Audio amps boost stereo effect for portables

EDN Europe, 17 Dec 2007

For designers working with compact, personal audio products who need to create convincing stereo sound, Maxim has introduced the MXA9775 and 9776, “audio subsystems”. These parts are high-efficiency class-D amplifiers with a mixer and other signal-path components that provide enhancement of the stereo effect where the speakers are close together. To emulate stereo sound in such situations, the chip interference effects to partially cancel the left channel in the vicinity of the listener's right ear, and vice versa (See Figure). This 3D enhancement technique yields an "apparent" separation between the speakers that is a factor of four or greater than the actual physical separation.
The 9775 integrates five output amplifiers: a mono, DirectDrive, 80mW receiver amplifier; a stereo, DirectDrive, 80mW headphone amplifier; and a high-efficiency, Class D, stereo 1.5W audio power amplifier. The MAX9776 has four output amplifiers: the mono, 80mW receiver amplifier; the stereo, DirectDrive 80mW headphone amplifier; and a mono Class D amplifier. Techniques that Maxim has applied in the design include spread-spectrum modulation, active emissions limiting (AEL), Class D filterless amps, and DirectDrive Technology. Spread-spectrum modulation and third-generation filterless AEL technology reduce EMI radiated emissions. Above a few MHz, the wideband spectrum looks like white noise for EMI purposes. Proprietary techniques ensure that the cycle-to-cycle variation of the switching period does not degrade audio reproduction or efficiency. The Class-D speaker amplifiers exhibit greater than 70% efficiencies. The devices' stereo headphone amplifier and receiver amplifier both feature the DirectDrive architecture, which uses a charge pump to produce an internal negative supply voltage. This allows a ground-referenced output, almost doubling the dynamic range while operating from a single supply. The chips incorporate a user-defined input architecture that features an input mixer and multiplexer. Three flexible inputs are available as single-ended stereo inputs or differential mono inputs. All input signals are summed into three unique signals before being routed to the amplifiers. The MAX9775 costs $1.45, and the MAX9776 is $1.25 (10,000)