REV. A

SSM2160

–13–

Controlling Stereo Headphones Level and Balance

Figure 14 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, which is deter-

mined by

= 1+

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

V

R

=

2

PmW

03

600

015

2

.

.

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

the voltage required to produce the desired power of 25 mW

VPR

VV rms

=

=×

=

0 025

600

3 9

..

The gain of the amplifiers must then be

A

A

R

R

R

R

R

R

V

V

==

=+

=

==

=

389

03

13

1

2

1

2

1

12

1

2

12

6000

12

500

.

.

Ω

If lower impedance headphones were used, say 30

Ω, the voltage

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

R1= 2.5 k

Ω and R2 = 5 kΩ.

The 100 pF capacitor, C2, in parallel with R2, creates a low-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 frequency

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

CH1 OUT

CH2 OUT

V–

DGND

V+

AGND

SSM2160

SSM2135-A

SSM2135-B

+5V

+

–5V

+

R1*

500

R1*

500

C2 100pF

R2 6k

150

150

R2 6k

C2 100pF

+5V

– 5V

50k

50k

15 F*

15 F*

LEFT

HEADPHONE

600

RIGHT

HEADPHONE

600

*SEE TEXT FOR ALTERNATE VALUES

1

2

4

21

12

13

Figure 14. 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 a

PC. The software may be downloaded from the Analog Devices

website at www.analog.com. The evaluation board comes com-

plete with the necessary parallel port cable and telephone type

plug that mates with the evaluation board.

Power Supplies

The evaluation 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 evalu-

ation board is per Figure 2.

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 pro-

vided 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 13. 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 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

Enter. If you downloaded the software to your hard disk from

the Analog Devices website to, for example, C:\SSM2160, on

the command line type C:\SSM2160\SETUP and press Enter.

The software will be automatically installed and a SSM2160

start-up icon will be displayed. Double-click the icon to start the

application. Under the menu item Port, select the parallel port