U47/U48 Tube/Valve Microphones vs economical replacements On November 02, 2019
History of the Neumann U47/U48 Tube/Valve Microphones
IF YOU HAVE FOUND THIS PAGE THEN YOU ARE MOST LIKELY LOOKING FOR HISTORY OF THE NEUMANN U47/U48 MICROPHONE :
The Neumann U47 is a microphone that has a prestigious place in the history of the recording industry. The legendary Neumann U47 was introduced at the 1947 Berlin trade show and was immediately accepted based on its forward looking design and innovative technological standards. The U47 came to North America in 1949 by the distributor Telefunken and quickly eclipsed the RCA 44bx as the first choice studio vocal microphone. The U47 was immediately embraced by artists like Frank Sinatra, Ella Fitzgerald and the Beatles along with recording engineers Tom Dowd, Bill Putnam and George Martin. 
How did the U47 design come about?
The condenser element was invented somewhere between 1916-1919 by E.C. Wente, an engineer at Bell Labs in the United States. Wente, developed the first working model of what he called the “Condenser Transmitter”. This new device consisted of a tightly stretched metal membrane placed over a stationary back-plate with just a few microns of clearance. After several experiments with varying dimensions, tensions and grooving of the back plate, he eventually filed for a patent for the "Telephone Transmitter". The United States Patent and Trademark Office granted patent #1,333,744 on March 16th 1920 for Wente's invention. 
Here is the Western Electric 47a amplifier with the #394 condenser capsule was produced in 1928. The 47a weighed 12lbs
Western Electric held all of the patents on the condenser element and several companies including AEG in Germany built versions of the 394 under license from Western Electric. AEG was a Company where the young George Neumann was working. During the mid 1930's ribbon microphones where very popular in the US, condenser microphone production in the US dramatically declined due to lack of demand. Georg Neumann became obsessed with the possibilities of manufacturing a commercial condenser microphone with a frequency response that would surpass that of those microphones he was developing at AEG. In 1928 George left AEG with Erich Richmann and started the Neumann Company.
The first tube microphones were built using the Neumann M7 capsule specifically designed and developed to get around the patents held by Western Electric on condenser capsules. 
The M7 capsule used a much thinner diaphragm than the 394 (12 microns vs 25 microns) and instead of solid aluminum foil the first M7 used PVC with a gold dusted layer that was later changed to a sputtered “disc” on PVC.
The other major difference was that the M7 featured a diaphragm on both sides of the back-plate enabling the capsule to pick up sound 360 degrees around the mic. Thanks to the dual diaphragm of the M7, the U47 was the first microphone to feature a switchable polarity pattern (cardioid and omni) on the microphone as opposed to changing the capsule assembly.
The 1936 Olympics in Berlin provided the “testing ground” for the classic bottle microphone circuit and M7 capsule designed by George Neumann. The “Neumann bottle” could be equipped with different capsules in order to change patterns. Hitler’s propaganda machinery immediately capitalized on the “Bottle Microphones” ability to capture Hitler’s speech dynamics. The “Neumann Bottle” was the sound of Hitler on News Reels and can be seen in many pictures of Hitler addressing German crowds. 
After the war, the Allies traded the Thueringen region to the Soviets in exchange for West Berlin. The Soviets & East Germans seized all businesses in the territory and prevented any money from being exported out of the country. Telefunken, who had been the distributor for Neumann microphones before the war, helped Georg Neumann set up a new company in West Berlin after the war in 1945 and became 25% owners of the new Neumann Company as well as continuing the distribution for their products. The U47 was developed from roughly 1945 to 1947 when it was introduced at the Berliner Funkausstellung electronic exhibition in 1947.
It is quite likely that Telefunken had some hand in the development of the U47 given the fact that the VF14 tube was specifically developed and produced by Telefunken for use in the U47 and had no other previous commercial use.
VF14M Tube
Before 1947, there is no mention of the VF14 tube in any design documents or any catalogs by Telefunken. The original design documents for the VF14 have no dates on them, but it does specifically state that the VF14 was designed for use in microphones. The very first microphone that it was used in was the U47 and later in the U48. Telefunken records show that they continued to produce the VF14 tubes up until 1958.
It has been over 60 years since the introduction of the U47 tube microphone and has certainly passed the test of time.
In 1958 Neumann starting fitting the K47 in the U47, M49 and introduced the new 2-pattern U48 microphone built with the same circuitry,M7 capsule and body as the U47 but with a FIG 8 option.
By 1958 Neumann believed the M7s capsules were no longer an option for serious recordings. Even, disregarding the early deterioration of M7 diaphragms, the new K47 capsule had a noise floor 4dB lower than that of an M7. This of course is due to the PVC materials that were used in the M7. The audible changes and drying-up tendencies of the M7 pretty much started as soon as it was poured, and never stopped until it had deteriorated beyond usability.
With its high quality components, “modern” design, head basket, good looks and the distribution might of the prestigious Telefunken Company; the U47 quickly became the microphone to use in nearly every recording and broadcast application. Perhaps the most surprising aspect of this microphone is the simplicity of its electronic design. It is amazing how good it sounds with such a simple and uncluttered design. Its only drawback was that it sold for around $400 in the early 50’s. This is about $3,600 in today’s currency! The same price as a U87 sells for today. At that time, the U47 was priced about three times higher than the price of the best ribbon microphone available on the market. But the price didn’t stop the U47 from being purchased by all the best recording studios during the 1950’s and early 60’s. Neumann stopped production of the CM47 & CM48 in the early 1960’s but with a modest supply of VF14 tubes. To this day Neumann have never made a reissue or replica of the U47. Over the years Neumann have occasionally created special versions of the U47 for people in the industry. For example, Bryan Adams received a pair of custom built U47’s in the early 90s.
