The Teletronix LA-2A Optical Limiter

The Warmth of a Beloved Compressor

One of the most iconic compressor/limiters of all time is the Teletronix LA-2A. The unique warmth of its compression, especially on the human voice, has made it one of the most popular designs ever made. The complex physics of how it compresses an audio signal is matched only by the complexity of all the revisions it has undergone since its creation.

Thanks to several changes in ownership, the LA-2A has seen a large volume of refinements in its basic design. This has made it extremely difficult to know the differences between the various versions that have been released over the years.

Here at Vintage King, we have serviced and sold a multitude of units, and have seen every variety possible. Read on to better understand the nature of each particular unit and learn more about one of the greatest compressors in history.

An old advertisement for the LA-2, predecessor to the LA-2A

The Early Days of The LA-2A

The model name LA-2A stands for Leveling Amplifier, version 2, first revision. The original amplifier of this type was designated the LA-1, invented by Jim Lawrence, an electrical engineer and founder of the Teletronix Engineering Company in Pasadena, California in 1958.

Teletronix made radio broadcast equipment, and the original limiting amplifier was intended to level out the audio signal for broadcast. Its innovation lay in using an electro-optical sensor (designated the T4) to detect signal changes that would trigger compression or limiting.

It was a simple design of which maybe a hundred were made before improvements led to the second design, the LA-2. This version used an improved photo sensor, the T4A, and the unit became popular in radio stations across the country. By 1962, further improvements led to the first revision of the LA-2, dubbed the LA-2A.

Different Versions of The LA-2A

Starting with the first revision, there have been seven major iterations of the LA-2A. These different designs include the two main versions, two sub-versions, and two reissues. Finally, there is the current resurrection of the design that is still being manufactured by Universal Audio. All of the major technical and cosmetic changes are detailed below.

The Original Revisions

Teletronix LA-2A Revision 1 'Grayface'

Revision 1

The Original Version, a.k.a. "Grayface"
Serial Numbers 001-572 (approx.)
1962-1965

  • Note: First revision of LA-2 limiting amplifier, thus "LA-2A"
  • Faceplate: Painted battleship gray, with red "Teletronix Engineering Co." logo
  • Company Nameplate: A metal plate on the rear panel; says "Produced by Teletronix in Pasadena, CA (sometimes labeled Los Angeles, somemes labeled Sunnyvale)
  • Type / Serial No. Plate: On the rear panel; a metal plate with engraved numbers
  • Rear Panel Graphics: Hand stenciled or ink stamped
  • Octal Jumper Socket: Located before the input transformer for pad/pre-emphasis modules
  • Input Transformer: UTC HA-100X
  • Electro-Optical attenuator cell: T4A
  • Variable Pre-emphasis Filter, R37: Installed on later units (after #361 minimum)
  • 230 volt operation: None
  • Slightly smaller VU meter than all later versions

Revision 2A

The Babcock version, a.k.a. "Silverface"
Serial Numbers 573-1000 (approx.)
1965-1967

  • Note: Teletronix company and its patents were acquired by Babcock Electronics Corporation of Costa Mesa, California in 1965
  • Faceplate: Brushed aluminum with red "Teletronix Div. of Babcock" logo (some early units are painted battleship gray, with the original red "Teletronix Engineering Co." logo - these were presumably leftover stock)
  • Company Nameplate: A metal plate on the rear panel; says "Teletronix | Division of Babcock Electronics Corp. | Broadcast and Communication | Equipment | Costa Mesa, California - Made in U.S.A."
  • Type / Serial No. Plate: On the rear panel; a metal plate with engraved numbers.
  • Rear Panel Graphics: Hand stenciled or ink stamped on early units (screen printed starting around number 630)
  • Octal Jumper Socket: Located before input transformer on early units (compress/limit switch installed in place of octal on rear panel, starting with number 573, but some later units still do not have it)
  • Input Transformer: UTC HA-100X
  • Electro-Optical attenuator cell: T4A
  • Safety Switch on front hinged panel: None
  • Variable Pre-emphasis Filter (R37): Installed on this and all later units o 230 volt operation: yes; possible on this and all later units

Revision 2B

The Licensed version, a.k.a. "Silverface"
Serial Numbers 1001-1200 (approx.)
1967

  • Note: Teletronix patent is licensed from Babcock Electronics by Bill Putnam's Studio Electronics Corporation of North Hollywood, California
  • Faceplate: Brushed aluminum with red "Teletronix" logo
  • Company Nameplate: A metal plate on the rear panel; earlier plates still used, showing “Teletronix”, covered by metallic sticker reading "Manufactured in USA by | Studio Electronics Corporation | North Hollywood, Calif. | under license agreement with | Babcock Electronics Corp."
  • Type / Serial No. Plate: On the rear panel; a metal plate with engraved numbers
  • Rear Panel Graphics: Screen printed
  • Compress/Limit Switch: On rear panel of unit
  • Input Transformer: UTC HA-100X
  • Electro-Optical attenuator cell: T4A
  • Safety Switch on front hinged panel: None

Revision 2C

The UREI version, a.k.a. "Silverface"
Serial Numbers 1201-1800 (approx.)
1967-1969

