Click-and-Pop Suppression

The output-coupling capacitor is a major contributor of

audible clicks and pops in conventional single-supply

headphone amplifiers. The amplifier charges the cou-

pling capacitor to its output bias voltage at startup.

During shutdown the capacitor is discharged. This

charging and discharging results in a DC shift across

the capacitor, which appears as an audible transient at

the speaker. Since the MAX9723 headphone amplifier

does not require output-coupling capacitors, no audi-

ble transients occur.

Additionally, the MAX9723 features extensive click-and-

pop suppression that eliminates any audible transient

sources internal to the device. The Power-Up/Power-

Down Waveform in the Typical Operating Characteristics

shows that there are minimal transients at the output upon

startup or shutdown.

In most applications, the preamplifier driving the

MAX9723 has a DC bias of typically half the supply.

The input-coupling capacitor is charged to the pream-

plifier’s bias voltage through the MAX9723’s input

shift across the capacitor creates an audible click/pop.

To avoid clicks/pops caused by the input filter, delay

BassMax (Bass Boost)

Typical headphones do not have a flat-frequency

response. The small physical size of the diaphragm

does not allow the headphone speaker to efficiently

reproduce low frequencies. This physical limitation

results in attenuated bass response. The MAX9723

includes a bass boost feature that compensates for the

headphone’s poor bass response by increasing the

amplifier gain at low frequencies.

The DirectDrive output of the MAX9723 has more head-

room than typical single-supply headphone amplifiers.

This additional headroom allows boosting the bass fre-

quencies without the output-signal clipping.

Program the BassMax gain and cutoff frequency with

external components connected between OUT_ and

BB_ (see the Functional Diagram/Typical Operating

command register to enable/disable the BassMax circuit.

BB_ is connected to the noninverting input of the output

amplifier when BassMax is enabled. BB_ is pulled to

SGND when BassMax is disabled. The typical application

of the BassMax circuit involves feeding a lowpass version

of the output signal back to the amplifier. This is realized

using positive feedback from OUT_ to BB_. Figure 2

shows the connections needed to implement BassMax.

Maximum Gain Control

The MAX9723A and MAX9723B have selectable

maximum gains of -5dB or 0dB (see Table 5) while

the MAX9723C and MAX9723D have selectable maxi-

mum gains of +1dB or +6dB (see Table 6). Bit 5 in the

command register selects between the two maximum

gain settings.

Volume Control

The MAX9723 includes a 32-level volume control that

adjusts the gain of the output amplifiers according to

the code contained in the command register. Volume is

programmed through the command register bits [4:0].

Tables 7–10 show all of the available gain settings for

the MAX9723A–MAX9723D. The mute attenuation is

typically better than 100dB when driving a 32

Ω load.

Serial Interface

2-wire serial interface consisting of a serial data line

(SDA) and a serial clock line (SCL). SDA and SCL facili-

tate communication between the MAX9723 and the

master at clock rates up to 400kHz. Figure 3 shows the

2-wire interface timing diagram. The MAX9723 is a

receive-only slave device relying on the master to gen-

erate the SCL signal. The MAX9723 cannot write to the

SDA bus except to acknowledge the receipt of data

Stereo DirectDrive Headphone Amplifier with

______________________________________________________________________________________

11

C3

R2

R1

R

R

OUT_

BB_

AUDIO

INPUT

MAX9723

BassMax

ENABLE

Figure 2. BassMax External Connections