Occasionally, a piece of music gear gains so much cultural significance that it transcends utilitarian function and becomes an icon. The Fender Stratocaster, the Akai MPC3000, and the Moog Model D all signal their respective subcultures visually, before anyone hears a note. But rack equipment, that mass of rectangular boxes that cluster together in studio furniture, rarely becomes larger than life. If any rackmount box could make a claim to “iconic” status, it would probably be the omnipresent LA-2A, with its striking “Teletronix” logo, central illuminated VU meter, and Daka-Ware knobs.
If a “compressor” emoji existed, it would look like an LA-2A. If a film set designer were choosing artifacts to signal that an onscreen studio was a serious professional facility, that familiar 3U faceplate would be a must, and for good reason! It has been a fixture of real studios for more than six decades, owing to some unique behaviors that have become indispensable to generations of engineers and producers.
How Does an Optical Compressor Work?
If you hang around recording engineers enough, you’ll have heard reference to several different compressor technologies: FET, VCA, diode bridge, and vari-mu, each associated with particular sounds and behaviors. One of these flavors (optical, or “opto”) is essentially shorthand for “LA-2A, or one of its descendants.” It was the first widely available compressor to make use of light in its gain reduction element.
When radio hobbyist James F. Lawrence began developing its predecessor, the LA-1, in the 1950s, virtually all compressors used the same basic technology: a special type of vacuum tube called a “variable mu” tube. While most tubes are optimized to offer a fixed amount of gain, vari-mu tubes can amplify a lot or a little, and exactly how much depends on the bias voltage they see. By changing the tube’s bias point dynamically in response to signal, a vari-mu tube becomes a compressor: it amplifies less and less as the input signal gets louder and louder.
But the LA-1 worked on an entirely different principle, and its inventor was well qualified (his day gig was at a Jet Propulsion Lab in Pasadena, working on Cold War-era aerospace projects). Perhaps it was this environment that inspired the use of optical sensors and electroluminescent (EL) panels, both pioneering technologies that hadn’t yet penetrated the mainstream in the 1950s.
Electroluminescence, a process by which a substance emits light when an electric current passes through it, is everyday stuff now. The common light-emitting diode (LED) has brought it into virtually all aspects of modern life. But in the 1950s, when almost all light was either incandescent or fluorescent, EL panels felt positively futuristic. Their perfectly uniform blue-green light, flat profile, and zero-heat operation made them ideal for things like military aircraft instrument displays and space mission guidance computers. Eventually, the excitement spilled into the mainstream, brimming with connotations of techno-optimism. When Chrysler introduced the “AstraDome” dashboard with “panelescent” lighting in 1960, it must’ve felt like you could take your daily driver straight to the moon. Even decades later, in the 1990s, electroluminescent displays were being portrayed as cutting-edge (notably by Timex in their “Indiglo” wristwatches).
The other key ingredient in the LA-1’s operation: the cadmium sulfide (CdS) photocell charted its own course to consumer relevance. Also called a “light-dependent resistor” (LDR), a CdS photocell is just what the terminology implies: a resistor that changes its value when exposed to light. As early as the late 1950s, municipalities were beginning to employ CdS cells to automate streetlamp systems. By the mid-1960s, consumer cameras were using them as automatic light meters, making it easier for novices to take quality photographs. Marketing departments called them “electric eyes.”
James Lawrence was inspired to package the two together in a prototype component that formed the heart of the LA-1: the T4 opto-attenuator (later refinements produced the T4A, and finally the mature T4B). The principle of operation is simple: the EL panel lights up in response to an incoming audio signal, which causes the CdS photocell to change its resistance. The photocell then acts as the shunt resistor in a simple attenuator (it’s essentially an automatic volume knob, controlled by light).
In short, an optical compressor turns audio level into light, and light into gain reduction: the louder the signal, the brighter the panel, and the more the photocell turns the volume down.
Why Does the LA-2A’s Release Sound So Musical?
But when implementing a new technology, the devil is always in the details. A few quirks of mid-century EL panels and CdS photocells ended up imbuing Lawrence’s new device with some unintended, but desirable behaviors. An old saying, attributed to 1930s Yankees pitcher Lefty Gomez: “I’d rather be lucky than good.” Well, sometimes you can be both. Even brilliant aerospace engineers occasionally stumble into greatness!
EL panels and CdS cells, particularly older ones, aren’t ideal manifestations of textbook concepts. They are real-world devices that are bound by the laws of physics and materials science, and one particular complication is non-linear behavior over time (or lag). The CdS photocell’s behavior, in particular, is highly dependent on both the intensity and the duration of the light source. If illuminated for a long time, the CdS cell’s resistance might take longer to recover than it would with a shorter burst. Bright blasts can likewise lead to a longer recovery (If a camera flash has ever gone off in your eye, leaving a ghostly blip in your vision that decays over a few seconds, you’ll be able to imagine this behavior!)
What this adds up to in practice: a compressor that doesn’t have a single release time constant, but rather, a complex, multi-stage recovery characteristic that’s highly program-dependent. The original Teletronix LA-2A manual specified approximately 60mS for “50% release,” and “0.5-5 seconds for complete release depending upon amount of previous reduction.” But even these are estimates. Later in the same manual, the second phase of release is described as “a period of 1-15 seconds.”
