Tletronix History

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A History of the Teletronix LA-2A Leveling Amplifier

by Lynn Fuston

Three generations of Teletronix LevelingAmplifiers: LA-2A Gray,

LA-2A Silver, and original LA-2.

In the early 1950s, Jim Lawrence, a young University of Southern California-degreed electrical engineer,

was quietly asked to join Cal Techs Jet Propulsion Laboratory (JPL), where he was assigned the task of

developing optical sensors for the Titan Missile Program. Little did he know at the time that his work on this

Cold War-era project would spawn theTeletronix LA-2A Leveling Amplifierone of the most iconic

compressors in history.

The Birth of the Optical Limiting Amplifier

Jim Lawrence was born in 1924 and served in the Navy during WWII as a radar operator, studied electricalengineering and worked at JPL in his younger years, but he was also able to apply his skills to his true

passion radio and broadcasting. During a stint at KMGM in Los Angeles, he became frustrated with

having to ride the gain to maintain a constant signal level on the air. From that frustration was born the

idea of a leveling amplifier.

His revolutionary idea was to create the worlds first leveling amplifier utilizing optical sensors. Lawrence

drew from his background with military optical sensors to design a circuit which would level the incoming

audio signal. He combined a luminescent panel with photo resistors (whose impedance changes relative to

light intensity) and sealed them in a vacuum-tube-sized metal canister. This optical attenuator, known as

the T4, is what gives the LA-2A its gentle, program dependent optical compression, revered to this day by

audio professionals worldwide.
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The Sound of The LA-2A

Brochure for the Putnam-eraLA-2A Leveling Amplifier.

Why has the LA-2A remained so popular for recording engineers of all stripes, even fifty years after its

original design? Its all about the slow, gentle, and versatile multi-stage release time. It treats your signal

so lovingly, says Bill Putnam Jr., CEO of Universal Audio. Its inspiring to sing through psychologically. It

responds especially well to the human voice in a way that inspires performance.

The T4 optical attenuator in the LA-2A is both program and frequency dependent, allowing the

compression ratio to vary greatly based on the source material. The average attack time is fixed at 10

milliseconds, but its unique release characteristic is what gives the LA-2A its definitive sound. While the

initial release time is about 60 milliseconds for 50% of the release, the following 50% happens gradually

over a period of one to 15 seconds before the signal comes back to 100%.

This timing of the release depends heavily on the length and strength of the incoming signal. If

compression is heavy or the signal is above the threshold for a long duration, then the release will be

slower. If the signal is below the threshold, then the release will be faster. Users may notice that it takes

quite some time for the meter to return to 0 VU (no compression) after a long period of heavy

compression.

How The LA-2A Works

The LA-2A Leveling Amplifier has only a single gain stage and is one of the simplest compressors to

operate with two knobs: Gain and Peak Reduction. The fact that it offers 40 dB of gain from input to output

allows it to also be used as a preamp and compressor.
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The inside of a T4A optical attenuator. Note the

electroluminescent panel on the top.

Lets look inside the T4 optical attenuator, which is the heart of the LA-2A. The T4 cells primary

components include an electro-luminescent panel and dual cadmium-sulfide photo resistors, one of whichcontrols gain reduction, and the other the VU meter in Gain Reduction mode. How does it work? As the

input signal increases, the light panel gets brighter. As the light gets brighter, the photo resistors

impedance increases which reduces the gain. As the input level decreases, the light dims and the

resistors impedance decreases. Whether by serendipity or intent, Jim Lawrences T4 design had a

musical response that allowed the LA-2A the sonic and technological longevity it still retains.

The heart and soul of the LA-2A is in the T4 cell, explains Bill Putnam Jr. All of the characteristics are

right there. The photocell and the physics behind it are what immediately result in the two-stage release.

Though the T4 cell makes the LA-2A shine, the overall transformer and tube amplifier design of the LA-2A

also plays a huge part in its longevity and popularity. Though there have been reissues of the LA-2A, verylittle has changed from the original design, which is a testament to its reliability and linear response.

Evolution and Revisions

In 1958, Jim Lawrence founded the Teletronix Engineering Company in Pasadena, California,

manufacturing broadcast products such as transmitter tubes, emergency tone generators, and full-scale

radio transmitters. His first leveling amplifier was the Teletronix LA-1, of which only about one hundred

units were ever made. One got into the hands of the legendary singing cowboy Gene Autry, who loved it

and used it for his radio program and recording sessions. Autrys fondness for the LA-1 was instrumental

in its gaining acceptance.

