User`s guide

will 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.

Figure 4 - Using a FET as a voltage-variable resistor. The combination of

R5 and Q1 acts as a voltage divider which controls the gain.

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 non-

linear 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

Output Amplifier

The output amplifier is a Darlington pair followed by a class A stage based on a 2N3053

transistor. The 1176 output stage was essentially the same as the Universal Audio 1108

pre-amplifier.

The output transformer is a custom transformer designed by Bill Putnam Sr. Aside from

offering output impedance matching, the transformer forms an integral part of the

feedback network used to stabilize the output stage. Note that later revisions (‘F’ and

beyond) used a push-pull (class AB) output stage.

Revision E was essentially the same as revision D. Revision E added 220 Volt operation as well as a

10M

Ω

resistor across the ratio switch to avoid ‘pops’ while changing between compression ratios.

Signal

Out

GR Control

Voltage

Signal

Out

27k