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MAX9706/MAX9707
3-Channel, 2.3W, Filterless Class D Amplifiers
with Active Crossover
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17
Mono Output
The left and right channels are summed and passed
through a lowpass filter to generate the mono output.
The mono speaker gain offset is an attenuation of the
selected speaker gain. The MAX9706/MAX9707 offer
three options for this summing gain. Select mono out-
put gain by setting MGAIN high, low, or leave floating
(see Table 3).
The left- and right-speaker impedance should be twice
that of the MONO channel (8Ω L/R, 4Ω MONO), then
from the same voltage swing, the mono speaker will
have 2 times the power. Over the left and right mono
channels, a 1.5dB increase improves matching
between the high- and low-frequency drivers.
Crossover Frequency
The MAX9706/MAX9707 feature an internal active filter
with adjustable crossover frequency (fXO) for use with a
low-frequency transducer. The crossover filter consists of
a complementary 2nd-order lowpass and 2nd-order
highpass Butterworth filter (Figure 6). Crossover fre-
quency is variable over the 800Hz to 2133.3Hz range to
accommodate different speaker types. There are four
selectable crossover frequencies selected by FS0 and
FS1 (Table 4).
The BTL outputs provide the option of phase-inverting
the mono (LF) output with respect to the main (L/R) out-
puts. Depending on the speaker placement and dis-
tance from the listener, this can smooth the crossover
transition between low and high frequencies.
Headphone Amplifier (MAX9706)
In conventional single-supply headphone amplifiers,
the output-coupling capacitor is a major contributor of
audible clicks and pops. Upon startup, the amplifier
charges the coupling capacitor to its bias voltage, typi-
cally half the supply. Likewise, during shutdown, the
capacitor is discharged to GND. This results in a DC
shift across the capacitor, which in turn appears as an
audible transient at the speaker. Since the MAX9706
headphone amplifier does not require output-coupling
capacitors, no audible transients appear.
The MAX9706 offers 0dB and 3dB headphone amplifier
gain settings controlled through the GAIN2 gain-select
input (see Table 2).
DirectDrive
Traditional single-supply headphone amplifiers have
outputs biased at a nominal DC voltage (typically half
the supply) for maximum dynamic range. Large cou-
pling capacitors are needed to block this DC bias from
the headphone. Without these capacitors, a significant
amount of DC current flows to the headphone, resulting
in unnecessary power dissipation and possible dam-
age to both headphone and headphone amplifier.
Maxim’s DirectDrive architecture uses a charge pump
to create an internal negative supply voltage. This
allows the headphone outputs of the MAX9706 to be
biased at GND, almost doubling dynamic range while
operating from a single supply (Figure 7). With no DC
component, there is no need for the large DC-block-
ing capacitors. Instead of two large (220F, typical)
Table 3. Mono Speaker Gain
MGAIN
MONO SPEAKER GAIN
OFFSET (dB)
GND
-4.5
FLOATING
-6.0
VDD
-7.5
Table 4. Crossover Frequency Selection
FS0
FS1
CROSSOVER
FREQUENCY (fXO) (Hz)
0
800
0
1
1066.7
1
0
1600
1
2133.3
AMPLITUDE
(dB)
fX
FREQUENCY (Hz)
HP FUNCTION
2nd-ORDER SLOPE
LP FUNCTION
2nd-ORDER SLOPE
Figure 6. Crossover Frequency