Universal Audio 2-1176 Dual 1176LN - Manual (Page 31)

The Technical Stuff
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As shown in Figure 3, gain reduction is achieved by a Field Effect Transistor (FET) which is used as a
variable resistor. In the 1176, the FET acts like a resistor whose resistance is controlled by the voltage
applied to its gate. The higher the voltage applied to the gate, the smaller the drain-source resistance
will be.
Figure 4 - Using an FET as a voltage-variable resistor. The combination of
R5 and Q1 acts as a voltage divider which controls the gain.
Figure 4 shows how the FET’s resistance determines the gain of this section. Resistor R5 and the FET
essentially comprise a voltage divider circuit. The lower the FET’s resistance, the less gain this stage
will have. The FET acts like a variable resistor, where the resistance is determined by the control
voltage that is applied to it. Note that the greater the voltage applied to the gate of the FET, the less
resistance, hence large signals cause the FET to reduce the gain. Larger input signals
result in a
higher voltage from the gain control circuit, which will lower the gain, hence reducing the signal level.
This is the basis of the limiting action. Note that the 1176 is a feedback style compressor since the
sidechain circuit samples the signal level after the gain reduction.
LN Circuitry
The LN circuit, which appeared in revisions ‘C’ and later, was designed to reduce the distortion that
the FET introduced in the gain reduction stage. FETs are inherently nonlinear devices, and any non
linear device will introduce signal distortion. The LN circuitry was designed to ensure that the FET
stayed as much within a linear region as possible, thus reducing unwanted distortions. Much of what
is now known about the operation and design guidelines of FETs was very new at the time the 1176
was designed. Initially, the decision was made to try to keep the ‘LN circuit’ a secret and file for a
patent. In order to accomplish this it was decided to build the ‘LN’ circuit in a separate module. This
module was then attached to the circuit board. The first revision to have the LN circuitry was revision
C. This was accomplished by attaching an ‘LN module’ to the revision B circuit board. This module
turned out to be a problem to manufacture and the decision was made to revise the circuit board to
accommodate the LN circuitry without the module. This then became revision D
1
.