Datasheet
The circuit in Figure 6 uses inverting amplifiers for their
ease in biasing. Note the phase labeling at the outputs
has been reversed. The resistors should be 1% or better
in tolerance and the capacitors 5% tolerance or better.
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 causes some high-frequency sound to
be sent to the subwoofer.
The circuit in Figure 6 drives a pair of MAX9709 devic-
es similar to the circuit in Figure 5. The inputs to the
MAX9709 still require AC-coupling to prevent compromis-
ing the click-and-pop performance of the MAX9709.
The left and right drivers should be at an 8Ω to 12Ω imped-
ance, whereas the subwoofer can be 4Ω to 8Ω depending
on the desired output power, the available power-supply
voltage, and the sensitivity of the individual speakers in
the system. The four gain settings of the MAX9709 allow
gain adjustments to match the sensitivity of the speakers.
Figure 5. Multiple Amplifiers Implement a 2.1 Audio System
MAX9709
MAX9709
HIGHPASS
FILTER
8Ω
FULL-
RANGE
SPEAKER
8Ω
FULL-
RANGE
SPEAKER
4Ω OR 8Ω
WOOFER
RIGHT
AUDIO
LEFT
AUDIO
HIGHPASS
FILTER
LOWPASS
FILTER
∑
V
DIGITAL
OUTR+
OUTR-
OUTL+
OUTL-
OUTR+
OUTR-
OUTL+
OUTL-
INR+
INR-
MONO
INL+
INL-
INR+
SYNCIN
SYNCOUT
INR-
MONO
INL+
INL-
MAX9709 25W/50W, Filterless, Spread-Spectrum,
Stereo/Mono, Class D Amplier
www.maximintegrated.com
Maxim Integrated
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