UREI LA-4 Optical Limiter Expedition
ã 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. This op-amp sucks 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 better? 

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 high frequencies.

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, U4’s input and /or the socket and the circuit board layout) also slows the rise-time of the square wave. I am not bandwidth obsessed, it’s just a troubleshooting technique.

See Figures-2a thru -2d to see the before and after via o'scope.


Figure-1: A simplified schematic of the LA-4’s front end.

Figure-2a: Stock LA-4 Zoomed OUT. 
Note the the vertical transition is nearly invisible 
on the source wave and quite obnoxious when 
the RC-4136 is driven to high levels.
Figure-2b: Stock LA-4 Zoomed IN

Figure-2c: LA-4 with OP275 opamps. 
Note that it is possible to determine the SLEW RATE,
approximately 20 volts per microsecond.
Figure-2d: LA-4 with BB2604 opamps. 
Nearly the same Slew Rate as the OP275, 
confirm with the specs in Table-1.


What would normally be a simple upgrade is, in this case, hampered by the pin configuration — the RC4136 is not compatible with most standard quad opamps, such as the TL074. (Figure-3 details the pin-out variations.) Finding a good opamp with the same low current consumption — 6.8mA — is a challenge that requires the following warning.

NOTE: When considering an opamp upgrade, it is important to know how to monitor IC current consumption of both the original component as well those you will be auditioning. It is not safe to assume that the power supply — for a piece of outboard gear or a console channel strip — will have adequate reserve.  In too many cases the power supply is already operating too close to capacity. See Table-1 for a quick comparison.

Slew rate


1.5 mA per amp

3mA total

13 V/m S

at unity gain



2.5 mA per amp

5mA total

22V/m S


4.5 mA per amp

9mA total

9V/m S


3 mA per amp

6mA total

25V/m S


< 2mA per amp

3.4mA for 2 amps

1.7V/m S

TABLE-1: Opamp Comparisons. 
Note: the current requirements are "per amp," the target being the RC 4136 which consumes less than 2mA per amplifier, less than 8mA total.

Figure-3: Pin-out comparison of the RC4136 and the more typical TL074

Are You Ready To Duel?

Since the RC4136 has no modern pin-compatible quad alternative I decided to use two dual opamps, increasing the options. In order to experiment it was first necessary to create an adapter to re-configure the pins for the dual opamps. After creating a "map" of the here-to-there, a "wire-wrap" IC socket allowed its long legs to be twisted into place.  It's ugly but only temporary, so don't look at Image-4a and -4b too long or too hard.   There’s an easier solution. Please feast your eyes on Image-5a and -5b...

Image-4a: The pin reconfiguration from Standard Dual Opamp to RC-4136.

Image-4b: The wire-wrap socket as seen from above. Note those sexy long legs.
OK.  You can open your eyes now. Below is a thing of beauty called the EC-4136.  You can buy it plain and put your favorite opamps on it.  I am currently using the Burr-Brown OPA2604 and have used the OP275 (as shown). BUY NOW!

Image-5a: Bottom View of EC-4136 retrofit adapter

Image-5b: Top View of EC-4136 retrofit adapter

LA-4 Upgrade Pricing and Options

LA-4 Opto Experiments