AD8531/AD8532/AD8534
This active crossover exhibits less than 0.4% THD+N at output
levels of 1.4 V rms using general-purpose, unity-gain HP/LP stages.
SINGLE-SUPPLY HEADPHONE AMPLIFIER
Because of its speed and large output drive, the AD8531/
AD8532/AD8534 make an excellent headphone driver, as
illustrated in Figure 44. Its low supply operation and rail-to-rail
inputs and outputs give a maximum signal swing on a single
5 V supply. To ensure maximum signal swing available to drive
the headphone, the amplifier inputs are biased to V+/2, which
in this case is 2.5 V. The 100 kΩ resistor to the positive supply
is equally split into two 50 kΩ resistors, with their common
point bypassed by 10 μF to prevent power supply noise from
contaminating the audio signal.
In this 2-way example, the LO signal is a dc-to-500 Hz LP woofer
output, and the HI signal is the HP (>500 Hz) tweeter output.
U1B forms an LP section at 500 Hz, while U1A provides an HP
section, covering frequencies ≥500 Hz.
500Hz
C1
0.01µF
R3
49.9Ω
R1
31.6kΩ
AND UP
270µF
+
HI
V
S
C2
0.01µF
100kΩ
U1A
AD8532
3
2
V
1
IN
R
IN
R2
31.6kΩ
The audio signal is then ac-coupled to each input through a
10 μF capacitor. A large value is needed to ensure that the 20 Hz
audio information is not blocked. If the input already has the
proper dc bias, the ac coupling and biasing resistors are not
required. A 270 μF capacitor is used at the output to couple the
amplifier to the headphone. This value is much larger than that
used for the input because of the low impedance of the head-
phones, which can range from 32 Ω to 600 Ω. An additional 16 Ω
resistor is used in series with the output capacitor to protect the
output stage of the op amp by limiting the capacitor discharge
current. When driving a 48 Ω load, the circuit exhibits less
than 0.3% THD+N at output drive levels of 4 V p-p.
V 5V
4
100kΩ
C
IN
10µF
DC –
R5
31.6kΩ
R6
31.6kΩ
R4
49.9Ω
500Hz
270µF
+
LO
C3
0.01µF
R7
15.8kΩ
100kΩ
V
S
6
5
C4
0.02µF
100kΩ
100kΩ
7
U1B
AD8532
10µF
V
S
5V
0.1µF
100µF/25V
TO U1
COM
V 5V
1µF/0.1µF
50kΩ
Figure 45. A Single-Supply, 2-Way Active Crossover
270µF
10µF
16Ω
1/2
AD8532
LEFT
HEADPHONE
The crossover example frequency of 500 Hz can be shifted
lower or higher by frequency scaling of either resistors or
capacitors. In configuring the circuit for other frequencies,
50kΩ
LEFT
INPUT
50kΩ
10µF
100kΩ
complementary LP/HP action must be maintained between
sections, and component values within the sections must be in
the same ratio. Table 6 provides a design aid to adaptation, with
suggested standard component values for other frequencies.
V
50kΩ
50kΩ
For additional information on the active filters and active crossover
networks, refer to the data sheet for the OP279, a dual rail-to-
rail, high output current, operational amplifier.
270µF
10µF
16Ω
1/2
AD8532
RIGHT
HEADPHONE
RIGHT
INPUT
50kΩ
Table 6. RC Component Selection for Various Crossover
Frequencies1
Crossover Frequency (Hz)
10µF
100kΩ
R1/C1 (U1A)2, Rꢀ/C3 (U1B)3
160 kΩ/0.01 μF
Figure 44. Single-Supply, Stereo Headphone Driver
100
200
319
500
1 k
80.6 kΩ/0.01 μF
49.9 kΩ/0.01 μF
31.6 kΩ/0.01 μF
16 kΩ/0.01 μF
SINGLE-SUPPLY, 2-WAY LOUDSPEAKER
CROSSOVER NETWORK
Active filters are useful in loudspeaker crossover networks
because of small size, relative freedom from parasitic effects, the
ease of controlling low/high channel drive, and the controlled
driver damping provided by a dedicated amplifier. Both Sallen-
Key (SK) and multiple-feedback (MFB) filter architectures are
useful in implementing active crossover networks. The circuit
shown in Figure 45 is a single-supply, 2-way active crossover
that combines the advantages of both filter topologies.
2 k
5 k
10 k
8.06 kΩ/0.01 μF
3.16 kΩ/0.01 μF
1.6 kΩ/0.01 μF
1 Applicable for Filter A = 2.
2 For Sallen-Key stage U1A: R1 = R2, and C1 = C2, and so on.
3 For multiple feedback stage U1B: R6 = R5, R7 = R5/2, and C4 = 2C3.
Rev. F | Page 15 of 20
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