Stereo systems for multiple speakers put the left and right speakers on opposite sides of the room. Personal systems use left and right ear buds or headphones to separate the sound. What can designers offer in the way of stereo on a cellular handset or other compact personal listening device that uses speakers? Maxim Integrated Products says its highly efficient MAX9775 class D audio subsystems are the answer. They enable the use of wave interference to cancel the left channel in the vicinity of the listener’s right ear, and vice versa, yielding an “apparent” separation (see the figure). The speakers seem to be four times farther apart than they actually are.
The MAX9775 integrates two 80-mW class ABs—one monophonic and one stereo (for ear buds). Both use Maxim’s DirectDrive (no output coupling capacitor) technology. For the stereo speakers, the chip has a 1.5-W class D stereo amp. The pre-amp for this implements the wave-cancelling circuitry. The similar MAX9776 only has a mono class D speaker amp, enabling cell-phone makers to offer a range of products on a common platform.
The wave-cancelling stereo enhancement implements a fairly well known psychoacoustic effect that is based on adding a bit of the right-channel signal to the left channel while inverting its phase, and vice-versa on the left channel. The result is an apparent 6-dB gain increase on each side.
This gain accounts for the sensation of greater channel separation. To fine-tune the effect, Maxim allows circuit designers to select the top and bottom of the frequency range that is affected through external resistors and capacitors and to an offsetting 6-dB reduction in overall gain.
The MAX9775 and MAX9776 incorporate a number of other proprietary Maxim technologies. For example, where traditional class D amplifiers require an output filter to recover the audio signal from the amplifier’s pulsewidth modulation output, the Maxim chips rely on the inherent inductance of the speaker coil and the natural filtering of both the speaker and the human ear to recover the audio component of the square-wave output. This does require some care in speaker selection.
The technique works because the class D switching frequency is far beyond the bandwidth of most speakers. However, Maxim recommends that speakers have a series inductance of at least 10 µH. Most 8-O speakers have two to 10 times that series inductance. But the datasheet warns that there have been problems with similar chips when makers of budget cell phones have tried to use them with ultra-cheap speakers.
A proprietary architecture that’s another class D enhancement reduces electromagnetic- interference radiated emissions and ensures acceptance by international communications standards organizations. The MAX9775 is priced at $1.45 in lots of 10,000 or more.
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