LA-3A Audio Leveler
It might not look like it but there’s a mini discotheque going on inside these units.
Following on from my last post about the UREI 1176 peak limiter here we have the LA-3A Limiter.
In the 50s and 60s Bill Putnam’s manufacturing arm of his recording studio business United Recording Corporation were buying up small electronics manufacturing companies to help him build recording equipment for his studios.
In 1967, one of these acquisitions was the broadcast division of Babcock Electronics which included Teletronix and the patent rights to the electro-optical LA-2A leveling amplifier.
With the LA-2A, Teletronix used an unconventional and innovative solution to the challenge of levelling audio signals.
Leaning on a background in aerospace (he worked at Cal Tech’s Jet Propulsion Laboratory), designer Jim Lawrence repurposed the #electroluminescent technology that was being used in the newest aircraft instrument displays at the time.
Known these days simply as an optical compressor, Lawrence paired an electroluminescent panel with a photoresistor to create an optical photocell (in this instance the T4). The LA-2A would use the audio signal to make light with the EL panel, and the photoresistor would sense how much illumination was occurring (and for how long). The more light detected by the photoresistor, the more reduction in gain is caused by the unit.
The attack and release times of the gain reduction is therefor wholly dependent on the time it takes for the EL panel to light up and then to die away again with the rise and fall of current, which in both cases is not an instant occurrence.
Furthermore, the longer and more-intensely the panel is lit, the longer it takes for the residual light to die away. This means that release time is lengthened even further after long, sustained periods of gain reduction.
This seeming lack of versatility is the characteristic that gives optical compressors their musical appeal.
The attack and release times cannot be set by the user but by the time it takes for the light to rise and fade away again, the result being a very desirable auto-attack and release which sounds great in the studio.
The units in the pictures are reproductions of the solid-state LA-3A which has a few differences to its predecessor the LA-2A, the main one being that the LA-2A uses valves in its amplifier stages but there are other differences to the circuit which means the photocell in the LA-3A lights faster and stronger resulting in a slightly faster attack time.
The differences between the two are more noticeable on large, sudden peaks but gentler levelling and release characteristics are quite similar to the LA-2A.
The LA-3A lends itself to use on electric guitars more than the LA-2A but also sounds great on many sources from bass to drums to vocals to pianos it has a full, robust and musical sound.
