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1176 compressor9/2/2023 ![]() ![]() Let’s look inside the T4 optical attenuator, which is the heart of the LA-2A. The fact that it offers 40 dB of gain from input to output allows it to also be used as a preamp and compressor. ![]() 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. 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. If the signal is below the threshold, then the release will be faster. If compression is heavy or the signal is above the threshold for a long duration, then the release will be slower. This timing of the release depends heavily on the length and strength of the incoming signal. 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%. The average attack time is fixed at 10 milliseconds, but its unique release characteristic is what gives the LA-2A its definitive sound. 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. It responds especially well to the human voice in a way that inspires performance.” “It’s inspiring to sing through psychologically. “It treats your signal so lovingly,” says Bill Putnam Jr., CEO of Universal Audio. Why has the LA-2A remained so popular for recording engineers of all stripes, even fifty years after its original design? It’s all about the slow, gentle, and versatile multi-stage release time. 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. 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. Lawrence drew from his background with military optical sensors to design a circuit which would “level” the incoming audio signal. His revolutionary idea was to create the world’s first leveling amplifier utilizing optical sensors. From that frustration was born the idea of a “leveling amplifier.” 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. Jim Lawrence was born in 1924 and served in the Navy during WWII as a radar operator, studied electrical engineering and worked at JPL in his younger years, but he was also able to apply his skills to his true passion - radio and broadcasting. ![]() The Birth of the Optical Limiting Amplifier Little did he know at the time that his work on this Cold War-era project would spawn the Teletronix® LA-2A Leveling Amplifier - one of the most iconic compressors in history. In the early 1950s, Jim Lawrence, a young University of Southern California-degreed electrical engineer, was quietly asked to join Cal Tech’s Jet Propulsion Laboratory (JPL), where he was assigned the task of developing optical sensors for the Titan Missile Program. ![]()
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