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Microphone Preamplifier with

Variable Compression and Noise Gating

*Patents pending.

FEATURES

Complete Microphone Conditioner in a 14-Lead 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)

Low Cost

APPLICATIONS

Microphone Preamplifier/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 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 opti-

mized signal levels prior to digitization, thereby eliminating the

need for additional gain or attenuation in the digital domain

that could 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, such as the AD1845 and AD1847.

The device is available in a 14-lead SOIC package, and is guar-

anteed for operation over the extended industrial temperature

range of –40

°C to +85°C. For similar features and performance

in an 8-lead package, please refer to the SSM2165.

INPUT – dBu

–70

0

–60

–50

–40

–30

–20

–10

10

0

–30

–40

–50

–60

–10

–20

RATIO = 10:1

RATIO = 2:1

RATIO = 1:1

Figure 1. SSM2166 Compression and Gating Char-

acteristics with 10 dB of Fixed Gain (The Gain Adjust

Pin Can Be Used to Vary This Fixed Gain Amount)

CONTROL

AUDIO

+IN

0.1 F

6

BUFOUT

53

10 F

*

13

OUTPUT

9

NOISE GATE

SET

V+

11

ROTATION

POINT SET

10

8

1

GND

22 F

COMPRESSION

RATIO

SET

25k

2

2.3k

14

500k

17k

1k

7

LEVEL

DETECTOR

BUFFER

VCA

1k

SSM2166

4

10 F

R1 = 10k

R2 = 10k

1 F

+

12

POWER

DOWN

AVG

CAP

+

++

V+

VCA GAIN

ADJ

–IN

Figure 2. Functional Block Diagram and Typical Speech Application