I'm an entry- to mid-level electronics hobbyist and I've been tinkering around with a simple active noise cancellation circuit (audio frequencies) based in principle on the schematic here: http://headwize.com/?page_id=684
I've obtained fairly decent performance prototyping with some spare op-amps. The biggest problem (for me) is that the circuit depends on a potentiometer to manually adjust the volume of the inverted noise signal before it is summed back into the noise signal. In real life, this requires constant manual adjustment to zero out the noise as the noise level volume fluctuates over time.
EDIT: Although the original design allows for the anti-phase noise signal to be summed with a desired audio signal (ie, music), I am using the headphones only to attempt to create artificial silence.
So I want to improve the design to auto-control the inverted signal gain so as to always sum as close to zero as possible. Does anyone have good ideas on how to accomplish this (without using a microcontroller)? I was thinking of some type of comparator circuit or maybe I should move directly to a voltage-controlled amplifier for the the inverting op-amp? I really don't have any familiarity with these sort of circuits.
If anyone has a relevant circuit schematic or op amp model recommendations, it would be much appreciated.
Best Answer
One analog approach is to add an Automatic Gain Control stage to the inverted signal path. The responsiveness of the AGC can be tuned to provide just enough tracking for the envisaged purpose.
If the AGC is placed early enough in the signal path, it will not need to handle much power, making design simpler.
This is a common enough requirement in analog-based noise cancellation designs that several manufacturers have published relevant app-notes about it. For instance, this page on Mouser might be a useful place to start reading up on it.
An AGC can be constructed using op-amps, Variable Gain Amplifiers (VGAs), or dedicated audio-use AGC ICs. The specific approach would be determined by preferences such as whether surface mount ICs are an option, and level of distortion that is acceptable.