Fuse Audio Labs - VCL-373 1.0.0 VST, VST3, AAX x86 x64 [08.2018]

Variations of the U73 Compressor
TAB ended up making 5 different versions of the U73 compressor for Telefunken. The U73, U73a, U73b, U73bt, and the U73ts. All of the units shared similar design aspects but each had it's own characteristics based on the build or the final intended usage.
U73: The standard U73 unit was designed for talk radio use and does not come equipped with any controls for overall compression levels or the ability to adjust the attack and release times for the unit.
U73a: Because the units are forward triggered, the term "compression ratio" is not completely accurate when discussing the control of compression with these units. The U73a allowed you to set the "compression stroke" but since it is forward triggered the ratio was constantly changing pending the dynamic signal that came before it making it a non-linear compressor. Most of these units used EF83 tubes and are very rare.
U73b: This is the unit that is most widely known and was most commonly used for producing audio recordings. Designed for the studio, the U73b not only allows one to change the level of compression but also has a control on the outside to change the Release time and a control on the inside to change the Attack time as well. These units used E99F tubes and therefore had a slightly different sound and schematic from the U73a.
U73bt: The U73bt came out after the U73b. The U73bt provided the same controls as the U73b but the amplifier utilized a mixture of tubes, nuvistor tubes and transistors to achieve its sound.
U73ts: The U73ts was the first Telefunken compressor to use only transistors. The unit is very similar to the later U373b made by TAB except that it came in the U73 sized housing with its own power supply.
U83: This was a limiter like the U73 that came with a different preset threshhold and no ability to change them like the U73.
TAB in conjunction with EMI (a subsidiary of Carl Lindström AG) made several mini versions of the U73 that could be inserted into the effect loop of the V76S, the studio version of the V76. This mini U73 does not work for the V76/80 or V76/120 versions.
Racked pair of U373a units
Other U Series Compressors

With the arrival of transistors, Telefunken and its manufacturing partners embraced transistors into their designs in order to create compressor modules that could take up less space and generate less heat.
In 1960, Siemens introduced the U273 the world’s first solid state compressor. There is a dispute over whether or not Neve based their 2254 compressor on this module. There are striking similarities to the schematics but one of the main differences is that the Neve uses Marinair transformers while the U273 used Haufe. The Siemens U273 is one of the best sounding transistor compressors ever made.
In 1963 TAB introduced the U373, an upgrade version of the diode bridge design, made with transistors that took the edge off the distortion. TAB built several different versions; the U373a/b/c, the U273a/b version for Siemens and the U373TS version of Telefunken. The U373 group is the only non-VCA compressor that works similar to a remote cut-off tube compressor.
Neumann, Filtek (BFE/Siemens) made several spin-offs off the U373, some with DBX or other VCA chips. These are reasonable units but not nearly as good sounding as the historic TAB U73 or U373 types.