Analog Noise-Cancelling Audio Amplifier

Summary

An analog op-amp circuit built on the principle behind noise-cancelling headphones. The incoming audio signal is split into matched-gain inverting and non-inverting amplifiers, then recombined in a summing amplifier so the 180° out-of-phase copies cancel.

Design and Implementation

Designed and built for BME 294L (Circuits, Instruments, and Measurement). The input feeds two sub-circuits, an inverting amplifier and a non-inverting amplifier, whose resistor ratios are chosen so both stages have the same gain magnitude. Their outputs are equal in amplitude but 180° out of phase, so an inverting summing amplifier adds them to cancel the signal. Resistors were sized to keep each stage within its 2–5 mA current budget, and the LM358P op-amps run from a split positive/negative supply referenced to earth ground. Theoretical node voltages and branch currents were derived with the ideal op-amp rules and nodal analysis, then checked against oscilloscope and multimeter measurements on the breadboard. Swapping the function generator for a microphone and the output for a speaker turned it into a working audio demo.

Technical Details

• Three-stage LM358P design: inverting amp, non-inverting amp, and inverting summing amp
• Gains matched in magnitude so the two branches are equal and 180° out of phase, so they cancel at the summer
• Resistors sized for a 2–5 mA current budget per stage
• Split ±V supply built from two series bench supplies referenced to earth ground
• Validated on the oscilloscope: overall gain, every node voltage, and every branch current, theory vs measured
• Extended to live audio with a microphone input and speaker output

Results

• Matched-gain inverting + non-inverting stages summed for cancellation
• Theory vs measured node voltages and currents compared