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Neumann U47 FET

Capacitor Microphone
Published July 2015
By Hugh Robjohns

Neumann U47 FET

Nearly 30 years after production of the original U47 FET ceased, Neumann have revived this iconic large-diaphragm capacitor mic.

The Neumann company have always been known for pouring their considerable resources into advancing microphone technology. Each new Neumann model has brought some worthwhile technical improvement, incrementing the industry forward. For example, the original U47 was the first large–diaphragm mic with a switchable polar pattern (1949), the KM84 was the first ever phantom–powered microphone (1966), and the TLM170 was the first mass–market transformerless microphone (1993). Neumann also introduced the first digital microphone, the Solution–D, in 2003. Of course, not everyone appreciates these technical advances — as evidenced by the great KM84 vs KM184 debate — but that’s a different matter!

However, for only the second time to my knowledge (the other case being a limited edition U67 reissue in 1993), Neumann have uncharacteristically decided to revisit their past by recommencing the manufacture of a 46-year–old design which ceased production some 28 years ago. As a result we can now — for a short time, anyway — purchase a brand-new ‘Collectors Edition’ U47 FET.

After 17 years of continuous production, the last U47 FET was built in 1986, by which time the U87 had evolved into the U87Ai as the default ‘studio microphone’ of choice, and the TLM170i had become the company’s flagship model. So why break with tradition and reintroduce the U47 FET? Well, Neumann’s press release for this reissued mic reflects on the fact that it played a significant role in shaping the sound of popular music in the ’70s and beyond, and that it is still a very common choice in today’s digital audio world. Perhaps it also has something to do with the burgeoning market for classic microphones and the remarkable prices people are willing to pay! The ‘Collectors Edition’ tag certainly suggests that, but whatever the underlying reason, Neumann are now building the U47 FET again, which is surely something to celebrate.

Importantly, the Collectors Edition U47 FET is manufactured directly from the original production documents and schematics, so this is not some kind of faux reissue with redesigned and modernised internals; Neumann genuinely have resumed production of the microphone as it was being manufactured from 1980. This is an important caveat because the original U47 FET went through a number of manufacturing variations over its production lifetime (as all models tend to do). Consequently, there are a few very minor variances to accommodate modern production requirements, and I’ll detail some of those in a moment, but rest assured that in every way that matters, this new U47 FET is exactly the same as the old one.

History

The U47 FET was conceived in 1969, the driving impetus being that Telefunken had stopped production of the VF14 valve that was employed as the impedance converter in the original U47 microphone. With the rapid advances being made in the late 1960s in the quality and capability of silicon transistors, Neumann decided to re–engineer the U47 as a solid–state, single-pattern microphone, using the recently perfected field–effect transistor (FET) to provide the high–impedance interface with the capsule. The resulting microphone, which entered mainstream production in early 1972, was called the U47 FET, although it is often referred to simply as the ‘fet47’. The original model employed a Tuchel output connector, but that was changed to an XLR and the model rebranded the U47 FET i. The internal construction also changed slightly at the same time, with a plastic inner chassis instead of the traditional brass-plate-and-pillars assembly.

A whole generation of solid–state microphones using this new FET technology was introduced under the family name of the ‘FET–80’ range. This included such revered microphones as the KM84 (and its siblings, the KM83 and KM85), the KM86, the U89, the now–legendary U87, the stereo USM69 and, of course, the U47 FET. Interestingly, the U47 FET’s impedance converter circuitry was far more complex than any of the other mics in the range, and this complexity was only shared with the U89 (and some KMR shotgun models). In most FET–80 microphones the FET impedance converter feeds the output transformer directly, but in the U47 FET (and U89), the FET buffer is followed by a further five bipolar–junction transistors, which essentially form a discrete op–amp to drive the output transformer. A sixth transistor serves as a voltage regulator to ensure a stable 43V power rail from the phantom supply.

Although the FET model incorporated exactly the same K47/49 capsule (see box) as the original U47, the revised body and grille metalwork, and especially the radically reinvented internal electronics, all contributed to a different, rather less 3D and organic sound character. Nevertheless, the U47 FET became a highly regarded and immensely popular mic in its own right: different from an original 47, of course, but a useful and reliable workhorse all the same.

