There is no doubt about it: The U17 is one of the rarest analog compressors ever built. Once in a blue moon, you will find one in a studio or private museum. Apart from the unit unexpectedly acquired for the NEOLD collection on a very fortunate day, we have never seen a single piece of the hardware up for sale. This device carries the serial number 14. Only a few dozen in total were ever manufactured.
So what else do we know about this ethereal device? Introduced in 1954, the U17 was custom built for the post-war NWDR broadcasting network. Based on circuitry developed by its own central engineering department, the executing manufacturer was Allgemeine Telefon-Fabrik GmbH, based in the northern German city of Hamburg.
We don't know much about this manufacturer other than it was founded in 1889 and that its main business was developing and building all kinds of telephones and other early communication devices. When it came to delivering equipment to the technically demanding broadcasting stations, ATF ranked on the same level as other legendary manufacturers of the time such as TAB, Maihak, and Malotki.
The "Braunbuch" description
As with every essential NWDR studio hardware, the U17 can be found inside the storied "Braunbuch”, which is an internal anthology of the broadcasting network, consisting of functional and technical descriptions as well as circuit diagrams. The color of this set of four highly sought-after ring binders is reddish-brown, hence the name “Braunbuch”. It is remarkable that just like the U17, many of the devices inside this catalog have been created according to the network's very own technical designs.
All “Braunbuch” texts have been written from a 100% engineering perspective, so they can be a little dry to read, to say the least:
“The U17 dynamic compressor is mainly intended for use with transmitters whose low levels are to be increased in order to improve the signal-to-noise ratio. Compression is achieved by means of a gain control that depends on the input level and causes a narrowing of the dynamic range.”
While the text above may not sound overly exciting, the technical implementation of all this is a completely different story and actually quite spectacular. What becomes obvious, at first sight, is the state-of-the-art build quality of this compressor. No cost and effort were spared, only the best and most reliable components were used, custom parts all over the place, beautiful hand wiring throughout; every single component is marked with its own numbered sticker.
The topology and circuit layout of the U17 are well worth mentioning. The highest respect must go to the engineers who built it. We marvel at how they used their modest means (from today's perspective) to create outstanding audio devices which still work incredibly well many decades on. The special highlight of the U17 is the combination of a solid-state diode bridge using Germanium crystal semiconductors with all-tube and massive transformer-coupled amplification — the best of both worlds!
The U17 circuit employs six DS62 Germanium crystal diodes: Four of them are wired in a bridge configuration to implement the floating gain reduction circuit. This can also be found in later units such as the Siemens U273 or the iconic Neve 33609. When the U17 was introduced in the early 1950s, remote cut-off tubes were the only other technology that could achieve voltage-controlled amplification. This ground-breaking design allowed for a lower component count and required minimum maintenance. The other two diodes can be found in the envelope detector and time-constant network, regulating the gain controller in a feed-forward configuration.
The audio amplifier as well as the sidechain driver feature an ECC81 double triode, which is strapped in a clever negative feedback configuration in both cases to allow for maximum linearity. The U17 was a broadcast device designed to improve the SNR in AM transmission paths after all.
The compressor includes plenty of custom iron: The audio and sidechain signals are tapped from the split secondary coil of the input transformer. The second ECC81 stage of the sidechain amplifier is loaded by a major choke and drives into another iron that feeds the envelope and time constant generator. Then there is another 1:7 interstage transformer that steps up the signal output by the floating diode bridge loop to give enough level to drive the audio amplifier, which eventually drives the output transformer using a second choke to bring the signal back to a nominal level of +6 dBu.
A one-trick pony?
Apart from all the technical glory, what does the unit actually sound like? And how does its compression behave? Judging from a single big black control knob (which is actually just an active/bypass rotary switch) on the front panel, you may wonder if the U17 is the ultimate one-trick pony, but the answer to this question is yes AND no.