Unworkable band pass filter due to misplacing

Discussion in 'The Projects Forum' started by elnino86, Feb 8, 2009.

  1. elnino86

    Thread Starter New Member

    Dec 1, 2008
    Hello n shalom everyone,

    Im designing a pre amp,bandpass filter and power amp.At first of my project,im constructing a ECM pre amp using TS974.It work well.Then i combine it with power amp using TDA2822M which then fed to 32ohm earphone with 90mW power rating.They work well with total gain 10000(80dB).
    Then,next thing i had do is to design a bandpass filter(200-20kHz) which require me to use 4th order sallen key filter due to its simplicity and sharpness.I test the filter.It work well as in simulation.

    Then,i place it as you can see on my post diagram(between pre amp and power amp).Then,i inject 0.5mV input with 100Hz ferq.,as i measure output of the power amp,the ouput is about approx. 0.15V,which mean im still can hear the sound at 100Hz.I thought my bandpss filter doesnt work well.After several days spend on time thinking of it,i just realize that the signal was amplify at pre amp about 100 gaind,and been attenuate at bandpass filter then amplify again at powere amp which indicate that there nothing much happen on my bandpass filter.

    I want reduce,in other way to silent the sound at outer freq. of my bandpass.How do i want to achieve this?My idea are to place the bandpass filter
    i)after TDA2822M or
    ii)before TS974.

    **My first idea seem to be impossible since the TL071 will not able to drive 32ohm earphone.If i want to move on with my 2nd idea,the problem that i thought i will face is mismatch impedance.

    Does anyone can help me in this prob?

    -elnino wesbo-
  2. mik3

    Senior Member

    Feb 4, 2008
    It doesn't really matter where you place the filter. If you place it at the output it will attenuate a more powerful signal and the gain at 100 Hz will be the same.
    You can hear the 100Hz tone because a filter is not ideal. It doesn't attenuates completely the frequencies out of its bandwidth but they still have some small gain.

    Do you have the bode plot of the filter?
  3. Audioguru


    Dec 20, 2007
    As I said on the other forum, the filter is "only" 4th-order.
    If it is a Butterworth type and has its cutoff frequency at 200Hz then 100Hz is only 24dB down. To be inaudible the filter needs to be 10th-order or better.
  4. elnino86

    Thread Starter New Member

    Dec 1, 2008

    Oo.Quite understand what u trying to explain.Yes.Of course i have the bode plot.Here it is.

    Like Audioguru said,i need 10th order band pass filter.Do i able to work with it?i mean,if i really need 10th order band pass,i need like 10 TL071 or using quad op amp but hard to interface with passive element.