One of its particular strengths was an ability to cope with extremely high sound-pressure levels — up to 147dB SPL with the 10dB pad switched in (for comparison, the original U47 was getting pretty crunchy by 120dB SPL). The FET 47’s high SPL-handling capability quickly made it a firm favourite for capturing kick drums, bass instruments, guitar amps, brass and many other demanding instrumental applications. Although never as popular as a vocal mic as the original U47 — it sounds a little more ‘closed–in’ and flat than the original 47 (which has a much broader 5dB presence boost) — the FET 47 was still quite usable in this role too, particularly for male vocals and speech. Its modest 2dB of presence boost between about 2 and 5 kHz is enough to aid clarity and intelligibility in a mix, while the gentle proximity boost adds a degree of body and warmth.

Collectors Edition

Supplied in a traditional dark wood case with a sculpted foam interior and a brass model badge on the top surface, this new Collectors Edition microphone ships with its operating manual inside a cardboard wallet, alongside an individual serial–numbered certificate signed by Wolfgang Fraissinet, President of Neumann GmbH. The review model was serial number 10083, but I believe standard production units are all over 10100.

For anyone not familiar with the U47 FET, I think it is fairly described as a ‘chunky’ microphone, measuring 160mm high by 63mm in diameter, and weighing a substantial 710 grams. The new microphone is only available with the traditional satin nickel finish, whereas the original could be obtained with either matte black or nickel finishes (although I’ve never seen a black one!).

An integral standmount is included, comprising a simple arm permanently attached and pivoted from the side of the mic body. The stand–adaptor thread is 5/8–inch, but the mic ships with an adaptor for both 3/8–inch and 1/2–inch standard mic–stand threads. Usefully, the base of the stand adaptor also incorporates a grip for mic cables up to 5mm in diameter (secured with a thumbwheel), which helps to isolate mechanical vibrations travelling along the cable.

Neumann describe the U47 FET as a fixed–pattern cardioid microphone, although it actually has a distinctly hypercardioid response for all frequencies above about 500Hz. The nominal sensitivity is a very modest 8mV/Pa, and this can be further reduced in two ways. The gain of the internal electronics can be attenuated by 10dB through a recessed slide–switch on the rear of the mic. Internally, this increases the amount of negative feedback around the FET, thus reducing the risk of front–end overload. In addition, the output transformer can be reconfigured with a slide–switch at the base of the mic, rearranging the transformer secondary windings in parallel rather than series to decrease the output level by 6dB. The aim is to reduce the risk of overloading the subsequent console or preamp, but this option was removed from the original FET 47 for the last couple of years of its production run.

Self–noise is given as 18dBA, which, combined with the normal maximum SPL rating of 137dBA, gives an impressive potential dynamic range (A–weighted) of 119dB, rising to 129dB with the 10dB attenuation switched in. The microphone requires standard 48V phantom power, but has an extremely low current requirement of just 0.5mA. A switchable high–pass filter is also provided, via another rear–panel slide–switch, to help control the mic’s proximity effect. This raises the low frequency roll–off point from 40 to 140 Hz.

Differences

To the casual eye, the Collectors Edition and original (post-1980) U47 FET microphone are absolutely identical — as they should be given that Neumann have ‘recommenced production’ rather than created a ‘new’ version! The body is the exact same size, shape and weight, with the same purple Neumann lozenge and the same three–layer (coarse, medium, fine) mesh grille.

However, in comparison to early FET 47s, one immediately obvious indication is that the milled polar–pattern symbol just below the grille is inverted. The new Collectors Edition model has the pattern null facing downwards (as it did in the post–1980 models), whereas it faces upwards on the earlier originals. The mounting arm’s threaded stand–adaptor section is slightly smaller than early original models, too, again being the same revised design as employed on the post–1980 mics (and the TLM170, in fact).

Inside the mic every part — with the exception of a new and improved version the of sealed slide–switch — remains directly interchangeable with the original model. Even the hand–wired ‘rat’s nest’ of electronic components located around the switches under the top circuit board is made in exactly the same way as the original was (in the precise form that it was being built around 1972 — apparently later editions employed a PCB for these components!).