  • Note: Teletronix patent acquired in 1967 along with the broadcast division of Babcock Electronics by Studio Electronics Corporation (later renamed United Recording Electronics Industries - UREI)
  • Faceplate: Brushed aluminum with red "Teletronix" logo (brushed aluminum with black "UREI" logo starting around number 1750)
  • Company Nameplate: A metallic sticker on the rear panel; says "Teletronix | United Recording | Electronics Industries | North Hollywood, Calif. | Made in U.S.A." with unit Type and Serial No. on lower portion (later units had a silver metallic sticker on the rear panel; says “United Recording Electronics Industries” in blue, with a blue UREI logo, along with Model and Serial No., starting around number 1750
  • Type / Serial No. Plate: No longer separate
  • Rear Panel Graphics: Screen printed
  • Compress/Limit Switch: On rear panel of unit
  • Input Transformer: UTC HA-100X (Input Transformer is changed to UTC A-10 starting around number 1640)
  • Electro-Optical attenuator cell: T4A (opto cell is changed to T4B starting around number 1640)
  • Safety Switch on front hinged panel: None

The Reissues

Teletronix LA-2A Revision 2C 'Silverface'

Reissue 1

The UREI Limited Edition version
Serial Numbers 101-400 (approx.)
1979

  • Note: Due to consumer demand, UREI resurrected the LA-2A for a limited edition of approximately 300 units
  • Faceplate: Brushed aluminum with red "Teletronix" logo
  • Company Nameplate: A metallic sticker on the rear panel; shows UREI logo with no address, and unit Model and Serial No. (100-300 approx.)
  • A larger metallic sticker says: "Model - LA-2A | Serial - xxx | UREI | United Recording | Electronics Industries | Sun Valley, Calif. USA" starting with number 300 (approx.)
  • Rear Panel Graphics: Screen printed
  • Compress/Limit Switch: On rear panel of unit
  • Input Transformer: UTC A-10
  • Electro-Optical attenuator cell: T4B
  • Safety Switch on front hinged panel: Power shuts off when front panel is opened

Reissue 2

The Harman Limited Edition version
Serial Numbers 3000-3235 (approx.)
1992

  • Note: Circa 1985 UREI was sold to Harman/JBL, and years later reissued a limited edition of approximately 235 units, using all available parts remaining
  • Faceplate: Brushed aluminum with red "Teletronix" logo
  • Company Nameplate: A metallic sticker on the rear panel; says “Manufactured in the U.S.A. By | Harman Electronics Inc.” with stamped model and serial numbers, and a date code
  • Rear Panel Graphics: Screen printed
  • Compress/Limit Switch: On rear panel of unit
  • Input Transformer: UTC A-10
  • The T4A opto module is replaced with a T4B after the first few units
  • Optical attenuator cell: T4B
  • Safety Switch on front hinged panel: Power shuts off when front panel is opened

The Current Iteration

Teletronix LA-2A Reissue 3, the current model

Reissue 3

The Universal Audio version
2000 to present

  • Note: Universal Audio was resurrected by Bill and James Putnam in 1999 and the reissue of the LA-2A was the second product released
  • Faceplate: Painted battleship gray with red "Teletronix Engineering Co. " logo, identical to the original on Revision 1
  • Company Nameplate: None
  • Type / Serial No. Plate: A metallic sticker on the rear panel
  • Rear Panel Graphics: Screen printed
  • Compress/Limit Switch: On front panel of unit
  • Input Transformer: Custom copy of an original UTC HA-100X
  • Optical attenuator cell: T4B
  • Front panel is no longer hinged for user access
  • XLR In and Out on rear panel

It stands to reason that the official Universal Audio LA-2A reissue is the closest to the original spec that one could hope for, utilizing a custom copy of the original transformer, as well as the T4B opto-cell from the 60s.

Inspired by the LA-2A, Acme Audio's Opticom XLA-3 MKII is an optical limiter that combines the best characteristics of three separate compression curves into a single unit. If you're looking for something for your 500 Series rack, the brand also makes a smaller version called the Opticom XLA-500 that still features a tube in its design.

The Doublewide from Retro Instruments is a compressor that gives off LA-2A vibes, but its small size also allows it to fit into your 500 Series rack. Unlike other Retro products, this one is designed completely from scratch and features four tubes hidden inside and two timing modes for a wide range of compression options.

Another classic optical compressor comes from Tube-Tech in the form of the CL 1B. This compressor has truly become a favorite in the modern studio setting as it is being used on hit records in the worlds of hip-hop and pop every single day. It's design features two time-controllers, a gain-reduction element and a tube-based push-pull amplifier.

IGS Audio makes several LA-2A reproductions that run the gamut of variations on the original design. The IGS Audio One LA is the classic version of the LA-2A, while the IGS Audio One LA 500 is a 500 Series size version and the 576 Blue Stripe takes design cues from the classic Revision A.

One last option comes from Anthony Demaria Labs, who have created a vacuum tube mono compressor/limiter known as the ADL C/L 1000. With a hand matched optical attenuator and custom in/out transformers, this compressor can give you everything from warmth to straight up crunch during tracking and mixing.