Teletronix LA-2A Leveling Amplifier
It’s not every day we can say this, but our story begins with the Titan Missile Program. Invented by electrical engineer Jim Lawrence, the LA-2A derives much of its character from its T4 ‘optical attenuator capsule’, the design for which was based on work Lawrence had done for the defence industry at Cal Tech’s Jet Propulsion Laboratory during the 1950s.
The LA-2A was born out of a practical need. Whilst working at a radio station in LA, Lawrence grew weary of riding the gain knobs and began contemplating a means by which the task could be automated. Having designed optical sensors at JPL, it wasn’t beyond him to control the dynamics of an audio signal.
Released under the name Teletronix in 1958, Lawrence’s first leveling amplifier, the LA-1, found its way into the hands of singing cowboy Gene Autry, who spread the word about this marvellous new tool. Around a hundred LA-1s were built before the design was improved with a bigger meter and the now-famous T4 optoattenuator capsule. Next, the LA-2 found favour with entertainment luminaries, ending up in the racks of RCA’s New York and Nashville studios.
In 1962, the LA-2 was quickly superseded by a further improved model offering a new front panel, better wiring and lower noise. This new take, the LA-2A, would become the standard against which all opto compressors would be measured for decades. Soon after, Lawrence sold Teletronix to Babcock Electronics who then sold the brand to Bill Putnam’s Studio Electronics Corporation (now Universal Audio).
The secret to the LA-2A’s famous response comes in large part from that T4 capsule, which consists of an electro-luminescent panel coupled with a pair of photo-sensitive resistors; one for the gain reduction, the other for the meter (when in Gain Reduction mode). The louder the signal passing through, the brighter the electro-luminescent panel. The brighter the panel gets, the higher the photo resistor’s impedance, which in turn reduces the gain.
By design, the T4’s attack time is fixed at 10ms; however, while the initial release time is 60ms during the first half of the release, the rest of the release is gradual and varies from between 1-15 seconds, depending on the length and volume of the incoming signal. Longer, louder signals decay more slowly than weaker signals. This is the signature of the LA-2A and one prized by engineers the world over.
Additionally, the LA-2A can provide up to 40dB of tube-amplified gain from input to output, meaning that it also acts as a preamp as well as a compressor.
With only the merest of controls available, the LA-2A is a doddle to use and virtually incapable of producing a bad sound. Peak Reduction and Gain knobs are used to dial in the amount of processing and signal level, and a switch is provided to select the role of the VU meter, while another toggles between compression and limiting.
The LA-2A would be replaced by a solid-state descendent in 1967 and only built in limited numbers until revived by Universal Audio in 2000. Much as the Minimoog is to synths, the LA-2A is something of a sonic holy grail for compressor designers and developers looking to create software and hardware clones.