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 critera 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.
Phase Control: (All-pass filter) http://sound.whsites.net/project103.htm