AD8531/AD8532/AD8534

REV. D

–12–

A Single-Supply Headphone Amplifier

Because of its speed and large output drive, the AD8531/AD8532/

AD8534 makes an excellent headphone driver, as illustrated in

Figure 40. 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 head-

phone, the amplifier inputs are biased to V+/2, which in this

case is 2.5 V. The 100 k

W resistor to the positive supply is

equally split into two 50 k

W resistors, with their common point

bypassed by 10

mF to prevent power supply noise from contami-

nating the audio signal.

The audio signal is then ac-coupled to each input through a

10

mF 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

mF 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

W to 600 W. An additional

16

W resistor is used in series with the output capacitor to pro-

tect the op amp’s output stage by limiting capacitor discharge

current. When driving a 48

W load, the circuit exhibits less than

0.3% THD+N at output drive levels of 4 V p-p.

1/2

AD8532

16

50k

270 F

LEFT

HEADPHONE

10 F

50k

50k

100k

10 F

LEFT

INPUT

1/2

AD8532

16

50k

270 F

RIGHT

HEADPHONE

10 F

50k

50k

100k

10 F

RIGHT

INPUT

V

V 5V

1 F/0.1 F

V 5V

Figure 40. A Single-Supply, Stereo Headphone Driver

A Single-Supply, Two-Way Loudspeaker Crossover Network

Active filters are useful in loudspeaker crossover networks for

reasons 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 use-

ful in implementing active crossover networks. The circuit

shown in Figure 41 is a single-supply, two-way active crossover

which 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 two-way example, the LO signal is a dc-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 a HP section, covering frequencies

≥500 Hz.

3

2

1

U1A

AD8532

4

R1

31.6k

C1

0.01 F

C2

0.01 F

R2

31.6k

R5

31.6k

R6

31.6k

R4

49.9

HI

LO

500Hz

AND UP

DC –

500Hz

6

5

7

C3

0.01 F

U1B

AD8532

C4

0.02 F

R7

15.8k

R3

49.9

270 F

270 F

100k

10 F

100k

100k

10 F

100k

0.1 F

100 F/25V

TO U1

5V

COM

+

100k

+

Figure 41. A Single-Supply, Two-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 II provides a design aid to adaptation,

with suggested standard component values for other frequencies.

Table II. RC Component Selection for Various

Crossover Frequencies

Crossover

R1/C1 (U1A)

1

Frequency (Hz)

R5/C3 (U1B)

2

100

160 k

W/0.01 mF

200

80.6 k

W/0.01 mF

319

49.9 k

W/0.01 mF

500

31.6 k

W/0.01 mF

1 k

16 k

W/0.01 mF

2 k

8.06 k

W/0.01 mF

5 k

3.16 k

W/0.01 mF

10 k

1.6 k

W/0.01 mF

NOTES

Applicable for filter

a = 2.

C4 = 2C3.

For additional information on the active filters and active cross-

over networks, please consult the data sheet for the OP279, a

dual rail-to-rail high-output current operational amplifier.