Compressor: Urei LA-4
UREI/Universal Audio is known for a long line of popular compressors and limiters. It's a rare sight to see a studio without some sort of 1176 or clone, and that's surprising to absolutely nobody. But I'm not talking about the 1176, this page is about one of Urei's later and less well-known solid state compressors: the LA-4.
It's my assumption that the majority of LA-4s were used mostly for a primary stage of program limiting in applications like radio stations, where if you exceed your allotted FM bandwidth, the FCC will come knocking with some paperwork and a hefty fine. LA-2As have also been used for this purpose, but on their own they aren't fast enough to prevent Overs. Our pair came racked together, although the way it was done made it impossible to rack the two, as you can see by the shelf they sit on, and even though the pairing would normally imply that they were matched, they are two very different compressors. The No.1 compressor pictured is gentle and smooth, and part of my secret sauce on vocals. A gentle knee to both 2:1 and 4:1 ratios contributes immensely to this end. Some people say these units sound dark, but all that means in practice is that I might be able to skip de-essing... but probably not. The unit also has a comfortable amount of headroom, unlike the No.2 compressor, which is quite dark and has a hyperactive O/L bulb. The two units have different faceplates, the no.1 having a black outline, and the no.2 having a printed white box in its place. There are also black LA-4s that came later and physically look much closer to the later (and very different) LA-5 limiter.
So... about how these things work. Like the LA-2A and LA-3A before it, the LA-4 is a compressor with an electro-optical sensor in the detector. It's the first of the series to use a different optical unit, which changes the sound a bit, but I was curious what kind of compressor the LA-4 actually was.
A compressor can't just be an "optical compressor". The optical sensor doesn't do the gain reduction. An LA-2A is unique in the series in that it uses a triode vacuum tube to perform gain reduction, as they are just "JFETS with pilot lights" (Dave Jones, sometime in the 2000s probably) and are effectively designed to decrease transmitted signal when voltage is applied onto one of their pins. A quick peek at the LA-4's schematic (readily available if you know what to ask) reveals that while it contains several op-amp ICs, the final attenuation stage is a pair of transistors, making it a FET compressor circuit.