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December 20, 2021

Fab Dupont how optical compressors work




The LA-2A compressor/limiter is a long-time favorite of recording engineers and producers. With its warm tube sound and distinctive, fixed attack and release times, this iconic processor—available as in hardware form or software plugin emulation—is great for vocals, bass and more. It’s easy to use, with only two main controls, so you can add it to many types of sources for instant improvement.

That said, it’s also easy to overly squash the audio if you’re not careful with your settings. In this excerpt from “How to Listen: LA-2A Edition,” Fab Dupont offers insights into how—and how not—to set the LA-2A. He also explains about the photocell that governs the compression in the unit

Good Optics

The LA-2A is an optical compressor, which means that it uses light to control the attenuation it applies. Like most compressors, the input signal gets split into two feeds: The main one goes to the compression circuit and eventually passes through the output. The other goes into the internal sidechain to trigger the compressor.

That sidechain signal feeds a T4 photocell, which converts the input signal to light. The brighter the light gets, the more the unit attenuates the primary signal.

The controls of an LA-2A are incredibly simple. A front-panel Peak Reduction knob works similarly to a Threshold control, and determines the amplitude at which the compression will kick in. The Gain knob is a makeup gain that has no impact on the amount of compression. That’s pretty much it except for switching between Compression and Limiting, which lets you choose between a 4:1 and Infinity:1 ratio, although it’s not quite as consequential as that might sound. The small control labeled “Emphasis” governs a filter on the internal sidechain input.

The LA-2A has no adjustable attack and release controls, those parameters are fixed, and part of what gives the unit its distinctive sound. In a hardware LA-2A, the output circuitry features a tube stage, which adds warmth to the audio. You’ll notice in the excerpt that Fab’s LA-2A has a custom bypass switch on the front panel.

Fab’s LA-2A.

Bass in Your Face

In the excerpt, Fab observes that a lot of people set their LA-2A in a way that over-compresses the source, resulting in a loss of dynamics on the track. A combination of hot input levels and high settings on the Peak Reduction knob make for a condition where the photocell in the unit stays on constantly. When that happens, Fab explains, the LA-2A is no longer functioning as a compressor; it’s just a glorified volume control that’s turning down the level. On the bass example he’s using for the video, the attenuation level is close to -20dB.

The bass is attenuated by almost -20dB.

He points out that some people might intentionally set it like that to imbue their tracks with the sound of the unit’s tubes and transformers. If you want its characteristic compression, however, go easy on the Peak Attenuation knob and the input level feeding the plugin. 

He demonstrates this with both a UAD LA-2A Gray plugin and his hardware LA-2A. He suggests that it’s better to compress less and turn the Gain knob higher. That will preserve your dynamics and still give you that silky LA-2A sound.

He also points out that the hardware reacts more slowly than the software and provides more of a clicky sound on the transients. That said, the comparison between the UAD plugin and the actual hardware shows you how closely the plugin mimics the behavior and sound of the real thing.

His closing point is that if you use extreme attenuation with the LA-2A, you’ll only be hearing part of what it can do. With more moderate settings, you get both the tone and the unique dynamics of the legendary compressor.

Heading to LA

One of the most impressive things about the LA-2A is how versatile it is. While it’s probably best known as a vocal compressor, for its ability to control dynamics and add it’s warm and shiny sound, it’s frequently used to smooth out and fatten up electric bass.

The LA-2A is also great with a lot of other instruments. We’ll check it out on several different ones in the following examples, all from the same song. We’ll use the UAD Teletronix LA-2A Gray on several of the tracks. For each example, we bypassed the LA-2A for the first four measures and turned it on for the last four.

First, here’s a lackluster-sounding track from a virtual drummer. With the Peak Reduction set at 40, which is on the low side, the LA-2A will attenuate moderately, about 4-5dB. But it’s enough to reduce the dynamic range and bring up the room sound on the track. The drums sound fuller and more energetic.

The compression on the drum track helps bring up the room tone.

When working with the LA-2A, it’s good to always remember that the input level affects the amount of compression on it. In Pro Tools, the channel inserts are pre-fader, so the fader position does not affect their input levels.

However, if you wanted to change the input level going into the plugin, you could adjust the track’s Clip Gain because the inserts are after it in the signal chain. If another plugin is before the LA-2A on a channel, its output level will affect the level coming into the compressor. If the track was recorded at a moderate level, but you’re getting what seems like more compression than you should on the LA-2A based on your setting, you could look to turn that output down.

For most cases, it’s probably easier to just adjust the Peak Reduction knob to get the amount of compression you want and not worry about the level coming into the plugin, unless it’s clipping. If you’ve got a hot signal coming in from the channel, use a lower Peak Reduction setting and vice versa.

The following example features the bass. Because it was recorded with a hot level into Pro Tools, it will hit the LA-2A’s input hard. As a result, it was possible to get a decent amount of attenuation with a Peak Reduction setting of only about 30.

The setting for the bass example.

In the final example, you’ll hear the melody guitar, which is playing arpeggiated chords. Again, the peak reduction is set pretty low—30 in this case—but the attenuation is up to about 7dB, which is relatively high. That’s happening because the amp simulator before the LA-2A on the channel has a high output. Besides controlling the guitar’s dynamic range, the LA-2A adds a slinky tone to it.

The lead guitar setting.