Early U47 capsule suspension with M7
For recording engineers, the U47 is the most recognizable microphone:
Even the average person will recognize the U47 due to the fact that so many famous recording artists have been pictured with the microphone throughout the years. The U47 can be found in virtually every book on recording applications, biographies of singers, documentaries as well as videos and even immortalized in song lyrics. It simply is one of the vintage microphones that everyone wants to have. If imitation is truly the sincerest form of flattery, then the U47 is admired by folks around the globe. There are more clones and derivatives based on the U47 design than any other microphone in recording history!
So, is it truly special or is it a status symbol? The answer is both. To a skilled engineer who understands microphone placement and optimum gain structure, there definitely is some magic that happens with the U47. The combination of capsule, tube, amplifier circuit and transformer creates harmonic overtones characteristics that are pleasing to the human ear. This effect provides warmth, richness and thickens the sound of the source being recorded. For example, during the height of his greatest 1950's and 1960's recordings, Frank Sinatra recorded almost exclusively on Telefunken branded microphones.
But the U47 was limited to production of an estimated 5000-6000 units. While that sounds like a lot, when you consider the number of recording studios around the world and the number of home studios that are in use, there is simply not enough left to go around. As a result, a lot of people see the U47 as the ultimate status symbol. If you have one, you’re viewed as a serious, professional (and successful) studio that can produce quality recordings. Additionally, as engineers, producers and artists specifically request the Neumann U47, it has become one of the “must have” microphones to secure named artists to work at one’s studio. The U47 is a 'go to' mic for vocals, acoustic guitar, kick drum, bass amp and many other recording applications.
The U47/U48 circuit is elegant in its simplicity:
It involves a pentode tube wired as a triode. This lowers the plate impedance of the tube circuit allowing the tube to drive the BV8 6.5:1 output transformer without any significant tube loading effects. The VF14 tube was designed to run from a 60v heater (filament) supply. Neumann chose to power the filament at about 34v fed by R4 which slowly bring the voltage up to 34v and this is why a U47 must be powered up for at least 45 minutes before the circuit stabilizes and the cathode is brought up to the correct temperature which creates and efficient electron “cloud”. There is some belief that this under powering of the filament voltage contributes to the U47 sound. However, it is this writer’s opinion from his 45 year of experience with tube circuits that once the cathode is brought up to the correct temperature it will react as the same tube with 60v on the filament. It just takes it much longer to achieve this optimum temperature and this is what contributes to the longevity of the VF14 tube that has been operating in some U47 microphones for 60 years.
There are only two capacitors in the audio chain of the U47. C1 bypasses the audio to ground so the back-plate can remain polarized by R1. C2 couples the output of the tube to the output transformer blocking the plate voltage from magnetizing the output transformer.
U47 schematic
1: What are the design considerations to produce a similar circuit?
The first consideration is to separate myth from fact regarding the circuit and capsule construction to understand the reasons for some of the unique U47 design features:
There is no reason to under power the filament with today’s solid state regulation which will hold the filament at a constant voltage + or – 2%.
This was necessary in the late 40’s and early 50’s in order to lengthen the life of the tubes filament. Today, solid state regulators hold the filament voltage in check no matter how much the A/C line voltage might change.
2: Does the VF14 provide a sound signature of its own?
There is no proof of this other than the original tube had an internal capacitance that was higher than a more modern sub-miniature pentode like the GE/Jan 5654w. The internal capacitor of the tube according to the Miller Effect states that the internal capacitance of the tube is multiplied by the gain of the tube. So, the VF14 will roll out the very high frequencies above 15khz earlier than a circuit using a more modern 5654W(6AQ5) low plate voltage tube having a similar gain structure. This would not be noticeable on most program material unless it had very hf content. The output impedance of the 5654w is very close to the venerable VF14 as the 5654W was designed to work on a lower plate voltage more like the VF14 compared to other modern tube types.
3: Can self-bias (cathode bias) be used and get the same sonic result?
Cathode bias requires larger bypass capacitors in the order of 220ufd to produce the same low frequency response as the original U47. In 1947 a 220ufd capacitor would be larger than U47 body. However, today as a result of the space race smaller, larger value high quality capacitors are readily available. For, example the .5 ufd transformer coupling capacitor can be increased to a 2.2ufd Wima metal film and is ¼ the size of the original capacitor in the U47/U48 microphones.
4: can a modern capsule be manufactured to reproduce the frequency response and sound signature of the original k47 type capsule?
Today modern CNC milling machine can produce exacting capsule metalwork identical to the original K47 form. These can be skilfully skinned with 6 micron mylar and will produce a response curve and sound signature within the tolerances set for original K47 capsules.
Can, a high quality vintage sounding tube microphone alternative be built that can be afforded by a wide section of amateur recorders?
Yes, this can be accomplished by using microphone bodies already manufactured for our CM87 and CM48FET microphones along with their 3 polar pattern switching circuit boards. Now we only have to fit a new tube circuit board for the economical GE/JAN 5654w tube, a 7 pin connector and our BV18 transformer used in the CM251 and CM67se microphones.
This allows us to produce a world class microphone with 3-patterns that sells for $595 USD including case, shockmount and power supply.
CM48T in shockmount GE/JAN5654W NOS Tube in socket BV 18 transformer
