The Technical Stuff
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29
1176LN Circuit Details
The fundamental problem facing Bill Putnam Sr. when he began designing the 1176 limiter was how
to keep its FET operating within its linear region in order to keep distortion sufficiently low. After much
experimentation he eventually hit upon the simple and elegant idea of using the FET as a voltage-
controlled variable resistor, forming the bottom half of a voltage divider circuit, across which the audio
signal was applied. He then placed his voltage-controlled attenuator ahead of a solid-state
preamplifier stage and line driver, and derived its control voltage from a relatively conventional
level-sensing circuit monitoring the output.
The output stage of the 1176 was a carefully crafted class A line level amplifier, designed to work with
the then standard load of 600 ohms. The heart of this stage is the custom output transformer
developed by Putnam; its design and performance is critical to the sound of the device. This
transformer was distinguished by the fact that it used several additional sets of windings to provide
feedback (a practice widely used in the tube amplifiers of the era), which made it an integral
component in the operation of the output amplifier. Putnam spent a great deal of time perfecting the
design of this tricky transformer and carefully qualified the few vendors capable of producing it.
Figure 1 - Block Diagram of the 1176LN
Figure 1 shows a block diagram of the 1176LN. Signal limiting and compression is performed by the
gain reduction section. Before the signal is applied to the gain reduction section, the audio signal is
attenuated by the input stage. The amount of attenuation is controlled by the Input control
potentiometer. The amount of gain reduction as well as the compressor attack and release times are
controlled by gain reduction control circuit. After gain reduction, a pre-amp is used to increase the
signal level. The Output control potentiometer is then used to control the amount of drive that is
applied to the output amplifier.
Let’s take a closer look at each stage within the 1176LN circuit.
