UREI LA-4 Optical Limiter
2000, 2001, 2002 & 2003 by Eddie Ciletti
The RETRO audio craze has turned everything
old into gold. But is this old stuff really so cool? Did we
somehow forget how to make great sounding gear? What's the difference
between cool old gear and stuff that needs a technology transfusion?
The answer to these questions can be
found in the simple investigative process of troubleshooting no matter
whether the goal is a repair or an upgrade in this instance, by using
a stock UREI LA-4 as an example. The old fashioned optical limiter
is nearly foolproof, the added RATIO control makes the LA-4 more versatile
than its predecessors the Teletronics LA-2A and LA-3A without the added
confusion of Attack and Release controls introduced on "modern" optical
limiters. Remember, part of the beauty of OPTOs in general and the
LA-2 / LA-3 / LA-4 in particular is their simplicity.
Unlike its respected Tube and discrete
Transistor ancestors, the LA-4 uses an early quad opamp called the RC-4136.
because, as the input level increases the LA-4
becomes quite literally dark sounding. .In geek speak, this is called "slew
rate (speed) limiting". With vintage gear, the technician always
has an option to "just" fix stuff and leave whatever creates "character"
as part of the box OR take the inquisitive approach, "what makes this box
work?" Where's the magic and where's the funk? Can I make it
It all started out when a minor repair
to a UREI 1176 turned into a "subtle improvement tweak." This intrigued
the customer enough to bring over a pair of LA-4s (of which he has eight)
for similar treatment. To backtrack, I have this technique of using
a square wave oscillator to find bad capacitors, the type of component
failure that causes loss of low frequencies. (Read this link: Hunt
for Bad Capacitors to learn more.) I used the same technique on
the 1176, the added benifit fo the square wave technique is that it also
reveals high frequency anomalies like transformer ringing. In this
case, I found an Impedance Sensitive part of a circuit, realized that one
option was to use a jigher grade of shielded cable so as to avoid attenuating
The same approach was applied the LA-4,
tracing the signal from each stage, watching it slow down considerably
after passing through four op-amps. Figure-1 shows the input stage
of the LA-4. Via two amplifiers, U1 and U2, the balanced (differential)
input stage eliminates the need for (and the sound of) an input transformer.
U3 can be switched for either normal (unity) or high (30dB) gain.
It is then followed by a "build-out" resistor R13 that when combined
with the photo-resistor, creates a voltage divider / gain manipulator.
R13 is a rather high value 82kW
large enough so that any "stray" capacitance (from the opto, U4s input
and /or the socket and the circuit board layout) also slows the rise-time
of the square wave. I am not bandwidth obsessed, its just a troubleshooting