User Manual

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 1176LN, 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 FETs resistance determines the gain of this section. Resistor R5 and the FET

essentially comprise a voltage divider circuit. The lower the FETs 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 1176LN 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 D1.