SSM2165(1999) Просмотр технического описания (PDF) - Analog Devices

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производитель

SSM2165
(Rev.:1999)

Microphone Preamplifier with Variable Compression and Noise Gating

Analog Devices 

SSM2165

C2

10␮F

+5V

+

GENERATOR

AND AC

VOLTMETER

C1

0.1␮F

V+ BUFOUT

+1

BUFFER

VCAIN

VCA

AC VOLTMETER

AND OSCILLOSCOPE

+2V

2k⍀

LEVEL

DETECTOR

SSM2165-1

CONTROL

HEADPHONES

+ MICROPHONE

(ELECTRET)

GND

AVG CAP

C3 +

22␮F

200k⍀

1:1

R1 – COMPRESSION

15:1 RATIO SET

CW

Figure 8. Electret Microphone Preamp Example

Compression Adjustment—A Practical Example

To illustrate how to set the compression ratio of the SSM2165,

we will take a practical example. The SSM2165 will be used

interface an electret-type microphone to a post-amplifier, as

shown in Figure 8. The signal from the microphone was mea-

sured under actual conditions to vary from 2 mV to 30 mV.

The post-amplifier requires no more than 350 mV at its input.

We will, therefore, choose the SSM2165-1, whose “rotation”

point is 40 mV and whose VCA fixed gain is 18 dB (×8), thus

giving 320 mV at limiting. From prior listening experience, we

will use a 2:1 compression ratio. The noise gate threshold of the

SSM2165-1 will operate when the input signal falls below 500 µV.

These objectives are summarized in Table II. The transfer charac-

teristic we will implement is illustrated in Figure 10.

Table II. Objective Specification of Example

Input Range

Output Range

Limiting Level

Compression

Gain

Noise Gate

2 mV–30 mV

To 350 mV

320 mV

2:1

18 dB

500 µV

Test Equipment Setup

The recommended equipment and configuration is shown in

Figure 9. A low noise audio generator with a smooth output

adjustment range of 100 µV to 25 mV is a suitable signal

source. The output voltmeter should go up to 2 V. The oscillo-

scope is used to verify that the output is sinusoidal, that no

clipping is occurring in the buffer, and to observe the limiting

and noise gating “knees.”

Breadboard Considerations

When building a breadboard, keep the leads to Pins 2 and 3 as

short as possible. Use a central analog ground and decouple

power supply connections adequately.

SIGNAL

GENERATOR

SSM2165-1

AC

VOLTMETER

AC

VOLTMETER

OSCILLOSCOPE

Figure 9. Test Equipment Setup

STEP 1. Initialize Potentiometer

With power off, preset R1—Compression Ratio potentiometer

to zero ohms.

STEP 2. Check Setup

With power on, adjust the generator for an input level of

50 mV (–24 dBu), 1 kHz. The output meter should indicate

approximately 350 mV (–6.9 dBu). If not, check the setup.

STEP 3. Find the Rotation Point

Set the input level to 50 mV (–24 dBu), and observe the output

on the oscilloscope. The output will be in the limiting range of

operation. Slowly reduce the input signal level until the output

level just begins to stop limiting and follows the input down.

Increase the input so that the output is 320 mV (–7.7 dBu).

This is the knee of the rotation point.

STEP 4. Adjust the Compression Ratio

With the input set as in Step 3, note the exact value of the input

signal level just below the knee (around 40 mV (–26 dBu)). Next,

reduce the input to 1/4 the value noted, (around 10 mV (–38

dBu)), for a change of –12 dB. Next, increase the RCOMP potenti-

ometer resistance so the output is 160 mV (–13.7 dBu) for an

output change of –6 dB. You have now set the compression,

which is the ratio of input change to output change, in dB, to 2:1.

STEP 5. Confirm the Noise Gate Threshold

Set the input to 1 mV, and observe the output on the oscillo-

scope. A 20 dB pad between generator and input may facilitate

this measurement. Reduce the input gradually until the output

falls off more rapidly. This point is the noise gate threshold, and

should be approximately 500 µV (–64 dBu). The noise gate

threshold on the SSM2165 is fixed at 500 µV, a practical value

for many microphones. Should a different noise gate threshold

be required, consider using the SSM2166.

–8–

REV. B

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