Low attenuation, 1-pot tunable audio band reject filter

Discussion in 'The Projects Forum' started by Audion, Jul 4, 2013.

  1. Audion

    Thread Starter New Member

    Jul 4, 2013

    1st post. I would like to create a band reject filter which has far less attenuation than the designs I have found online thus far. The filters I see have 40 dB or more of attenuation which is far too much for my application. I also have not seen any such filter which is tunable with a single pot.

    What I want:

    Band Reject Filter
    Center Frequency of stop band (center of range) 350 Hz
    Maximum attenuation in stop band -6 dB
    -3 dB points for passbands: 300 Hz & 400 Hz (when tuned to 350 Hz)
    Frequency tunable by 1 pot (can be ganged up to 2) within 250 Hz to 450 Hz

    I am no expert. Call me an informed amateur. I have looked a fair amount online and all I can find are notch filters, which have way too much attenuation for my application.

    My application is an 'anti-mud' filter for electric guitar. The frequency region between 300-400 Hz is known in audio mixing circles as the 'mud' region, and is often attenuated during mix. But that attenuation is rarely much over 6 dB, which is a lot.

    If the design is unduly complicated by the variable tuning requirement, I can get by with fixed frequency as long as the attenuation is low and the Q is 1.0 or above.

    So, do I just make complementary shelving lowpass and highpass filters and use them in a parallel circuit? If that is the case, I need some help finding a design where I can control the amount of attenuation. I also don't know how to use these circuits in parallel. Just feed the same input into both filters and sum the outputs? Won't that create all kinds of nasty phase smearing? I am concerned about summing ANYTHING for the phase risk. I don't want to have to add an allpass filter just to fix the phase.

    Another idea I have seen is make a bandpass filter and use the filter output as negative feedback for the op amp. Ok. Sounds great. But there has to more to it than just that. Is there? Implementation?

    I apologize if what I have asked is already present on this forum. I have spent about 20 hours online researching this and I just read all the potentially applicable posts here I could find. You weren't my first option - I always try to figure things out myself first.

    I appreciate any help.
  2. RichardO

    Late Member

    May 4, 2013
    #12 likes this.