Analog Obsession: Stereo Linking LA-2As and LA-3As, Barrier Strips and Gain-Mod Info
by Will Shanks

If you are a seasoned studio engineer and have had lots of great gear at your disposal, then it is more than likely you've used LA-2As and LA-3As. It is also likely that you know strapping two LA-3As for stereo use is common, but strapping two LA-2As is rare. But what you might not know is that the units work the same way when stereo strapped. Why is the LA-2A less commonly strapped? Actually, I don't know. Feel free to email me if you think you have the answer! To dispel any myths: the LA-2As are just as capable as their solid-state (LA-3A) siblings of working as bus compressors or mix compressors and of working in stereo in general.

Racking the LA-3As
Unless you just returned from the Foreign Legion or some alien abduction, you're probably aware that Universal Audio is rereleasing the LA-3A under the "UA Classics" moniker; the LA-3A reissue is a clone that is as close to the original as possible. UA will offer two options: a single unit with a single unit rack kit, and a two-channel package that provides two units in one box with the two-channel rack system. For those who buy one channel now but plan on adding another unit in the future, don't worry: when the second unit is added, the rack system is complete. Each single channel package provides half the parts you need for the complete two-channel package.

“The LA-2As are equally capable as their solid-state (LA-3A) siblings of working as bus compressors or mix compressors and of working in stereo in general.”

Stereo Matched
As I mentioned, the LA-3As and the LA-2As should work well in a stereo configuration. However, any LA-2A or LA-3A pair will function even better in a stereo configuration with matched T4s (the electroluminescent/photocell compression element of optical compressors). UA is taking the extra step of providing tightly matched T4s in the two-channel LA-3A package. As a result, you get improved stereo imaging and response as well as improved metering in Gain Reduction view.

Stereo Setup and Calibration
To strap two LA-2As or two LA-3As (hereafter referred to as LA), the only additional equipment you'll need is two shielded wires no greater than two feet in length. The terminal strip is the only way to connect two LAs for stereo operation. Simply connect the Chassis (ground) connections of the two units with one of the wires, and connect the Stereo connections with the other wire. If you plan on bouncing between dual mono a stereo a lot, you can add a simple throw switch to the stereo connection to allow easy switching.

To calibrate your LA pair, take the following steps:

1. On the rear of both units, set the Stereo Adjust control to full clockwise (flat).
   
2. On the front panel, set the Peak Reduction knob to 0 (off). Now set the GR/Output switch to Output.
   
3. Connect a signal generator to the inputs of both LA units (400 to 1,000 Hz is recommended). Set the signal generator output level to produce a typical level for your application.
   
4. Adjust the Gain control on each unit so the outputs are equal at 0 dB.
   
5. Switch to GR (Gain Reduction metering) on both units. Turn the Peak Reduction knob on one of the units until the meter reads -5 dB.
   
6. Note which LA is reading the most gain reduction. On that unit, turn the rear-panel Stereo Adjust control until both LA meters read the same value.
   

You are now ready to use the two units as a stereo pair. Adjust the Gain and Peak Reduction knobs in tandem (visibly match them) to achieve the matched compression and output level desired. The stereo image should remain solid and both units should compress the stereo signal equally, regardless of which side (left or right) is triggering the gain reduction.

Bonus Feature with Barrier Strips
The terminal strip provides screw connections for input, output, stereo balance and chassis ground. The strip can be used instead of the XLR connections. In addition, you can use the XLR output and the terminal strip output simultaneously to "split" the output signal and feed two separate devices.

Mod Switch for LA-3A
In designing the LA-3A reissue, we considered many mods, as the LA-3A is a unit that lends itself to customization. After looking at the various circuit modifications commonly practiced, we felt that just one was essential. Commonly known as the "Evans Mod" and sometimes the "Clearmountain Mod," the modification controlled by the rear-panel Mod switch improves the signal-to-noise ratio of a "stock" LA-3A. To properly take advantage of the Mod functionality, both the Mod Switch and the Gain Switch should be set in the "up" position: + 50 on the stock switch (this equates to an unpadded input stage), and "Mod" engaged (this equates to a 24 dB pad on the output). In this configuration, the unit has a total gain of +26 dB and will be less noisy. The loss of gain is not a problem in most applications, because the LA-3A has about 50 dB of gain to begin with. The improvement in signal to noise is usually a desirable trade-off. We have supplied this modification as a switchable feature so you can operate with or without it. In addition to providing a lower noise floor, the mod allows greater gain-reduction range.