UREI LA-4 Optical
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
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
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.
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.