Filter help please

Discussion in 'The Projects Forum' started by Neil Groves, Jan 15, 2012.

  1. Neil Groves

    Thread Starter Member

    Sep 14, 2011
    ok i just transfered my circuit from breadboard to stripboard in the hope it was a bad connection, i have a better result but my bandpass filter doesn't seem to attenuate in the right place:

    I have only built the bass section so far and i get a signal from about 90Hz but it doesn't attenuate till i get to around 5Khz, i was expecting it to start tailing off at around 200Hz?

    also i am using a 1N914 instead of the specified 1N4002, will this have a detrimental effect?

    The chip i am using is LM324 since i had one laying around instead of MC33204, i don't know if this could be the issue?

  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Are the diode and the opamp the only parts that you changed in the circuit?

  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    It seems that you circuit work as it should.
    I also think that this circuit may be unstable too ,because he has high gain and Q factor.
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Wouldn't it have a better plot if it were a butterworth filter?

    Moving the lower 0.047u cap from input to ground?

    --ETA: Closer to what is shown in the inset boxes of "Original Circuit" on Page 1 of 75(!) of the attached PDF, but with the cap in the feedback loop as well.

    The LM324 sub may not be the best, it will have problems at higher frequencies, and with the gain alone could be showing a rolloff you are seeing @5kHz
    Last edited: Jan 15, 2012
  5. Neil Groves

    Thread Starter Member

    Sep 14, 2011
    When i get time i'll try another op amp, and if i get REALLY desperate i'll change the diode to what the circuit specifies.

  6. crutschow


    Mar 14, 2008
    I don't think it's the op amp or the diode. I would double-check all the circuit connections, particularly the Virtual Ground. Are you measuring the proper DC voltage at that point?
  7. Neil Groves

    Thread Starter Member

    Sep 14, 2011
    I am using a 9v supply instead of 12v and my virtual ground is 4.4v, i am going to check my connections though, as that is the most common reason why my circuits don't work first time.