However, there are a number of almost infinitesimally subtle constructional differences which would make a great ‘U47 FET nerd’ quiz! Examples include the knurled cable clamp wheel being plastic instead of metal, the detail of the output transformer’s construction, the inner mesh layer’s weave density, and the ‘off’ position of the new slide-switches. The last item is because the new switches seem to have a shorter slide travel, resulting in the ‘off’ position being noticeably closer to the centre of the microphone case’s switch slots than the bottom. Thankfully the ‘on’ position is still unambiguously at the end of the slot next to the legend, though. Another minor detail change from the early FET 47s is that the capsule has gained a protective nylon collar across the top, providing some protection to the diaphragms in case the mic gets dropped and the flexibly mounted capsule accidentally strikes the grille housing.

The capsule assembly plugs into a gold–plated seven–pin socket mounted on a PCB with the 2N3819 FET mounted all alone on the top surface. A number of components are soldered directly to the underside of the socket and to the two slide–switches, which are supported on a metal frame. The remaining electronic components are accommodated on a circuit board positioned below the output transformer, with the power regulator transistor and the four other audio transistors, as well as the output level slide–switch. Again, this board is identical to the post–1980 version of the U47 FET.

Conclusions

In every sensible interpretation of the term, Neumann really have ‘recommenced production’ of the U47 FET, but it should be noted that this new version isn’t actually identical to any specific original model, because it contains the best elements of several iterations drawn from across the 17–year production run. The new model should, therefore, actually be better than any original model — and not least because of the far tighter manufacturing tolerances of current K47/49 capsules.

To my ears, the new U47 FET retains all the character (or lack of it, some might argue!) of the original, with that same slightly two–dimensional sound, contrasting with the pronounced three–dimensionality of the progenitor U47. Personally, I think Neumann made a mistake in sticking with the U47 moniker; it should have been christened the U57 or something to help break the sonic link with the U47, in much the same way that the transistorised U67 became the U87.

That said, the U47 FET remains a very useful and sought–after microphone and the new version is every bit as accomplished as the original, with the benefit that it is available brand new for similar or less than a mint–condition original currently changes hands (especially in the US). Given the total authenticity of the new model’s construction, that surely makes it something of a bargain, even at its eye–watering price. And a small comfort is that its future value is likely to compare directly with the original models if the reissued U67 valuations are anything to go by. Neumann have not given any clear information on how many Collectors Edition mics it intends to produce, but it’s rumoured to be around 100 — so get your order in quickly!

Alternatives

There are several microphone homages based upon the Neumann U47 FET, including the Bees Neez Tribute 3 FET, the Bock Audio iFet, the Wunder Audio CM7, and the Lawson L47FET. However, for considerably less money, the Audio–Technica AT4047 is designed to deliver the U47 FET character too, albeit with a more modern styling.

The K47/49 Capsule

The double capsule number is a source of confusion to many. The K47 capsule design dates from 1958 and it was intended as a replacement for the M7 capsule employed in the earliest U47 mics (amongst others). It is a centre–terminated dual–diaphragm capsule built around a shared central backplate electrode, with screwed–down diaphragm tensioning rings (the M7 diaphragms were glued on).

A major problem with this type of shared backplate construction is ensuring that both diaphragms have identical performance, and that’s why Neumann’s later dual–diaphragm capsule designs were built in two identical halves, each with their own backplate electrode, and then screwed together. This approach made construction much easier, and allowed considerably better consistency in matching and performance between capsule halves. The first example of this approach was the K67 capsule built for the U67 mic (using a Telefunken EF86 valve), which was itself intended as a direct replacement for the U47 in 1960.

However, because of the matching consistency issues with the K47 design, Neumann started employing a second model number to identify the very best–matched capsules, which was K49. Consequently, these K49 capsules were specified for use in switchable pattern mics like the U47, U48, M49 and so on, where front and back capsule matching was critical to the overall performance.

Although it still uses a dual–diaphragm or Braunmühl–Weber capsule, the U47 FET has a fixed cardioid pattern, so front/back capsule matching was far less critical for this application. Neumann therefore denoted capsules with one perfect side and one less well–matched side as K47 FET capsules, specifically because they were only suitable for use in that model.

Today Neumann’s production tolerances for the K47 capsule — which is still used in several current production mics — are so tight that the company no longer makes performance distinctions. Either the capsule is up to standard or it isn’t (there is just a single grade of capsule called the K47/49), and that’s what is employed today in the M147 Tube, M149 Tube, TLM49, and now, once again, the U47 FET.

Published July 2015