This was, to borrow a phrase from television painting instructor Bob Ross, a happy little accident. It turns out this sort of behavior is incredibly musically intuitive, particularly on certain types of program material. Imagine a vocal performance with rhythmic, delicate delivery in the verses, but soaring, sustained high notes in the choruses. We might favor a shorter release in sections where intelligibility is paramount, but a longer release to level out the belted bits. The real-world implementation of Lawrence’s optical solution seems to have the very same instinct. It just works.
Who Made the LA-2A? From Teletronix to UREI to Universal Audio
Lawrence launched the LA-1 in 1958 as the flagship product of his new Teletronix Engineering Company, but very few units were sold before the introduction of the refined LA-2 (a subtle but substantial functional improvement). While the LA-1 had seen some adoption by area radio stations, the LA-2 caught the attention of major players: CBS and RCA. It wasn’t long before Teletronix was receiving orders from broadcast and recording studios across the country. In 1962, a few minor tweaks to the design and layout streamlined production and made for a more commercially viable, predictable product: the mature LA-2A.
By 1965, Lawrence was ready to exit the venture he’d founded, so he sold Teletronix to Babcock Electronics of Costa Mesa, CA. Two years later, Universal Audio founder Bill Putnam acquired all of Teletronix’s patents, hardware, and asset rights for his Studio Electronics Corporation. In 1968, Putnam rebranded Studio Electronics Corporation to UREI (United Recording Electronics Industries) and made the LA-2A for another couple of years before shifting focus to its new solid-state successor, the LA-3. Despite all of the ownership changes, the Teletronix branding remained.
The LA-3 (which used the same T4B optical element) is a great compressor in its own right, and was state-of-the-art for a few years. But it was barely a decade before the first LA-2A reissues started to emerge (in 1979), built to original specs. A second reissue in 1992 used up remaining stockpiles of the original parts. Since 1999 (when Bill Putnam’s sons re-launched Universal Audio), the LA-2A has remained in continuous production. The periodic reissues and eventual lasting return are hints that the recording industry’s appetite for vacuum tube optical compressors never fully went away, though it may have waxed and waned at times.
Part of the LA-2A’s legacy may indeed be the very notion of reissued audio gear. The 1979 batch occurred three years prior to Fender’s first series of “vintage reissue” Stratocasters and Telecasters. Aside from Gibson bringing back the original Les Paul body shape in 1968, it’s difficult to point to an earlier example of a music gear manufacturer revisiting their own history in such a prominent way. Something about the LA-2A just refused to be consigned to history.
Why Does the LA-2A Sound Warm?
Perhaps the LA-2A's enduring appeal comes down to the tone. While not a “vacuum tube compressor” in the same sense as the Fairchild 660, Gates Sta-Level, or Universal 176 (all vari-mu designs), the LA-2A does have tube amplifier stages. Old ones were built almost like guitar amps, with carbon composition resistors, Class 2 ceramic coupling capacitors, and electrolytic caps in the signal path. The vintage UTC transformers on the input and the output are part of the sauce, too. While not necessarily audiophile-approved, these components impart subtle colorations that we hear as character, as subjective “warmth.” But there’s a balance a good LA-2A strikes: it’s not exactly transparent, but neither is it so heavy-handed in its coloration as to invite typecasting. It’s not some kind of period piece, used to evoke prior eras. It’s an enduring, ever-present functional tool that has, in its own quiet way, shaped music and pop culture for generations now. The LA-2A seems to exist almost entirely independent of era.
LA-2A Reissues, Clones, and Plug-ins
Its relevance shows no signs of slowing down. Original production was a non-trivial 2,000-2,500 units, and that number has surely been vastly surpassed by the last quarter-century of reissue production. This is to say nothing of the numerous replicas, variations, and clones available (at virtually all price points). It’s a popular-enough DIY project that ready-made kits, enclosures, and transformer sets exist. At least one company, Kenetek, offers high-quality T4B opto-attenuator reproductions. Purchasers can select from a range of release time behaviors, accounting for the variations in vintage units made from old CdS cells with wide tolerances
And that’s only the hardware! Beginning with the Bomb Factory BF-2A in 1999, the LA-2A’s signature faceplate (okay, a GUI representation of the faceplate!) has been as omnipresent in DAWs as it has in physical control rooms. Dozens of different plug-in iterations exist. Universal Audio’s ecosystem has several by itself: a port of its primitive 2002 build, an LA-2, a gray-plate LA-2A (with a slower release curve), and a silver-panel LA-2A (faster release). The latter three are also available in their UADx native format, amounting to seven total representations of one piece of hardware from a single developer!
The LA-2A’s narrative unfolds in parallel with the history of record production, and both stories are still being written. A visionary technical insight from a moonlighting aerospace engineer combined with a bit of luck made a hell of a good compressor. Its commercial release just predated a shift in the landscape of popular music: the emergence of the pop record as a serious art form. When recording artists began treating albums as ambitious statements, James F. Lawrence’s innovation was already there, ready to help. At the risk of torturing a metaphor, there’s an obvious parallel to be drawn between the LA-2A’s defining technical behavior (the multi-stage release) and its cultural impact. During its initial production run, the metaphorical first 60 milliseconds, it burned as brightly as a mere technical tool ever could, and for longer than most. But that all-important second stage of release has proven much longer. The LA-2A has clamped down so hard on our collective workflows and aesthetic preferences that nearly seventy years later it still hasn’t let go.
Shop the Universal Audio LA-2A, explore LA-2A-style compressors, or talk to a Vintage King Audio Consultant about which opto compressor fits your studio.
