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LA-2A Tube Compressor Manual

In this article

With its gentle, program dependent optical compression, and meticulously designed tube amplifier, the LA-2A is a go-to compressor for professional mixers around the world. 


Teletronix LA-2A Tube Compressor Plug-In


The Teletronix® Story

Teletronix founder Jim Lawrence first used photocells for controlling audio gain in the early 1960s. His ingenious optical compression design was a technological breakthrough, far surpassing the stability and transparency of earlier circuits. Universal Audio founder M.T. "Bill" Putnam later purchased this patented technology, continuing to manufacture the LA-2A for years to come.



Operational Overview


In the 60s and 70s, the LA-2A and 1176 became inextricably linked as the must-have dynamic tools of the day. If the 1176 is to the Stratocaster in terms of immediacy and flexibility, then the LA-2A is to the Gibson Les Paul in terms of warmth and one-of-a-kind, magical sonic distinction. An important characteristic of the T4 photocell response is that it is both program and frequency dependent. The T4 cell has a multi-stage release, and can take a few minutes to fully recover from the incoming signal.

The primary use is as individual inserts for sources that require nominal transparent gain reduction, such as vocals, bass, strings or horns. These tools can also be used to isolate the "color" of the output amplifier by turning off the Peak Reduction control, even to extreme distortion settings. An interesting sidechain distortion can be achieved at the most extreme Peak Reduction settings, which primarily affects low frequencies.


No compressor is as easy to operate or instantly satisfying to use as the Teletronix LA-2A. Peak Reduction applies the compression threshold to the incoming signal up to -40 dB, while Gain amplifies the signal for level matching post Peak Reduction. Set the metering view on the compressor with either +4 or +10 dB Output Gain, or Gain Reduction. The compressor also includes the hardware's rear Limit/Compress switch. Power bypasses all processor use, providing a handy level matching tool not found on the original hardware.

Plug-In Presets

The Teletronix LA-2A Tube Compressor includes a set of useful presets, accessed from the plug-in's preset manager. For more information on how to use UAD plug-ins, go here.




LA-2A Controls

The LA-2A Tube Compressor has a simple control set. Note that the Gain And Peak Reduction values, which range from 0-100, are arbitrary and do not reflect any particular dB value.


Peak Reduction (1)

This control sets the amount of signal compression by adjusting the trigger threshold. Increasing the value lowers the threshold, and therefore increases the amount of compression. The available range is 0 dB (fully counter-clockwise) to -40 dB (fully clockwise).

Rotate this control clockwise until the desired amount of compression is achieved. To monitor the amount of Peak Reduction, set the VU Meter knob to Gain Reduction. Peak Reduction should be adjusted independently of the Gain control.

When Peak Reduction is set to its minimum value, no compression (or limiting) occurs but the signal is still colored by the circuitry and the output level can be adjusted with the Gain control.

Gain (2)

The Gain knob increases the output level by up to 40 dB to compensate for the reduced level that results from compression. Adjust the Gain control after the desired amount of compression is achieved with the Peak Reduction control. The Gain control does not affect the amount of compression.

Note: The front panel knob values, which range from 0-100, are arbitrary and do not reflect any particular dB value.

Meter Knob (3)

This rotary knob sets the mode of the VU Meter. When set to Gain Reduction, the VU Meter indicates the amount of compression in dB. When set to +10 or +4, the VU Meter indicates the output level in dB (when set to +4, a meter reading of 0 corresponds to an output level of +4 dB).

VU Meter (4)

This is a standard VU meter that displays either the amount of gain reduction, or output level, depending upon the setting of the Meter Function switch.

Compress/Limit (5)

This switch sets the compression ratio of the leveler. When set to Compress, the compression ratio is approximately 3:1. When set to Limit, the ratio is approximately infinity:1. However, the compression ratios are nonlinear and frequency dependent, so these figures are not absolute.

On/Power Switch (6)

Determines whether the plug-in is active. When the Power switch is in the Off position, the plug-in is disabled and CPU usage is reduced.




Historical Background

In the 1950s while at Parsons Electronics, Electrical Engineer Jim Lawrence was quietly asked to join the Titan Missile Program based at Cal Tech's Jet Propulsion Lab and was assigned to develop optical sensors for the program. Fortunately for everyone, the technology developed from Lawrence's work lead back to a more peaceful deployment of the optical sensor, as the detector in his future Leveling Amplifier. The interactions of the luminescent panel with the photo resistors in his T4 design are predominantly what gives the Teletronix Levelers their signature sound.

Lawrence later broke out on his own to start Teletronix, setting up shop in his hometown of Pasadena, California in 1958. Among the Teletronix line of products were transmitter tubes, multiplex generators, to full-scale radio transmitters. Jim's first pass at his Leveling Amplifier was realized as the Teletronix LA-1; Around 100 units were made. Lawrence then updated the design to the LA-2 with improved specs and circuit layout, then moved quickly to the industry standard LA-2A. In 1965, just three years after the incarnation of the LA-2A, Jim Lawrence sold the company to Babcock Electronics. Enter Bill Putnam. Putnam picked up Babcock's broadcast division including Teletronix, and rolled it into his parent company, Studio Electronics in 1967. From there, Universal Audio resumed manufacturing of the LA-2A, and Putnam began using the optical detector for new designs.

Whether serendipity or by intent, Jim Lawrence's Teletronix Levelers and his T4 design had the right musical response that allowed the LA-2A the sonic and technological longevity it still retains. Universal Audio spent a long time getting the T4 right for their hardware LA-2A reissue and the plug-in. But what was special about it wasn't fully understood until UA began the research to model the LA-2A for the UAD-1. Modern photocells are designed to be as fast as possible, but they don't have the right multi-stage response they need to sound like a Teletronix design. Our DSP research helped us understand how the original T4 worked at the quantum physics level. This not only allowed us to develop an accurate DSP model of the gain reduction behavior, it also helped us make our hardware T4 more consistent. This involved studying the original photocell formula, working with both modern device physicists and the people who developed the original photocells, locating the special equipment originally used to manufacture these back in the ‘60s, and re-qualifying the manufacturer. Whether hardware or DSP, it is this special qualified manufacturing process and "recipe" UA re-established that gives the LA-2A its unique, musical sonic quality to this day.


Teletronix LA-2A Compressor Hardware