The active noise reduction circuit is divided into three stages, each of which contains an amplifying circuit that changes the noise analog signal in a unique way。 

      The first is pre-amplification, which does not reverse the signal; Then, phase reversal, configuration of the op amp; Finally, the signal is superimposed and amplified.
      Noise-cancelling headphones include two separate channels in order to maintain stereo sound. The two channels are independent and processed in parallel. Noise elimination is the analog audio signal through the three parts of the circuit processing to achieve.
      preamplification

      The pre-amplifier circuit is an op-amp circuit that does not reverse the audio and looks like pre-amplification. This circuit is necessary because the audio signal of this noise is very weak after coming from the microphone and must be amplified before it can be used. The pre-amplified gain is determined by the ratio of R8 to R6, and the amplitude of the audio signal can be controlled by changing these two resistance values, which will be fed into the phase reversal circuit.
      Phase reversal

      This part of the circuit changes the phase of the signal by changing the voltage polarity. By changing the ratio of resistors R12 and R10, the amplitude of the op amp circuit is changed.
      This is where the potentiometer (in the separation component scheme) comes in.
      Signal superposition

      Here the reversed noise signal will be superimposed with the original music signal and output to the speaker. The gain of the superimposed signal is controlled by the ratio of R19 to R15.