REV. B

SSM2166

–13–

STEP 5. Adjusting the Rotation Point

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

oscilloscope. Adjust R3—ROTATION PT ADJ CW until the

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

is 500 mV. The limiting has now been set 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. The compression, which is the ratio

of output change to input change, in dB, has now been set 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.” The noise gate has now been set to 100

µV. The range

of the noise gate is from 0.3 mV to over 0.5 mV relative to the

output of the buffer. To fit this range to the application, it may

be necessary to attenuate the input or apportion the buffer gain

and VCA gain differently.

STEP 8. Listening

At this time, it may be desirable to connect an electret micro-

phone 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). Experi-

ment 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. The 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. Recording Values

With the power removed from the test fixture, measure and

record the values of all potentiometers, including any fixed

resistance in series with them. If the averaging capacitor, C4,

has been changed, 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

background noise will be minimized. Above it, the output will

increase 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

increase. The SSM2166 processes the output of the buffer,

which in our example is 20 dB, or 10 times the input level. Use

the oscilloscope to ensure that the buffer is not being driven into

clipping 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. Customized

parts are available for large volume users. For further informa-

tion, contact an ADI sales representative.