REV. A

SSM2165

–8–

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 16. 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 18.

Table II. Objective Specification of Example

Input Range

2 mV–30 mV

Output Range

To 350 mV

Limiting Level

320 mV

Compression

2:1

Gain

18 dB

Noise Gate

500

µV

Test Equipment Setup

The recommended equipment and configuration is shown in

Figure 17. 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 volts. The

oscilloscope is used to verify that the output is sinusoidal, that

no clipping is occurring in the buffer, and to observe the limit-

ing and noise gating “knees.”

Breadboard Considerations

When building your 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

OSCILLOSCOPE

AC

VOLTMETER

Figure 17. Test Equipment Setup

+1

+5V

V+

+

C2

10 F

BUFFER

LEVEL

DETECTOR

CONTROL

SSM2165-1

C1

0.1 F

GENERATOR

AND AC

VOLTMETER

200k

C3

22 F

AVG CAP

1:1

R1 – COMPRESSION

15:1 RATIO SET

GND

+2V

2k

MICROPHONE

(ELECTRET)

+

CW

HEADPHONES

AC VOLTMETER

AND OSCILLOSCOPE

+

VCA

Figure 16. Electret Microphone Preamp Example

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 your 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). You

have located 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

potentiometer resistance so the output is 160 mV (–13.7 dBu)

for an output change of –6 dB. You have now set the compres-

sion, 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 you require a different noise gate

threshold, consider using the SSM2166.

STEP 6. Listen

At this time, you may replace the signal generator with a

properly powered electret microphone and listen to the results

through a set of headphones. The microphone’s internal FET

usually requires around +2 V through a 2 k

Ω resistor; this varies

with the manufacturer. Experiment with the compression ratio

value and averaging capacitor size. More compression will keep

the output steady over a wider range of microphone-to-source