Member

    Jan 19, 2009
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    What's the best way to approach this...I seek a fairly flat audio preamplifier, but would like to have a fixed boost at 330hz midpoint, where I seek about 3db of extra gain. (sloping off 100hz either side of this midpoint to the non boosted level)

    Haven't got a cliue how to approach this (filter n00b)...but would like to do so with as few components as possible...and with opamps!

    What do you guys propose?
     
  2. BillO

    Well-Known Member

    Nov 24, 2008
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    The filter you are looking for is a band-pass filter. Do a google search.
     
  3. peskywinnets

    Thread Starter Active Member

    Jan 19, 2009
    47
    0
    Is it a band pass here?


    I want 30Hz thru 8000hz to pass (which I guess would be the main 'band' to pass), *but* within that band I want 330hz to be boosted by 3db ...with a tail 100Hz either side (so a slow rising amount of gain from 230Hz to 330hz & vice versa on the other side down to 430hz.

    Would a notch filter be what I'm after?

    Edit: Apparently a band boost might be a better way of expressing...

    http://www.ecircuitcenter.com/Circuits/op_bandcut_boost/op_bandcut_boost_LCR.htm
     
    Last edited: Apr 24, 2012
  4. crutschow

    Expert

    Mar 14, 2008
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    A normal band-pass filter, by itself, will roll off all frequencies above and below the bandpass which you do not want.

    One way is to design a band-pass filter for the frequency you want (perhaps from 320Hz to 340Hz) and then connect that into the input of an op amp summing amp along with the unfiltered signal to a second summing input. You then select the relative gains so that the BP filter adds 3dB to the unfiltered signal at 330Hz.

    An easy way to design active filters is with the free FilterPro software from TI.
     
  5. BillO

    Well-Known Member

    Nov 24, 2008
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    Basically the band-stop and band-pass filters are much the same thing. It's just used in different ways. It can be used to attenuate all frequencies except the band or attenuate only the band. Or, as in these circuits you have found, used to cut or boost the band. There are several ways to implement a band filter, depending on the 'Q' factor you need. In the most common form, the series element, the basic elements are tuned to be a low impedance over the band. A parallel element configuration can be tuned to be a high impedance over the band, but is less frequently employed. In the circuits you found the RC is tuned to begin passing at a lower frequency than the LR is tuned to start blocking.

    Technically the notch filter is a very narrow band filter. The attenuation analogy of the band-pass filter is the band-stop filter.
     
  6. bertus

    Administrator

    Apr 5, 2008
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