REV. E

SSM2135

–9–

APPLICATION CIRCUITS

Low Noise Stereo Headphone Driver Amplifier

Figure 1 shows the SSM2135 used in a stereo headphone driver

for multimedia applications with the AD1848, a 16-bit stereo

codec. The SSM2135 is equally well suited for the serial-bused

AD1849 stereo codec. The headphone’s impedance can be as

low as 25

W, which covers most commercially available high fidel-

ity headphones. Although the amplifier can operate at up to

±18 V

supply, it is just as efficient powered by a single 5 V. At this

voltage, the amplifier has sufficient output drive to deliver

distortion-free sound to a low impedance headphone.

GND

AD1848

40

35/36

32

41

10k

8.66k

470 F

+5V

0.1 F

10 F

L CH.

R CH.

10k

8.66k

0.1 F

34/37

1

2

3

5

6

7

470 F

0.1 F

10 F

1/2

SSM2135

4

8

AGND

1/2

SSM2135

Figure 1. A Stereo Headphone Driver for Multimedia

Sound Codec

Figure 2 shows the total harmonic distortion characteristics

versus frequency driving into a 32

W load, which is a very

typical impedance for a high quality stereo headphone. The

SSM2135 has excellent power supply rejection, and as a result,

is tolerant of poorly regulated supplies. However, for best sonic

quality, the power supply should be well regulated and heavily

bypassed to minimize supply modulation under heavy loads. A

minimum of 10

mF bypass is recommended.

1

20

20k

0.010

100

1k

10k

0.1

0.0005

0.001

AUDIO PRECISION

THD+(%) VS FREQ(Hz)

Figure 2. Headphone Driver THD+N vs. Frequency into a

32

Low Noise Microphone Preamplifier

The SSM2135’s 4.7 nV/

÷Hz input noise in conjunction with

low distortion makes it an ideal device for amplifying low level

signals such as those produced by microphones. Figure 3 illus-

trates a stereo microphone input circuit feeding a multimedia

sound codec. As shown, the gain is set at 100 (40 dB), although

it can be set to other gains depending on the microphone output

levels. Figure 4 shows the preamplifier’s harmonic distortion

performance with 1 V rms output while operating from a single

5 V supply.

codec. The same voltage is buffered by the 2N4124 transistor to

provide “phantom power” to the microphone. A typical electret

condenser microphone with an impedance range of 100

W to 1 kW

works well with the circuit. This power booster circuit may be

omitted for dynamic microphone elements.

L CHANNEL

MIC IN

R CHANNEL

MIC IN

2k

+5V

10 F

10k

2N4124

100

LMIC

GND

AD1848

RMIC

2k

10 F

10k

10 F

0.1 F

100

10k

10k

SSM2135

0.1 F

10 F

+5V

2

3

4

1/2

1

8

SSM2135

5

6

1/2

7

+5V

29

35/36

34/37

32

28

Figure 3. Low Noise Microphone Preamp for Multimedia

Sound Codec

1

20

20k

0.1

100

1k

10k

0.010

AUDIO PRECISION

THD+(%) VS FREQ(Hz)