MAX4409
80mW, DirectDrive, Stereo Headphone
Amplifier with Common-Mode Sense
10
______________________________________________________________________________________
typically 0.5mV, which, when combined with a 32
Ω
load, results in less than 16A of DC current flow to the
headphones.
Previous attempts to eliminate the output-coupling capac-
itors involved biasing the headphone return (sleeve) to
the DC-bias voltage of the headphone amplifiers. This
method raises some issues:
When combining a microphone and headphone on
a single connector, the microphone bias scheme
typically requires a 0V reference.
The sleeve is typically grounded to the chassis.
Using this biasing approach, the sleeve must be
isolated from system ground, complicating product
design.
During an ESD strike, the driver’s ESD structures
are the only path to system ground. Thus, the driver
must be able to withstand the full ESD strike.
When using the headphone jack as a line out to other
equipment, the bias voltage on the sleeve may con-
flict with the ground potential from other equipment,
resulting in possible damage to the drivers.
Low-Frequency Response
In addition to the cost and size disadvantages of the DC-
blocking capacitors required by conventional head-
phone amplifiers, these capacitors limit the amplifier’s
low-frequency response and can distort the audio signal:
The impedance of the headphone load and the DC-
blocking capacitor form a highpass filter with the
-3dB point set by:
where RL is the headphone impedance and COUT is
the DC-blocking capacitor value. The highpass filter
is required by conventional single-ended, single
power-supply headphone drivers to block the midrail
DC bias component of the audio signal from the
headphones. The drawback to the filter is that it can
attenuate low-frequency signals. Larger values of
COUT reduce this effect but result in physically larg-
er, more expensive capacitors. Figure 2 shows the
relationship between the size of COUT and the result-
ing low-frequency attenuation. Note that the -3dB
point for a 16
Ω headphone with a 100F blocking
capacitor is 100Hz, well within the normal audio
band, resulting in low-frequency attenuation of the
reproduced signal.
The voltage coefficient of the DC-blocking capacitor
contributes distortion to the reproduced audio signal
as the capacitance value varies as a function of the
voltage change across the capacitor. At low fre-
quencies, the reactance of the capacitor dominates
at frequencies below the -3dB point and the voltage
coefficient appears as frequency-dependent distor-
tion. Figure 3 shows the THD+N introduced by two
different capacitor dielectric types. Note that below
100Hz, THD+N increases rapidly.
The combination of low-frequency attenuation and fre-
quency-dependent distortion compromises audio
reproduction in portable audio equipment that empha-
sizes low-frequency effects such as multimedia lap-
f
RC
dB
L OUT
-
2
3
1
=
π
LF ROLL OFF (16
Ω LOAD)
MAX4409
fig02
FREQUENCY (Hz)
ATTENUATION
(dB)
100
-30
-25
-20
-10
-3dB CORNER FOR
100
μF IS 100Hz
-15
-5
-3
0
-35
10
1k
33
μF
330
μF
220
μF
100
μF
Figure 2. Low-Frequency Attenuation for Common DC-Blocking
Capacitor Values
ADDITIONAL THD+N DUE
TO DC-BLOCKING CAPACITORS
MAX4409
fig03
FREQUENCY (Hz)
THD+N
(%)
10k
1k
100
0.001
0.01
0.1
1
10
0.0001
10
100k
TANTALUM
ALUM/ELEC
Figure 3. Distortion Contributed by DC-Blocking Capacitors