SSM2135

REV. D

–8–

APPLICATION CIRCUITS

A Low Noise Stereo Headphone Driver Amplifier

Figure 29 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

Ω, which covers most commercially available high

fidelity 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.

10µF

4

6

5

8

7

0.1µF

1/2

SSM2135

10µF

+5V

1/2

SSM2135

0.1µF

1

2

3

8.66k

Ω

10k

Ω

0.1µF

40

35/36

32

41

L CH

R CH

AGND

34/37

8.66k

Ω

10k

Ω

GND

AD1848

470µF

470µF

Figure 29. A Stereo Headphone Driver for Multimedia

Sound Codec

Figure 30 shows the total harmonic distortion characteristics

versus frequency driving into a 32

Ω 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

µF bypass is recommended.

Figure 30. Headphone Driver THD+N vs. Frequency into a

32

Ω Load (V

A 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 31 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 32 shows the preamplifier’s harmonic distortion

performance with 1 V rms output while operating from a single

+5 V supply.

AD1848 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

Ω to 1 kΩ works well with the circuit. This power booster

circuit may be omitted for dynamic microphone elements.

L CHANNEL

MIC IN

10µF

4

6

5

8

7

10µF

1/2

SSM2135

10µF

+5V

1/2

SSM2135

0.1µF

1

2

3

10k

Ω

10k

Ω

100

Ω

R CHANNEL

MIC IN

100

Ω

0.1µF

29

35/36

34/37

+5V

LMIC

GND

RMIC

AD1848

32

28

10k

Ω

10k

Ω

+5V

2N4124

10µF

2k

Ω

2k

Ω

Figure 31. Low Noise Microphone Preamp for Multimedia

Sound Codec