​The first wireless transmissions were of wireless telegraphs.  With the telegraph apparatus essentially being a binary sound source (on or off) it was quite simple to use the early broadcast equipment to dial in the appropriate volume level to appropriately hear the communication being sent.  Even during the earliest voice transmissions, given that they were either government or military transmissions to other government or military installations, the expectation for sound quality was quite simple; if you can understand the message correctly, the sound quality is good enough.

Once radio broadcasts began to be used for commercial purposes, the expectations of sound quality and needs for audio broadcasts began to change.  Listeners did not want to listen to radio broadcasts that jumped in various volume levels across a program as they didn’t want to have to sit next to the radio and “ride” the volume knob. Radios were also an expensive proposition at that time and new owners would not be pleased to have their speaker blown out due to these jumps in volume.   A piece of equipment needed to be created that could smooth out these transmissions and protect the listeners on the other end.

In 1953 Rhode und Schwarz developed the U23 and it was commercially introduced in late 1954.  Used as an overload protection device at radio stations, this unit was a technological achievement in studio equipment for the 1950s.  Using 3 EF804S, 2 EAF 42, 1 EB 41, and 1 ECC 40 tube, this unit is very similar to a Fairchild 660 in both design and sound.  If a signal came into the unit at +6 dB (or it could be set for +12 dB) the unit would kick in and reduce every 10 dB to 1.5 dB.

While the U23 was a quality compressor, it still did not conform to the high standards of the IRT.  Looking for a solution that was fully capable of handling the full frequency response of FM radio broadcasts, the IRT started development on the U73. Working again with the NWDR in Hamburg, the U73 was developed over a 3 year period.  Built into a double Danner cassette module chassis like the V76, the U73 shared significant circuit features with the V76 and used roughly 50% of the same components as the V76.  The U73 is a three stage push/push variable mu design with a theoretical compression ratio of 100 to 1.  The unit was yet another landmark design by Telefunken/AEG as it was the fastest tube compressor ever built due to its forward triggered side chain.

Once the design was finalized, it was licensed to the historic TAB corporation of West Germany who produced it for almost 30 years until finally discontinuing the model in 1980.  This was the last tube compressor made in the Telefunken/IRT gray series.  Due to its high price and fixed set-up, the U73 only found its way outside Germany in one specific application; as a final mastering compressor for disc cutting laths, mostly in conjunction with an automatic release trigger; the Varicom. Nearly every record recorded and cut in Europe from 1960 to 1980 was mastered through a stereo pair of U73’s.

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.