Microphone Preamplifier with

Variable Compression and Noise Gating

Data Sheet

SSM2166

Rev. E

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FEATURES

Complete microphone conditioner in a 14-lead SOIC package

Single 5 V operation

Adjustable noise gate threshold

Compression ratio set by external resistor

Automatic limiting feature—prevents ADC overload

Adjustable release time

Low noise and distortion

Power-down feature

20 kHz bandwidth (±1 dB)

APPLICATIONS

Microphone preamplifiers/processors

Computer sound cards

Public address/paging systems

Communication headsets

Telephone conferencing

Guitar sustain effects generators

Computerized voice recognition

Surveillance systems

Karaoke and DJ mixers

GENERAL DESCRIPTION

The SSM2166 integrates a complete and flexible solution for

conditioning microphone inputs in computer audio systems. It

is also excellent for improving vocal clarity in communications

and public address systems. A low noise, voltage-controlled

amplifier (VCA) provides a gain that is dynamically adjusted by

a control loop to maintain a set compression characteristic. The

compression ratio is set by a single resistor and can be varied

from 1:1 to over 15:1 relative to a user-defined rotation point;

signals above the rotation point are limited to prevent overload

and to eliminate popping. In the 1:1 compression setting, the

SSM2166 can be programmed with a fixed gain of up to 20 dB;

this gain is in addition to the variable gain in other compression

settings. The input buffer can also be configured for front-end

gains of 0 dB to 20 dB. A downward expander (noise gate) prevents

amplification of noise or hum. This results in optimized signal

levels prior to digitization, thereby eliminating the need for

additional gain or attenuation in the digital domain that may

add noise or impair accuracy of speech recognition algorithms.

The compression ratio and time constants are set externally. A

high degree of flexibility is provided by the VCA gain, rotation

point, and noise gate adjustment pins.

The SSM2166 is an ideal companion product for audio codecs

used in computer systems. The SSM2166 is available in a 14-

lead SOIC package and is guaranteed for operation over the

extended industrial temperature range of −40°C to +85°C.

10

0

–10

–20

–30

–40

–50

–60

–70

–60

–50

–40

–30

–20

–10

0

INPUT (dBu)

COMP RATIO = 10:1

COMP RATIO = 2:1

COMP RATIO = 1:1

Figure 1. Compression and Gating Characteristics with 10 dB of Fixed Gain (The

Gain Adjust Pin Can Be Used to Vary This Fixed Gain Amount)

FUNCTIONAL BLOCK DIAGRAM AND TYPICAL SPEECH APPLICATION

BUFFER

1kΩ

1kΩ

BUF OUT

V+

4

3

5

14

LEVEL

DETECTOR

VCA

CONTROL

8

1

12

6

7

2

13

9

11

10

SSM2166

POWER DOWN

GND

AVG CAP

COMP RATIO SET

GAIN

ADJUST

OUTPUT

NOISE GATE

SET

ROTATION

SET

–IN

AUDIO

+IN

0.1µF

R2

10kΩ

1µF

+

R1

10kΩ

10µF

10µF (OPTIONAL)

++

+

25kΩ

22µF

500kΩ

2.3kΩ

17kΩ

V+

Figure 2.