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Universal Audio LA-2A

LEVELING AMPLIFIER There are few pieces of equipment, new or vintage, that inspire as devoted a following as the Teletronix LA-2A. Ever since production

There are few pieces of equipment, new or vintage, that inspire as devoted a following as the Teletronix LA-2A. Ever since production of the LA-2A ceased 30 years ago, engineers eagerly paid premium prices on the secondhand market for this revered electro-optical tube compressor – when they could find one. Of course, as with all vintage gear, you never knew what condition the unit was in when you bought it. Not to mention how well that condition would hold up in the months or years ahead.

But there’s no need to guess any longer. Recognizing the continuing strong demand for the LA-2A, Universal Audio has brought the venerable dynamics processor back into production. And, at $3,495 list, it’s competitively priced with vintage, secondhand units.

What is so special about the LA-2A that engineers routinely choose it over scores of more modern compressors? To answer this question, let’s explore its unique design and then take it out for a test drive.

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.

Go to the Focus on Universal Audio page to see video and learn more about UA products

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.

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.

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.

My first task was every engineer’s nightmare: compressing a very undisciplined female vocalist who had a cutting attack, glaring tone and wildly fluctuating dynamics. Even a fat AKG C-12VR tube condenser mic, with its inherent upper bass reinforcement, could not warm up this banshee! But the LA-2A made it easy, providing completely transparent gain reduction and a velvety tone. The result was a vocal track that sounded dramatically smoother and warmer. And it took well under a minute to dial in the sound.

I got similar, if not quite as extraordinary, results compressing a trumpet. The LA-2A’s dynamics processing was smooth as could be, and the tone was slightly warmer and less piercing.

Limiting a kick drum track with the LA-2A, the dynamics were once again reined in tightly, yet so smoothly. Instead of providing a warmer, rounder tone, however, the LA-2A gave the kick a snappier sound – somewhat like the old trick of taping a quarter to the drum head where the beater strikes. The rigorously consistent output levels the LA-2A provides, along with the extra “point” on the sound, was just the ticket for this techno pop mix. I could crank the kick drum track to where it rocked hard without any need for rolling off excess bottom end or adding top.

Finally, I used the LA-2A to limit a DI’d electric bass guitar. As always, the dynamics processing was completely transparent. And the LA-2A put some much-needed “hair” on the relatively lean-sounding track, making it beautifully round and lush.

It’s easy to dial in great sounds with the LA-2A. Operation is so straightforward you’d have to be in a coma to get stumped.

The LA-2A’s uniquely transparent processing makes it incredibly effective for treating vocals. When you need to warm up a piercing or thin vocal and smooth its uneven dynamics, the LA-2A can work miracles. It also sounds awesome on bass guitar and, for a certain type of sound, on kick drum. And did I mention that it looks beautiful, too? If I were a mutant, I’d give the LA-2A three thumbs up!

Universal Audio, PO Box 3818, Santa Cruz, CA 95063-3818; 831/454-0630; fax 831/454-0689;