SSM2000 Analog, SSM2000 Datasheet - Page 11

• Manufacturer Part Number: SSM2000
• Description: HUSH Stereo Noise Reduction System with Adaptive Threshold
• Manufacturer: Analog

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Signal Chain Position
Figure 27 illustrates a common audio signal path. The SSM2000
is normally placed just before the volume control and power
amplifier. The reason the SSM2000 should be placed as far
down the signal chain as possible is that noise that is introduced
by the previous components will be operated on by the noise re-
duction system. On the other hand, the SSM2000 works best
at the 300 mV rms level and therefore the IC should be placed
before any significant volume control. To operate the SSM2000
in a post-volume control position, please refer to the Optional
Application Circuits–Using the SSM2000 in a post-volume con-
trol application section.
Figure 27. The Position of the SSM2000 in a Common
Audio Signal Chain
Powering the SSM2000—Dual Supply
The SSM2000 has a supply voltage range from +7 V to +18 V.
A well regulated (< 20 mV of ripple) 9 V supply is acceptable,
even though this is value is exactly equal to the absolute maxi-
mum rating of 18 V. The HUSH has been found to be opera-
tional down to a supply range of only 4.5 V. However, for
guaranteed performance the supplies must be within the speci-
fied supply ranges.
The power supplies of choice from a performance perspective
are quality linear supplies or batteries, because of their low
noise. If a switching supply is required, or the power supply
lines are noisy, then adequate filtering of the power supply lines
should be provided. It is common to place a power supply filter
at the power supply and another at the entry point of power to
the audio system, but only one filter is necessary if these two
locations are separated by less than a foot or so.
In normal dual supply applications ACOM should be connected
to the system ground. If ACOM is not attached to the system
ground, then please refer to the section on Powering HUSH—
single supply. V+ and V– should be connected to their respec-
tive power rails. V+ and V– will need to be properly decoupled,
because the IR drops in the power rails during large signal
swings in the HUSH chip can cause additional distortion. As
shown in Figure 28a, it is recommended that a aluminum elec-
trolytic bulk storage cap of 1 F be placed in the vicinity of the
HUSH chip and a 0.1 F ceramic capacitor be placed directly
at the HUSH chip. Both power rails should be decoupled this
manner.
CASSETTE
CD
MIC...
LP
TUNER
PREAMP,
MIXING,
EQ...
SSM2000
POWER AMP
VOLUME +
L
R
–11–
Powering the SSM2000—Single Supply
To operate HUSH in the single supply mode, a false ground
potential should be generated and attached to ACOM, as
shown in Figure 28b. This false ground is usually set at the
midpoint between the power rails. The false ground that will be
created for the HUSH chip must be able to both sink and
source current up to 10 mA. A low cost OP292 will perform
this function very well. Once again, a 1 F aluminum electro-
lytic capacitor and a 0.1 F ceramic capacitor should be use to
decouple both the false ground potential and positive supply
from the negative supply. It may be helpful to read the preced-
ing section “Powering HUSH—Dual Supply” for information
on power supply issues that affect both dual and single supply
applications.
Mute (Pin 17) and Bypass (Pin 16) potentials should be refer-
enced to DGND. Other inputs are either capacity coupled or
referenced to ACOM.
DETAILED PIN DESCRIPTIONS
Left Audio IN and Right Audio IN (Pins 1, 2)
L IN and R IN are the inputs for the Left and Right channels.
These are each single ended inputs with an input impedance of
10 k . The input driver for SSM2000 should be able to handle
this load level without significant harmonic distortion. These
inputs should be ac coupled with at least the minimum capaci-
tor value shown in the example circuits, which is 6.8 F. This
sets up a high pass filter with a corner frequency of less than 3 Hz.
Choosing a lower value capacitor than 6.8 F may result in the
loss of low frequency audio signal. On the other hand choosing
a large value will improve low frequency channel separation.
Figure 28a. The Dual Power Rail Connections for the
SSM2000
Figure 28b. Powering the SSM2000—Single Supply
V
CC
V
CC
1µF
1µF
1µF
0.1µF
10k
10k
5
6
2
3
V+
ACOM
SSM2000
OP292
1/2
1
GND*
V–
6
5
19
20
ACOM
V+
SSM2000
0.1µF
SSM2000
BYPASS
GND
MUTE
V–
19
20
1µF
V
EE