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.