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This continues in the same vein as my previous question - I'm working on an Active Noise Cancellation circuit to get rid of highway noises in my bedroom. Based off an ANC circuit pictured below, I intend to modify it in the following ways:

  • Replace the simple op-amp inverter with a phase controller

    • This will involve modifying the phase controller circuit to have an active range of 32-4096hz, discussed in the previous question linked above
  • Add in a low-pass filter
    • Some of the notes on the website where I found the circuit mention that it's not as effective above 1kHz, and I also recall reading about a hiss. Accordingly, I want to be able to eliminate this in a controllable way - so I want a variable low pass filter.

Step 2 is where I'm stuck - I'm not sure on A) component selection (High C, low R? High R, Low C? Variable R? Variable C?... I mean, probably variable R, but still) and B) filter placement.

I have 3300pF capacitors on hand, so I calculated that 1MOhm potentiometer would give me a range from 48hz to as high as I want for the cutoff (I mean, I imagine 10kHz would be more than enough). I was thinking of placing it immediately before the volume control (after the phase control) in my circuit, but I don't know how well that will line up with input characteristics of the op-amp I'm looking at using - or if there's a better place to put it? For the op-amp, I'm considering NE5532 (the datasheet I found said it had an input resistance of 30kOhm (min) and 300kOhm (average)).

I guess my question(s) (is/are), then:

  1. What should I use to judge the component value selection for this particular application? I'm still new to audio, and while I understand some of the selection criteria I'm not sure how it applies.

  2. Where should I place the filter? Will immediately prior to the volume control work, or should I look elsewhere for better results?

  3. Will my chosen op-amp work for this, or should I consider alternatives?

The diagrams I'm using are provided below, as well as links to where I found them.

ANC Circuit

Phase Control Circuit

ANC: https://web.archive.org/web/20110813221708/http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=noise_prj.htm

Phase Control: (All-pass filter) https://sound-au.com/project103.htm

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  • \$\begingroup\$ There are many unknowns in your design. attenuation and phase shift thru building, and spectral differences of microphone outside that needs to be matched to inside noise and inverted. All Pass Filter is good for 1 freq decade not (32~4k)as shown. Stereo? just do bass until it works \$\endgroup\$
    – Tony Stewart EE since 1975
    Commented Aug 30, 2018 at 19:22
  • \$\begingroup\$ I agree - there are a lot of unknowns in the system (my bedroom), so I'm trying to design a circuit that can respond to a wide variety of outside noise. This is why I'm modifying the all-pass filter circuit provided with a selector switch (for capacitors) and a 100Kohm pot - this will enable to me to control the 90-degree center of the shift. That was discussed in my previous question: this one is about the low-pass filter selection and placement. \$\endgroup\$
    – Helpful
    Commented Aug 30, 2018 at 19:25
  • \$\begingroup\$ My point is until you analyze with 2 mics one outside and one inside and compare you cannot design a filter. Once you see the spectral amplitude and phase difference then you match a filter to this then only use the outside mic and custom filter. Stereo will be impossible with all the wall echoes. The result ought to be LPF noise from 32 to 500 Hz. If you are getting noise to 4kHz, is your window open? \$\endgroup\$
    – Tony Stewart EE since 1975
    Commented Aug 30, 2018 at 21:13
  • \$\begingroup\$ I'm close to agreeing - for a single, specific situation that would be ideal: custom built specifically for it. I'm hoping to build something that can be applied to a couple different situations, however - and am therefore putting in adjustability via potentiometers so it can be dialed in to match various rooms. (For the record, I doubt the adjustability up to 4khz is necessary, but it doesn't change my design that much to make it possible). (Also, yes, I'm just going to make in mono rather than stereo). \$\endgroup\$
    – Helpful
    Commented Aug 30, 2018 at 21:40
  • \$\begingroup\$ Often the phase shift due to a room resonance cannot be equalize and as I recall some systems for a PA mic had tuneable graphic equalizers or notch filters to attenuate room resonances that have inherent sharp phase shift to avoid feedback oscillations \$\endgroup\$
    – Tony Stewart EE since 1975
    Commented Aug 30, 2018 at 21:45

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