20W/40W, Filterless, Spread-Spectrum,

Mono/Stereo, Class D Amplifier

______________________________________________________________________________________

17

Mismatch in the components can cause discrepancies

between the nominal transfer function and actual perfor-

mance. Also, the mismatch of the input resistors (R15,

R17, R19, and R21 in Figure 6) of the summing amplifier

and lowpass filter will cause some high-frequency sound

to be sent to the subwoofer.

The circuit in Figure 6 drives a pair of MAX9708 devices

similar to the circuit in Figure 5. The inputs to the

MAX9708 still require AC-coupling to prevent compro-

mising the click-and-pop performance of the MAX9708.

The left and right drivers should be at an 8

Ω to 12Ω

impedance, whereas the subwoofer can be 4

Ω to 12Ω

depending on the desired output power, the available

power-supply voltage, and the sensitivity of the individ-

ual speakers in the system. The four gain settings of

the MAX9708 allow gain adjustments to match the sen-

sitivity of the speakers.

Figure 5. Multiple Amplifiers Implement a 2.1 Audio System

MAX9708

MAX9708

HIGHPASS

FILTER

8

Ω

FULL-

RANGE

SPEAKER

8

Ω

FULL-

RANGE

SPEAKER

4

Ω OR 8Ω

WOOFER

RIGHT

AUDIO

LEFT

AUDIO

HIGHPASS

FILTER

LOWPASS

FILTER

Σ

OUTR+

OUTR-

OUTL+

OUTL-

OUTR+

OUTR-

OUTL+

OUTL-

INR+

INR-

MONO

INL+

INL-

INR+

SYNCIN

SYNCOUT

INR-

MONO

INL+

INL-