Historical perspective
Audio professionals passionate about their compressors revere the LA-2A. The original was immediately acknowledged for its natural compression characteristics. A unique electro-optical attenuator system allows instantaneous gain reduction with no increase in harmonic distortion – an accomplishment at the time, still appreciated today.

Universal Audio now announces the rebirth of the Teletronix LA-2A, a Universal Audio Classics product. Painstaking care has been taken to ensure that every new LA-2A provides the performance and characteristics of the original. Each unit is hand built, each component carefully evaluated for authenticity. No expense has been spared to guarantee that this LA-2A will bring that classic sound to your recording. Demand the original. Accept no copy.

A classic is reborn
Initially intended for broadcast applications, the LA-2A was first produced in the 1960s by Teletronix. Teletronix was later bought by Babcock Electronics Corporation, which was acquired, in turn, by Universal Audio (UA) in 1967. There were three versions of the LA-2A before it was discontinued in 1969. For decades, it appeared as if the beloved tube compressor had passed into history with UA's founder, the late M.T. "Bill" Putnam.

Thirty years later, Bill Putnam's sons reactivated Universal Audio. The first products to be released by UA are virtually exact replicas of the Teletronix LA-2A and UREI 1176LN.

Painstaking efforts were made to re-create an authentic replica of the vintage LA-2A, with point-to-point handwiring of original components. However, a couple of minor changes were made. The original Allen-Bradley input pots were only 20% tolerant, and were replaced with 10% tolerance pots from Precision Electronic Components (PEC). To conform to European safety standards, the large handscrews that provided tool-free access behind the unit's hinged front panel and into the high-voltage innards were replaced with ones requiring a screwdriver. Also, for convenience, a rear panel switch that toggled between compression and limiting modes was moved to the front panel.

But the heart and soul of the LA-2A is faithfully reproduced in the reissue. The original T4 electro-optical cell (containing an electroluminescent panel and photoelectric cell), HA-100X input transformer and UTC A-24 output transformer were either procured from outside sources or, if no longer available, built to specifications from scratch. The same 12AX7 and 12BH7A tubes drive the LA-2A's output. The compressor's sidechain incorporates a pre-emphasis circuit, originally designed for broadcast purposes, which is adjustable via a rear panel pot. Turning up this pot makes the unit increasingly more sensitive to higher frequencies. For music purposes, you'll want to keep it set to the flat factory setting.

T4 optical photocell
To a large degree, the T4 electro-optical cell gives the LA-2A its truly unique character. The T4 determines the program-dependent attack and release times; no manual user controls are provided for these. Inside the T4, the audio signal is applied to an electroluminescent (EL) light panel (essentially a night light), which shines brighter as the signal level increases. The EL panel shines on a cadmium sulfide photoelectric cell that, in turn, controls gain reduction with a lightning-fast attack time of 10 microseconds. But it is largely the photocell's two-stage release that makes the LA-2A's dynamics processing so incredibly transparent.

The photocell releases approximately half of its resistance in 40-80 milliseconds and the remainder over as much as several seconds. The duration of the second stage of release - ruled by the cell's "memory effect" - depends on how long light has been shining on the cell and at what brightness before it ceased. When compression is heavy and/or the signal has been above threshold for a long time, the LA-2A's release will be slower. This allows you to really hit the LA-2A hard with little or no audible amplitude modulation artifacts. It's these unique attack and release characteristics that give the LA-2A its legendary reputation for ultra-smooth control over the most unruly vocals.

Connections and controls
All I/O are found on the rear of the 3-rackspace chassis. Transformer balanced I/O are via XLRs (pin #2 hot) or barrier strips. It was no problem feeding unbalanced I/O signals to the LA-2A using custom cables with pins 1 and 3 shunted together. While few studios use barrier strips for input/output audio these days, it's nice to have such historical details presented on the reissue. Maximum input/output levels are specified as +16 dBm, which is a bit weak by today's standards. But UA made a conscious decision to be true to the LA-2A's original design and to not "improve" what has stood the test of time.

The rear panel barrier strips also provide facilities for strapping together two units for stereo processing applications. A three-prong IEC receptacle and detachable AC cord round out the rear panel. There's no need to delegate rackspace above or below the unit - all tubes are mounted horizontally on the outside rear panel, keeping the top and bottom chassis relatively cool.

The LA-2A's Spartan front panel controls speak to the unit's ease of use. A switch toggles between compression and limiting modes. Compression is soft knee, fixed at a 4:1 ratio. Limiting provides an infinity:1 ratio. The LA-2A can provide up to 40 dB of gain reduction. In practice, I found the differences between compression and limiting modes to be rather subtle. But, I always had a preference for one mode over the other for every application.

Like many opto-compressors, the LA-2A offers only two continuously variable control knobs. Turning the Peak Reduction knob clockwise increases gain in the sidechain circuit (not in the audio path), which effectively lowers the threshold. Simply turn the knob clockwise for more compression, or counter-clockwise for less. The Gain knob sets post-compression output level, or makeup gain. No bypass switch is provided, making instant A/B comparisons problematic. But at the risk of sounding ironic or indiscriminate, once you hear the processed sound you probably won't care to hear what choice "B" sounds like! It is so easy to dial in a gorgeous sound with the LA-2A.

A third knob switches the large VU meter to show gain reduction or output level, the latter referenced to either +4 dBm or +10 dBm. A beefy power switch is also provided on the front panel.

Key Features
True to the original in design, manufacturing and performance
Lag free, distortion free optical attenuator system
Distortion less than 0.5% THD
0 to 40 dB gain limiting
Balanced stereo interconnection
Frequency response 1 dB, 30 to 15,000 cps
Output level of 10 dBm and 40 dB gain
Low noise – less than 70 dB below +10 dBm output

Bruker-manual

LA2A brosjyre

LA2A recall sheet

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