Manual

9
The 1176LN Limiting Amplifier and LA-2A Leveling Amplifier
The LA-2A and 1176 compressor/limiters long ago achieved classic status. They're a given in almost any studio
in the world — relied upon daily by engineers whose styles range from rock to rap, classical to country and
everything in between. With so many newer products on the market to choose from, it's worth looking at the
reasons why these classics remain a necessary part of any professional studio's outboard equipment collection.
The basic concept of a compressor/limiter, is of course, relatively simple. It's a device in which the gain of a
circuit is automatically adjusted using a predetermined ratio that acts in response to the input signal level. A
compressor/limiter "rides gain" like a recording engineer does by hand with the fader of a console: it keeps the
volume up during softer sections and brings it down when the signal gets louder. The dynamic processing that
occurs at ratios below 10 or 12 to one is generally referred to as compression; above that it's known as limiting.
Modern day compressors offer a great degree of programmability and flexibility while older devices such as the
1176 and the LA-2A are more straightforward in their design. Perhaps it is this fact that has contributed to their
appealing sound and the longevity of their popularity.
The 1176LN
The original Universal Audio 1176LN 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. 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. 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. 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. 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 introduced
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®. Three different versions of the LA-2A were produced under
the auspices of these different companies before production was discontinued around 1969.