SSM2160/SSM2161

–13–

REV. 0

Controlling Stereo Headphones Level and Balance

Figure 26 shows how the SSM2160 can be configured to drive a

stereo headphone output amplifier. Note that the minimum

load specification precludes driving headphones directly. This

example assumes that audio left and right signals are being fed

into Channels 1 and 2, respectively. Additional amplifiers could

be connected to the outputs to provide additional channels.

The master control will set the loudness, and the channel

controls will set the balance. The headphone amplifiers may be

connected to the same power supplies as the SSM2160. The

stereo audio signals are directly coupled to the noninverting

input of both op amps. Depending upon the headphones and

the signal levels, the optional R1 may be selected to provide

additional gain. The gain is determined by:

R2

R1









As an example, suppose a high impedance headphone (600

Ω)

required a minimum of 25 mW to produce the desired loudness.

Further, suppose the system design made available an output

level from the SSM2160 of 300 mV. If the output were buffered

without gain and applied directly to the headphone, the power

would be:

P

=

R

P

=

600

= 0.15 mW

This is obviously too little power, so we solve the equation for

the voltage required to produce the desired power of 25 mW:

V

= PR

V

= 0.025 × 600 = 3.9 V rms

The gain of the amplifiers must then be:

3.89

0.3

= 13

R2

R1

R2

R1

= 12

R1

=

R2

12

=

6000

12

= 500 Ω

If lower impedance headphones were used, say 30

Ω, the voltage

required would be 0.9 V rms, so a gain of 3 would suffice, thus

R1 = 2.5 k

Ω and R2 = 5 kΩ.

pass filter with a cutoff above the audible range, reducing the

gain to high frequency noise. A small resistor within the

feedback loop protects the output stage in the event of a short

circuit at the headphone output but does not measurably reduce

the signal swing or loop gain. The dc blocking capacitor at the

output establishes a high pass filter with a –3 dB corner fre-

quency determined by the value of C1 and the headphone

impedance. With 600

Ω headphones, an output capacitor of 15

µF sets this corner at 20 Hz. Similarly, a 30 Ω headphone will

require 250

µF.

CAUTION: As with all headphone applications, listening to

loud sounds can cause permanent hearing loss.

+5V

– 5V

CH1OUT

CH2OUT

V–

DGND

V+

AGND

SSM2160

SSM2135-A

SSM2135-B

+5V

+

–5V

+

500

Ω

500

Ω

C2 100pF

150

Ω

150

Ω

C2 100pF

+5V

– 5V

50k

Ω

50k

Ω

15µF*

15µF*

LEFT

HEADPHONE

600

Ω

RIGHT

HEADPHONE

600

Ω

*SEE TEXT FOR ALTERNATE VALUES

1

2

4

21

12

13

Figure 26. Headphone Output Amplifier Configuration

EVALUATION BOARD FOR THE SSM2160

The following information is to be used with the SSM2160

evaluation board, which simplifies connecting the part into

existing systems. Audio signals are fed in and out via standard

RCA-type audio connectors. A stereo headphone driver socket

is provided for the convenience of listening to Channels 1 and

2. Microsoft Windows software is available for controlling the

serial data bus of the SSM2160 via the parallel port driver

(LPT) of an IBM-compatible PC. The software may be

downloaded from the Analog Devices Internet web site at

http://WWW.ANALOG.COM, or by requesting a diskette from

Analog Audio marketing by faxing (408)727-1550. The demo

board comes complete with the necessary parallel port cable and

telephone type plug that mates with the evaluation board.

Power Supplies

The demo board should be connected to

±6 V supplies for

initial evaluation. If other supply voltages are planned, they can

be subsequently changed. The power configuration on the

evaluation board is per Figure 14.

Signal Inputs and Outputs

Input load impedances are approximately 10 k

Ω, so the load on

the sources is relatively light. DC blocking capacitors are

provided on the evaluation board. The load impedance

connected to the outputs must be no less than 10 k

Ω and no

more than 50 pF shunt capacitance. This enables driving short

lengths of shielded or twisted wire cable. If heavier loads must

be driven, use an external buffer as shown in Figure 25. Note

that 50

Ω isolation resistors are placed in series with each

SSM2160 output and may be jumpered if desired.

Digital Interface

The interconnecting cable provided has a DB25 male connector

for the parallel port of the PC and an RJ14 plug that connects to

the evaluation board. This cable is all that is required for the

computer interface.

Software Installation

If installing the software from a diskette, and using Windows

version 3.1 or later, select the RUN command from the FILE

menu of the Program Manager. In the command line, type

a:\setup and press return. If you downloaded the software to