Whether you find yourself in airplanes, cars, or just next to that overly-chatty person, it's sometimes nice to experience some music absent the distracting—and often overwhelming— effects of ambient noise.

Enter the noise-canceling headphone, in this case the SBC HN100 from Philips, where the concept is simple: "listen" to outside noise and create electronic "anti-noise" to destructively interfere with ambient garbage. The elegance of the design lies in the use of some surprisingly simple but capable analog circuitry to deliver low manufacturing cost.

Microphones in each ear cup serve to generate the noise reference signal. In theory, what the two microphones hear is a stereophonic representation of noise which sneaks through your headphones to disrupt an otherwise serene listening experience.

Key to the design is a pair of Japan Radio Corporation dual amplifiers (#NJM2076) which implement an inverting and non-inverting amplifier in a common chip. One dual amplifier is used for each of the left and right audio channels.

Passing through an amplifier with a level-calibrating potentiometer, the microphone signal is sent to the noninverting amplifier input. Concurrently, the input source connects to the inverting amplifier through its own volume- adjusting potentiometer.

Normally, the NJM2076 implements a bridge-tied-load (BTL) amplifier, with each amplifier creating equal but opposite contributions to output signal swing. The Philips design keeps a BTL topology but uses half of the amplifier pair to create the anti-noise for cancellation.

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The entire circuit is powered by a single AAA battery and enclosed in a pod which receives microphone signals from the ear cups and sends the amplified/purified signals to conventional earpiece speakers.

Incremental cost to a standard $25-$30 pair of headphones is small, though retail price is able to double with the noise-canceling feature. In the end, it's slick analog design that makes for sweet music—and in this case sweet profit margins.

David Carey is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics. (www.teardown.com)