AD8531/AD8532/AD8534

Rev. F | Page 15 of 20

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.

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.

1/2

AD8532

16Ω

50kΩ

270µF

LEFT

HEADPHONE

10µF

50kΩ

50kΩ

100kΩ

10µF

LEFT

INPUT

1/2

AD8532

16Ω

50kΩ

270µF

RIGHT

HEADPHONE

10µF

50kΩ

50kΩ

100kΩ

10µF

RIGHT

INPUT

V

V5V

1µF/0.1µF

V 5V

Figure 44. Single-Supply, Stereo Headphone Driver

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.

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.

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.

3

2

1

U1A

AD8532

4

R1

31.6kΩ

C1

0.01µF

C2

0.01µF

R2

31.6kΩ

R5

31.6kΩ

R6

31.6kΩ

R4

49.9Ω

HI

LO

500Hz

AND UP

DC –

500Hz

6

5

7

C3

0.01µF

U1B

AD8532

C4

0.02µF

R7

15.8kΩ

R3

49.9Ω 270µF

270µF

100kΩ

10µF

100kΩ

100kΩ

10µF

100kΩ

0.1µF

100µF/25V

TO U1

5V

COM

+

100kΩ

+

Figure 45. A Single-Supply, 2-Way Active Crossover

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,

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.

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.

Table 6. RC Component Selection for Various Crossover

Frequencies1

Crossover Frequency (Hz)

R1/C1 (U1A)2, R5/C3 (U1B)3

100

160 kΩ/0.01 μF

200

80.6 kΩ/0.01 μF

319

49.9 kΩ/0.01 μF

500

31.6 kΩ/0.01 μF

1 k

16 kΩ/0.01 μF

2 k

8.06 kΩ/0.01 μF

5 k

3.16 kΩ/0.01 μF

10 k

1.6 kΩ/0.01 μF