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Headphone attenuator includes microphone monitor

Posted: 05 Jun 2006 ?? ?Print Version ?Bookmark and Share

Keywords:John Guy? Robert Nicoletti? Maxim? headphone? attenuator?

Headphones that fit into the ear, sometimes called ear buds, provide excellent fidelity and some intrinsic cancellation of ambient noise. Noise cancellation is useful on airplane flights and other situations in which the user wants to concentrate or sleep, but it makes talking difficult. Muting the music source provides some help, but the ear buds can still interfere with conversation.

Also, the minimum volume setting of the audio source is sometimes an issue. The headphone outputs of a modern airliner, for instance, are optimized for inexpensive disposable headphones. Because the sensitivity of low-cost headphones (the output sound pressure level, or SPL) for a given power input is lower than that of better-quality headphones, using high-quality headphones with the minimum volume on the control console can yield audio that is louder than tolerable.

Fortunately, both problems are solved with a single circuit (Figure 1). Resistors R1 and R2 reduce the input amplitude. The other circuitry amplifies the microphone input and injects that amplified signal into the left and right headphone-signal paths, producing an audible feedback when you speak.

Both the headphone amplifier (U1) and microphone amplifier (U2) connect directly to a 3V battery. During normal operation, both are in shutdown, drawing mere microamperes from the battery. To speak with someone, you need only depress the momentary switch SW1 to activate the microphone. That action enables the microphone amplifier's bias output, allowing it to amplify the external sound. The headphone amplifier connects to this amplified sound, and injects it into the headphone audio via resistors R3 and R4. As an added benefit of this mixing, the headphone amp's low-output impedance (when enabled) further attenuates the original audio stream.

Headphone attenuator with mic monitor

You adjust the monitor circuit's sensitivity by changing the values of R3 and R4, and the audio-input attenuation by changing the values of R1 and R2. Microphone, headphone jack, momentary switch, battery and other circuitry fits in an enclosure the size of a key fob, with an approximately 1m cable and headphone plug attached.

- John Guy, Robert Nicoletti
Maxim Integrated Products

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