Jim Lawrence's first limiting amplifier was the LA-1, ofwhich about only a hundred were made.

The LA-2 was the next iteration, featuring a larger VU meter than the LA-1, and the addition of the now

legendary T4A optical attenuator photocell. A variety of cosmetic changes were made as well including

changing the metering switch from a multiposition toggle switch to a rotary switch. Nowadays, this

predecessor to the LA-2A is extremely rare and hard to find, but is still revered for its own unique, slow,

time constant.


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An engineer by the name of Sid Feldman purchased an early LA-2 and showed it to the engineer on the

hugely influential and popular Ed Sullivan Show. That introduction lead CBS to buy a pair of LA-2s and

shortly after, RCA purchased LA-2s for its New York and Nashville operations. Soon these broadcast

compressors began showing up in recording studios around the country.

In 1962, Lawrence reconfigured the LA-2 into the first LA-2A, sporting a gray faceplate and a change tothe messy layout of the units interior wiring. For the LA-2A, turret boards were placed in a long strip at the

bottom of the chassis, helping tidy up the wiring inside the unit. This shorter, cleaner wiring enabled the

LA-2A to have a lower noise floor than its predecessor.

In 1965, Jim Lawrence sold Teletronix to Babcock Electronics of Costa Mesa, California. In 1967, Bill

Putnams company, Studio Electronics (eventually renamed UREI), picked up Babcocks broadcast

division, including the Teletronix brand. They continued manufacturing the LA-2A, swapping the original

gray faceplate with a silver one.

As the development of solid-state technology matured, UREI launched more limiting amplifiers with the

patented T4 optical attenuator, but without the space-consuming tube-based circuitry. As products like the

LA-3A, LA-4, and LA-5 gained popularity, sales of the LA-2A slowed until it was discontinued in 1969.

Turret boards were added to the LA-2A design, resulting incleaner wiring and a lower noise floor than the LA-2.

Its hard to imagine people throwing away vintage tube compressors backthen, but tubes were old

technology and solid state was all the rage. That is, until engineers began to miss the slow, unique

compression of the LA-2A and prices for used units started to climb. This prompted two limited edition

reissues one in the 1970s, and one in the 1980s and then a full resurrection of the LA-2A in 1999 by

Universal Audio.

The Rebirth of the LA-2A

When Bill Putnam Jr. refounded Universal Audio in 1999, reissuing the LA-2A once produced by his father

was at the forefront of this mind. But, it wasnt an easy task. Replicating the crucial T4 optical attenuator

for a faithful LA-2A reproduction involved a lengthy study of the original photocell formula including

working with both modern device physicists and the developers of the original 60s-era photocells.

The photocell is really hard to get right, recalls Bill Putnam, Jr. For our reproduction, we had a

challenging time developing that part. It required working with the original manufacturer to perfect it, to dial

it in to the specifications of the original photocells from the 1960s. The optical sensor T4 variant we worked

to develop is customized for us. No one else has it.


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For manufacturing, Universal Audio located the special equipment originally used to manufacture the T4

back in the '60s, and had to requalify the manufacturer to make sure it could meet UAs special production

needs.

To say it was hard is an understatement, says Universal Audio Product Manager Will Shanks, We were

asking a manufacturer to revert back to manufacturing methods that were archaic and a step backwards inthe technology.

Though Universal Audio spent a long time perfecting the T4 for their hardware LA-2A reissue, its unique

design wasnt fully understood until UA began the research to model the LA-2A for theUAD Powered

Plug-Insplatform in 2001.

Modern photocells are designed to be as fastas possible, but they don't have the right multi-stage

response needed to sound like a Teletronix design, explains Shanks. Our DSP research helped us

understand how the original T4 worked at the quantum physics level. This not only allowed us to develop

an accurate DSP model of the gain reduction behavior, it also helped us make our hardware T4 more

consistent.

Shortly after Universal Audio was refounded in 1999, theLA-2A was reissued with a design true to the original.

Whether hardware or DSP, it is this special qualified manufacturing process and recipe UA re-established

that gives the LA-2A its unique, musical sonic quality to this day.

I was very curious about the future of the LA-2A, so I asked Bill Putnam Jr. if the advent of plug-ins has

adversely affected the sales of the Universal LA-2A, since now people can easily put a digital LA-2A on

every single track and the emulations are quite similar to the originals.

