How to Eliminate Intercom Noise With a Capacitor

By Sean Lancaster

Use a bypass filter to eliminate noise from your audio signals.
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Much of the noise that occurs in intercom systems arises from the coupling of DC and AC signals. The elimination of the noise, while retaining the primary signal, is the goal of communication specialists. They use bypass capacitors to filter out the AC noise component of the intercom. A simple intercom circuit uses an operational amplifier and transistor to generate a signal large enough for the intercom. A capacitor placed at the noninverting input of the operational amplifier removes any noise component coupled with the signal before amplification. The filtering of the input to eliminate the noise component cleans up the transmission quality of the signal.

Evaluate the input signal to the operational amplifier. The line in or microphone line on the intercom system delivers the signal to the portion of the amplifier circuit that feeds signal to the speakers.

Monitor the output of the amplifier section of the intercom system. Look at the traces of the signal on the oscilloscope. The trace that you see is a combination of the real signal and the noise present in the circuit.

Find the proper capacitor value that damps the noise in the output signal. The selection of the proper capacitor occurs visually based on the effects observed on the oscilliscope. Place the capacitor between the input signal and ground to allow for signal filtering. This configuration allows the capacitor to pass through the signal and removes the high frequency noise. Use the frequency of the noise to aid in the selection of the value of the capacitor to use as a bypass filter. Higher frequency noise requires a smaller capacitor value for efficient filtering. Common values for bypass capacitors are 4.7 uF, 0.1 uF and 22 uF. Complex noise patterns may require multiple capacitors to provide satisfactory noise rejection.