Headphone attenuator with microphone monitoring function
This application note describes a circuit that can attenuate the input amplitude while amplifying the microphone input signal. This circuit is used in conjunction with earplugs. Even if cheap, low-sensitivity headphones are used, this circuit can provide high-quality voice signals. The circuit is implemented with MAX4411 headphone amplifier and MAX9812 microphone amplifier.
In-ear headphones can maintain realistic sound quality and can eliminate background noise to a certain extent. The noise canceling function is very useful in the environment where aircraft and other users need to concentrate or rest. But the noise canceling function will affect people's verbal communication. Muting the music may help, but the earplugs still prevent normal conversation.
The minimum volume setting will also cause certain problems. For example, the headphone output channel of a passenger aircraft is generally designed for cheap headphones. The sensitivity of inexpensive headphones (ie, the output sound pressure level at a given input power, SPL) is lower than that of high-quality headphones. Therefore, when using high-quality headphones, even if the volume is set to a minimum, it will produce an intolerable volume.
Fortunately, all these problems can be solved by the following circuit (Figure 1). R1 and R2 are used to reduce the input amplitude. Other circuits are used to amplify the microphone input signal and inject the signal into the left and right channels of the headset, which can provide voice feedback when you need to speak. 
Figure 1. The headphone attenuator / amplifier has a microphone monitoring function to facilitate conversations when wearing headphones.
Headphone amplifier (U1, MAX4411) and microphone amplifier (U2, MAX9812) can be directly powered by 3V battery. In standard mode, both devices are in shutdown mode, consuming extremely low battery current. When you need to talk to other people, just press the momentary switch SW1 to turn on the microphone. This will turn on the bias output of the microphone amplifier and amplify the external sound signal. Connect the amplified signal to the headphone amplifier and inject it into the audio signal of the headphone through resistors R3 and R4. Another benefit provided by this mixing method is that the low output impedance of the headphone amplifier (when turned on) can further attenuate the audio stream of the original headphone channel.
The sensitivity of the monitoring circuit can be adjusted by changing R3 and R4. The attenuation of the audio input can be adjusted by R1 and R2. Microphones, headphone jacks, transient switches, batteries and other circuits can be installed in a keychain-sized box with a 1 meter long cable and headphone plug.
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