SSM2160_03 Datasheet, PDF (13/16 Pages) Analog Devices – 6-Channel, Serial Input Master/Balance Volume Controls

SSM2160
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
A_V
=
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
V²
P=
R
(^0.3)²
P=
= 0.15 mW
600
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
A_V = _0.3 = 13
A_V
=1+ ^R2
R1
R 2 _{= 12}
R1
R 2 6000
R1 = ₁₂ = 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 kΩ 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
V+ ¹
AGND ²
+ _R1*
500⍀
4
CH1 OUT
SSM2160
CH2 OUT ²¹
SSM2135-A
SSM2135-B
C2 100pF
R2 6k⍀
+5V
15␮F*
150⍀
– 5V
50k⍀
+5V
150⍀ ^15␮F*
–5V
V– ¹² ₊
DGND ¹³
R1*
500⍀
– 5V
R2 6k⍀
50k⍀
C2 100pF
LEFT
HEADPHONE
600⍀
RIGHT
HEADPHONE
600⍀
*SEE TEXT FOR ALTERNATE VALUES
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 kΩ, 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 kΩ 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
REV. A
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