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Apollo Solo Manual Universal Audio History 261
1176LN
It was Bill Putnam himself who, in 1966, was
responsible for the initial design of the 1176.
Its circuit was rooted in the 1108 preamplifier
which was also designed by Putnam. As is
evident from entries and schematics in his
design notebook, he experimented with the recently developed Field Effect Transistor
(F.E.T.) in various configurations to control the gain reduction in the circuit. He began
using F.E.T.s as voltage variable resistors, in which the resistance between the drain and
the source terminals is controlled by a voltage applied to the gate. His greatest challenge
was to ensure that distortion was minimized by operating the F.E.T.s within a linear
region of operation.
After several unsuccessful attempts at using F.E.T.s in gain reduction circuits, Putnam
settled upon the straightforward approach of using the F.E.T. as the bottom leg in a
voltage divider circuit, which is placed ahead of a preamp stage.
The output stage of the 1176 is 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 output
transformer, whose design and performance is critical. Its primary function is to convert
the unbalanced nature of the 1176 circuit to a balanced line output, and to provide the
proper impedance matching to drive the line impedance of 600 ohms. These two jobs
are accomplished by the primary and secondary windings whose turns’ ratio defines the
impedance ratio.
This transformer is critical due to the fact that it uses several additional sets of windings
to provide feedback, which makes 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.
The first major modification to the 1176 circuit was designed by Brad Plunkett in an
effort to reduce noise--hence the birth of the 1176LN, whose LN stands for low noise.
Numerous design improvements followed, resulting in at least 13 revisions of the 1176.
Legend has it that the D and E blackface revisions sound the most “authentic”.
The original Universal Audio 1176LN designed by Bill Putnam was a major breakthrough
in limiter technology - the first true peak limiter with all transistor circuitry offering
superior performance and a signature sound. Evolved from the popular Universal Audio
175 and 176 vacuum tube limiters, the 1176LN retained the proven qualities of these
industry leaders, and set the standard for all limiters to follow.
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