1.2W, Low-EMI, Filterless,

Class D Audio Amplifier

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13

Stereo Configuration

Two MAX9746s can be configured as a stereo amplifier

(Figure 6). Device U1 is the master amplifier; its unfil-

tered output drives the SYNC input of the slave device

(U2), synchronizing the switching frequencies of the two

devices. Synchronizing two MAX9746s ensures that no

beat frequencies occur within the audio spectrum. This

configuration works when the master device is in either

FFM or SSM mode. There is excellent THD+N perfor-

mance and minimal crosstalk between devices due to

the SYNC connection (Figures 7 and 8). U2 locks onto

only the frequency present at SYNC, not the pulse

width. The internal feedback loop of device U2 ensures

that the audio component of U1’s output is rejected.

Designing with Volume Control

The MAX9746 can easily be driven by single-ended

sources (Figure 5), but extra care is needed if the

source impedance “seen” by each differential input is

unbalanced, such as the case in Figure 9, where the

MAX9746 is used with an audio taper potentiometer

acting as a volume control. Functionally, this configura-

tion works well, but can suffer from click-pop transients

at power-up (or coming out of shutdown) depending on

the volume-control setting. As shown, the click-pop per-

formance is fine for either max or min volume, but wors-

ens at other settings.

Figure 6. Master-Slave Stereo Configuration

IN+

IN-

OUT+

OUT-

SYNC

1µF

RIGHT-CHANNEL

DIFFERENTIAL

AUDIO INPUT

MAX9746

U1

U2

IN+

IN-

OUT+

OUT-

SYNC

1µF

LEFT-CHANNEL

DIFFERENTIAL

AUDIO INPUT

MAX9746

Figure 7. Master-Slave THD+N

100

0

0.1

0.2

0.3

0.4

0.5

10

1

0.1

0.01

0.001

TOTAL HARMONIC DISTORTION PLUS NOISE

vs. OUTPUT POWER

OUTPUT POWER (W)

f = 1kHz

SLAVE DEVICE

0

-120

-100

10

100

1k

10k

100k

CROSSTALK vs. FREQUENCY

-80

FREQUENCY (Hz)

-60

-40

-20

MASTER TO SLAVE

f = 1kHz

Figure 8. Master-Slave Crosstalk