Universal Audio LA-3A - Manual (Page 21)

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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. It was Bill Putnam himself who, in 1966, was respon-
sible 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 a voltage applied to the gate controls the
resistance between the drain and the source terminals. 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, de-
signed to work with the (then) standard load of 600Ω. 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 600Ω.
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 produc-
ing 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, result-
ing in at least 13 revisions of the 1176. The D and E ‘black-face’ LN revisions are widely considered to be the
best-sounding models; therefore Universal Audio modeled our reissue after these two models.
The LA-2A
The LA-2A leveling amplifier, a tube unit with hand wired components and three simple controls, was in-
troduced in the early 1960s. It utilized a system of electro-luminescent optical gain control that was quite
revolutionary; gain reduction was controlled by applying the audio voltage to a luminescent driver amplifier,
with a second matched photoconductive cell used to control the metering section. With its 0 to 40 dB of gain
limiting, flat frequency response of 0.1 dB from 30-15,000 Hz and a low noise level (better than 70 dB below
plus 10 dBm output,) the LA-2A quickly became a studio standard. Originally patented by Jim Lawrence, it was
produced by Teletronix in Pasadena, California, which became a division of Babcock Electronics Corp. in 1965.
In 1967 Babcock’s broadcast division was acquired by Bill Putnam ‘s company, Studio Electronics Corporation
shortly before he changed the company’s name to UREI(r). Three different versions of the LA-2A were produced
under the auspices of these different companies before production was discontinued around 1969.