User Manual

Arrow 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.