SSM2166

REV. A

–13–

STEP 5. Adjusting the Rotation Point

Set the input level to 15 mV, and observe the output on the os-

cilloscope. Adjust R3—ROTATION PT ADJ CW until the

output level just begins to drop, then reverse so that the output is

500 mV. You have now set the limiting to 500 mV.

STEP 6. Adjusting the Compression Ratio

Set the input signal for an output of 500 mV but not in limiting.

Note the value (around 15 mV). Next, reduce the input to 1/10

the value noted, (around 1.5 mV), for a change of –20 dB. Next,

adjust R6—COMP RATIO CW until the output is 160 mV, for

an output change of –10 dB. You have now set the compression,

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

STEP 7. Setting the Noise Gate

With the input set at 100

µV, observe the output on the oscillo-

scope, and adjust R7—ROT PT SET CCW until the output

drops rapidly. “Rock” the control back and forth to find the

“knee.” You have set the noise gate to 100

µV. The range of

the noise gate is from 0.3 mV to over 0.5 mV relative to the out-

put of the buffer. To fit this range to your application, you may

have to attenuate the input or apportion the buffer gain and VCA

gain differently.

STEP 8. Listening

At this time, you may want to connect an electret microphone to

the SSM2166, and listen to the results. Be sure to include the

proper power for the microphone’s internal FET (usually +2 V

to +5 V dc through a 2.2 k

Ω resistor). Experiment with the

settings to hear how the results change. Varying the averaging

capacitor, C4, changes the attack and decay times, which are

best determined empirically. Compression ratio will keep the

output steady over a range of microphone to speaker distance,

and the noise gate will keep the background sounds subdued.

STEP 9. Record Values

With the power removed from the test fixture, measure and

record the values of all potentiometers, including any fixed resis-

tance in series with them. If you have changed the averaging

capacitor, C4, note its value too.

SUMMARY

We have implemented the transfer condition of Figure 2. For

inputs below the 100

µV noise gate threshold, circuit and back-

ground noise will be minimized. Above it, the output will in-

crease at a rate of 1 dB for each 2 dB input increase, until the

500 mV rotation point is reached at an input of approximately

15 mV. For higher inputs that would drive the output beyond

500 mV, limiting will occur, and there will be little further in-

crease. The SSM2166 processes the output of the buffer, which

in our example is 20 dB or ten times the input level. Use the os-

cilloscope to ensure that you are not driving the buffer into clip-

ping with the highest expected input peaks. Always take the

minimum gain in the buffer consistent with the average source

level and crest factor (ratio of peak to rms). The wide program

range of the SSM2166 makes it useful in many applications

other than microphone signal conditioning.

Other Versions

The SSM2165 is an 8-lead version of this microphone preamp

with unity buffer gain and preset noise gate threshold. Custom-

ized parts are available for large volume users. For further in-

formation, contact your sales representative.