Actually, were selling more of the hardware LA-2As now than we ever have, Putnam replied. The fact

that demand is increasing even as the design recently passed the 50-year mark is a testament to the fact

that the LA-2A truly is a studio legend.
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Teletronix LA-2A Leveling Amplifier

INTRODUCTION

The Teletronix Leveling Amplifier will automatically reduce audio peaks which might

otherwise over drivebroadcast or recording equipment.

Automatic gain reduction is accomplished by the use of an electro-optical variable

attenuator, which isplaced ahead of the first amplifier stage. The attenuation is

controlled by the amplitude of the LA-2A inputsignal.

This system permits up to 40 DB of instantaneous gain reduction, yet causes no

wave form or harmonic distortion. The amplifier provides sufficient gain and output

level (10 DBM nominal) to be used as a line or program amplifier, or for direct

connection to the transmitter in the case of radio and TV operation. Provisions are

made for interconnection of the optical attenuators to provide equal gain reduction

in both channels when two of the LA-2A Leveling Amplifiers are used for FM stereo

broadcasting.

CIRCUIT DESCRIPTION

The LA-2A Leveling Amplifier will produce essentially instantaneous gain reduction

of over 40 DB with no increase in harmonic distortion.

Compressor action occurs from the breakaway point at -30 DB input and up to -20

DB, at which point the curve becomes horizontal to exhibit limiting action. The inputincreases an additional 20 DB, but the output increases less than 1 DB. Theleveling

amplifier thus combines the the characteristics of a compressor and a limiter. A

reasonable amount of care in gain riding will restrict normal operation to the

compression region, but uncontrolled output levels will be prevented by the limited

action.

The heart of the leveling amplifier is the electro-optical attenuator which is placed

ahead of the first amplifier stage. The actual stage gains and and tube operating

parameters are not varied, permitting the tubes to operate at optimum conditions

regardless of the amount of gain reduction. The optical attenuator consists of a

photo-conductive cell, which is optically coupled to an electroluminescent light

source. The electro-luminescent device provides a light intensity which isproportional to the audio voltage applied to its terminals.

Not unlike a capacitor in construction, the electro-luminescent lamp consists of a

plate of glass or plastic coated with a clear conducting material on one side and a

thin layer of phosphor on the other side. A metallic plate contacts the phosphor

coating. As alternating current is applied to the conducting plates thephosphors are

excited by the voltage across the dielectric and light is produced. The amount of

light depends upon the applied voltage and frequency. The gain or level controlling

element is the photo-conductive cell. The resistance of the cell decreases with an

increase in the impinging light. Since the light is produced directly from the audio

voltage, the response is instantaneous. Rectification and filtering of the audiotoproduce a control signal are not necessary as in the case of conventional limiters.


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This system results in automatic level control whose speed of operation is limited

only by the response of the variable resistance photo cell used.

A cell is selected which provides a minimum attack time, and a release time which

requires about 60 milliseconds for 50% release, and then a gradual release over a

period of 1 to 15 seconds to the point of complete release.

The input signal is applied directly to the optical attenuator from the high

impedance winding of the input transformer. The amount of introduced by the

optical attenuator is controlled by the audio voltage applied to the 6AQ5 which is

the luminescent driver amplifier. The amount of signal applied to the 12AX7A

voltage amplifier is also controlled by the manual gain control. The voltage amplifier

stage provides a gain of 40 DB. Overall amplifier feedback of approximately 20 DB

provides low distortion, flat response, and gain stability.

The output stage is somewhat unconventional in that a totem pole or double

cathode follower is used. This output stage can tolerate great amounts of output

impedance mismatch, but retains low distortion and flat frequency response.

For stereo broadcasting applications, a portion of the input signal is fed through the

gain reduction control to the 12AX7A control amplifier. The output at this stage is

applied to the stereo balance control and is brought out out to a terminal on the

chassis. For stereo operation, this terminal is connected to the same terminal on an

identical amplifier and control voltage becomes common to both units. A gain-

reduction control voltage in either amplifier will cause equal gain reduction in both

units. The control voltage is applied to the stereo balance control to the 6AQ5

driver amplifier. This stage provides the necessary voltage to operate the electro-

luminescent light source.

Foundational text courtesy of Teletronix Operating Manual and associated literature.

Pair of Teletronix LA-2A Leveling Amplifiers.


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Skip Juried, in Session, with four Teletronix LA-2A Leveling Amplifiers at Sound

Exchange Studios, NY. Photo Courtesy of Chris Juried


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