UREI LA-4 Optical Limiter Overview
ã 2000, 2001, 2002 & 2003 by Eddie Ciletti

I have always loved optical Limiters especially the classics — Teletronics’ vacuum-tube LA-2A and its "solid-state" sibling, the Universal Audio LA-3A. Both were minimalist in terms of user interface as well as the relatively non-destructive way they processed sound. With just two knobs — Input Level and Gain Reduction — and no Attack or Release controls, it’s hard to make a vintage Optical Limiter sound bad. 

Much of the secret to their simplicity and success is the optical transmitter and receiver — an Electro-Luminescent (EL) Panel as light source and Photo-Resistor as attenuator.  Unlike VCA, FET and Variable-MU designs, the EL panel is directly driven by an audio amplifier requiring no conversion from AC to DC with a PLUS of having a nearly instantaneous Attack Time. 

It is the Photo-Resistor that primarily determines the Attack and Release characteristic.  People think of Optical Dynamics processors as slow, but that is too simple an explanation.  You can think of them as quasi-intelligent because there is a memory effect combined with a "non-linear" release characteristic.  With just a little bit of Gain Reduction, the Attack and Release are fast.  Dig in a little deeper and the recovery time is quick at first, then slows down as if waiting for the next round of Gain Reduction commands.  Most users agree that optical is perfect for Vocals, Bass and Guitars. 

The large mechanical VU Meter is another endearing feature of all vintage processors. In many ways, it's response characterisitcs are similar to that of the photo-resistor.  "Similar," in that the meter is just enough faster than the opto to failry accurately reflect its response characteristic.  Most modern optical processor designs choose an extremely fast opto that then requires Attack and Release controls to tame it.  In addition, when in the "peppy response mode," the VU meter can no longer accurately display the actual processing being done.

Click here for more info about Dynamics Processors.

The only "problem" with great vintage gear is that the prices are out of this world. The new-retro versions are equally out of reach, though it’s easy to understand the prices — power and audio transformers are not cheap and much of the stuff is hand assembled. Even the LA-4, the bastard step-child of the family, is a bit pricey. Street price should be $400 to $600 tops but people are asking, and getting, more. What makes a stock LA-4 funky is that it uses an early quad opamp called the RC-4136. This op-amp sucks because, as the input level increases, it slows down, making the LA-4 quite literally dark sounding. 

Click here to read about the investigative process that led to the upgrade.