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
µ
V, 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
µ
V. 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 (u2 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
µ
V 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.
