The Secret Compressor
There is no doubt about it: The U17 is one of the rarest analog compressors ever built. Just once in a blue moon you will find one in a studio or private museum, and so far we have never seen a single unit up for sale, except the one we could unexpectedly acquire for the NEOLD collection on a very fortunate day. Our device carries serial number 14, and only a few dozen units have been manufactured in total.
But what do we know about this ethereal device other than it being rare like a unicorn? 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 which was based in the city of Hamburg, located in northern Germany.
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 kind of telephones and other early communication devices.
With delivering equipment to the technically highly demanding broadcasting stations, ATF ranked on a level with other legendary manufacturers from the time like TAB, Maihak and Malotki.
Like 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 a reddish brown, hence the name. It is remarkable that just like the U17, many of the devices inside this catalogue have been created according the network's very own technical designs.
All Braunbuch texts have been written from a 100% engineering perspective, so we need to understand they read a little… well: "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." And it goes on just as dry as this.
While this does not sound overly exciting yet, 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 spared, only the best and most reliable components used, custom parts all over the place, most beautiful hand wiring throughout, every single component marked with its own numbered sticker. Bam!
But also the topology and circuit layout of the U17 are well worth mentioning. We have the highest respect for the old engineers, and it's always a marvel to see how they used their (from today's perspective) seemingly limited means to create outstanding audio devices which still work great many decades later 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. 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 which can also be found in later units such as the Siemens U273 or the iconic Neve 33609.
With its introduction in the early 1950s, when remote cut-off tubes where the only other technology available in order to achieve voltage controlled amplification, this design allowed for a low component count and a minimum of required maintenance. The other two diodes can be found in the envelope detector and time-constant network regulating the aforementioned 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 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 so as to bring the signal back to a nominal level of +6 dBu.
But 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, the U17 seems to be the ultimate one-trick pony in the history of studio compressors? The answer is yes… and no.
Looking at the original hardware alone, it's yes all the way. You turn the big knob one click to activate the machine and receive the most beautiful sounding, ultimately smooth compression. Check out the characteristic curve to get the idea :-) Now, while this is really cool and useful, it just seems too limited for a modern DAW environment. This is why – completely based on the original topology of the U17 – the NEOLD team has added a couple of powerful setscrews, mutating the gentleman into a veritable two-face.
The Power of DAW
The first thing we did was to move the controls for the time constants from the inside of the unit to the front panel, adding more range and convenient Auto functions for Attack and Release. Then, the compressor was embedded into an Input (threshold) and Mix/Output frame for flexible level control. After this, the powerful Torque control combining ratio and knee was added.
Torque has an enormous range and can drive the compressor deeply into negative ratios as well – the original, very gentle compression curve of the U17 is located in the 20% position.
We then decided to give the compressor further flexibility by adding a control for continuously linking/unlinking the detectors in stereo mode, and a very powerful sidechain filter section based on just two controls. While doing so, the sidechain also received a monitoring function and a switchable input for external keying. Already now, the one-trick pony had transmuted into something completely different. But there is more…
Finally, we wanted to be able to add some grit to the very pleasant sound character of the original circuit if needed, and this is how the Clipper block with its Density and Emphasis controls came into play. This section uses the specific characteristics of the ECC81 tube when driven for taming incoming transients, achieving a louder signal and adding selective amounts of harmonic distortion.
An interesting aspect about the Clipper is that in our initial concept it was placed at the end of the signal chain, just as the original circuit demanded. In numerous tests, however, we got better results both in terms of dynamics and sound when placing it right in front of the compressor.
So in the end, we decided to skip "True to the Work" in favor of "True to the Sound". Anyway, given all this additional leverage, the plugin can blend from gentle and transparent to straight and aggressive.
Coming to an end, we hope this little report has provided some useful background info on one of the rarest compressors in audio history, and maybe we could get you interested in our plugin implementation as well. We took great care to emulate the real thing in all its technical finesse, while adding useful extras to make your U17 experience a complete and most satisfying one. In fact, this plugin combines everything we love about the golden era of analog design with a very modern and experienced approach to dynamics processing. Enjoy it!