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.
The world’s first limiter was the Telefunken U3. Designed in
the early 1930’s it saw its first use at the 1936 Olympics. Built on a circuit that was designed
to protect against frequency modulation distortion due to sudden input overloads to the system, it was used on the public
address system at the Olympics. Not many of these units were made and it is doubtful that many (if any)
have survived.
The U13 was the next generation of audio limiter. It was developed
based on the components and technology of the V72 in 1950 and was the first diode bridge compressor ever designed and made.
This unit is literally the grandfather of almost every modern non-tube compressor.
In 1953 Rohde und Schwarz introduced the U23. 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.
Other U Series Compressors
Several
different versions were made.
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.
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.
There are several WSW
(Siemens in Austria) solid state compressors based on the early U273 design that sound too good to be transistor based…
