Now, a change from records, and a post specially for the sound engineers out there, and the rest of us too. A guest post, by Neville Roberts. If the name is familiar, you are reading the right sort of magazine: Neville is a regular reviewer/ contributor to the audio magazine Hi Fi Choice. A Physics graduate, Chartered Scientist, Chartered Engineer, Chartered Physicist, Fellow of the Institution of Engineering and Technology, and generally all-round clever chap, he is a vinyl devotee, naturally, and has a particular interest in valve audio design.
The topic of the post was prompted by research Neville was doing on the legendary microphones which contributed so much to the quality of music recorded to vinyl in the ’50’s and beyond. The idea seemed a fitting postscript to the life’s work of Rudy Van Gelder.
Microphones were one of the LJC six essential enablers of modern jazz:
Microphones were the one I knew least about; I have a lot to learn, and Neville Roberts is just the right man to learn us. It’s not rocket science – if only it was that easy.
There will be a test at the end of the post, so pay close attention.
Microphones and Missiles: The Legendary Neumann U47 Microphone
Is there really a connection between microphones and long-range, guided ballistic missiles? Well, it is actually part of a story that led to one of the most successful microphones that are still in use today.
The early years
In the early days of radio and sound recording in general, carbon microphones were used to convert sound waves into electrical signals. These microphones resembled a shoe polish tin, partially filled with carbon grains, with small openings on one side to admit the sound. These openings were backed with fine gauze to prevent the carbon grains from falling out. A current was passed through the carbon grains and the sound waves would modulate the current as the resistance of the carbon capsule changes with the sound. By modern standards, the quality of these microphones was dreadful. The transducer principle used in these microphones was jokingly referred to as a “controlled loose connection”.
A certain Georg Neumann found that he could improve the quality of the microphone by stretching a rubber membrane over the carbon grains and in the early 1920s this led to the development of a microphone that had a linear frequency response between 50 Hz and 1 kHz, then dropped by around 10 dB at 4 kHz and by 15 dB at 10 kHz. Not exactly studio quality, but certainly a vast improvement on the basic carbon microphone. However, Neumann considered that a far better microphone could be produced based on the capacitive transducer principle. Up until then, these condenser microphones could only be manufactured under laboratory conditions, and Neumann realised that the technology would have to be developed to suit commercial manufacture.
He set up his own company in 1928 and produced the first ever mass-produced condenser microphone, the CMV3, which was far superior to carbon microphones and gained recognition under the nickname of the ‘Neumann Bottle’. It wasn’t exactly compact, measuring about 9 cm diameter and 40 cm high and weighing in at nearly 3 Kg!
The aftermath of World War Two
Right up until the end of World War II, the CMV3’s design remained virtually unchanged. It became the standard microphone for studio use and was used extensively in the 1936 Olympic Games in Berlin. The CMV3 also had a selection of exchangeable capsule heads with different polar patterns for use in different situations.
After the Second World War, the damage to German infrastructure was so severe that no nationwide electricity supply existed. Instead, many temporary and provisional mains electricity supplies were put in place – some provided AC and some DC. As part of post-war reconstruction, there existed a market for cheap radio sets which could operate on all these various supplies. This was quite an unusual requirement because the sets running on DC supplies couldn’t use transformers. To satisfy this demand, German valve manufacturers, in particular, Telefunken, developed valves with high filament voltages that could be run directly from the mains supply. Since two-valve radio-sets were the most common and 110V AC and DC were the most common supplies at the time, the voltage chosen for the filaments was 55V, so that the two valves’ filaments could be powered by wiring them across the radio-set HT supply in series!
One of the high filament voltage valves widely used in radios of the period was the VF14, a 55V heater version of the ubiquitous EF14 (an RF pentode) which had a conventional 6.3V filament.
The VF14 was very much the workhorse valve during the war. It found applications in everything from field radios to the Vergeltungswaffen Zwei (aka V2 rocket!)
The new German broadcast stations after the war had the same issue as civilians. Faced with a DC (110V) mains supply, Berliner Rundfunk (Radio Berlin) asked Georg Neumann to supply them with microphones which could be used without a transformer power supply and could therefore be powered directly from the mains (wouldn’t the Health and Safety people love that nowadays?). Neumann solved this problem in exactly the same way as the radio designers had, and opted to use the VF14.
Thus, in 1949, the Neumann company launched the microphone that has probably had the greatest influence on the development of modern studio microphone technology – the U47.
This microphone was the first switchable pattern condenser microphone and its impact, especially in America, was such that the dominance of RCA’s ribbon microphone was eclipsed by it as the studio standard. The U47 had a double diaphragm capsule and both diaphragms could be polarised with the same voltage or neutralised with respect to the centre electrode, so that the omnidirectional and cardioid characteristics were adjustable. A development of this microphone, the U48, was also produced where the diaphragms could be polarised with opposite voltages with respect to the centre electrode. This enabled switching between cardioid and figure of eight directional characteristics.
One problem for Neumann in developing the U47 was that the requirements for the valve in the head amplifier in a broadcast microphone and in the RF front-end of a cheap radio are very different. Whilst the pedestrian VF14 valve worked consistently in the latter, only a few examples worked in the former. Neumann’s solution was to set up an arrangement with Telefunken so that when they took delivery of a batch of VF14s, they selected suitable samples for use with microphones. Those that passed the Neumann test procedure were marked with an “M” on the side of the steel can (for Microphon). However, at least two-thirds of the valves off the production-line were rejected for inadequate performance in the U47 and U48 microphones.
By the late 1950s, Neumann could not get hold of reliable quantities of the VF14 for production, maintenance and repair. In fact, the situation was so serious that, by 1960, a major German broadcasting company refused to invest further in Neumann equipment unless a solution could be offered that would justify their substantial investment in U47s.
By the end of the fifties, the Neumann design team knew that no more VF14 tubes existed anywhere in the world that would measure sufficiently well to be used in their microphones, and extensive research had revealed that no direct substitute tube existed. Their response was to re-design the associated circuitry, and particularly the output transformer, and to release a new unit to replace the U47 and U48. This was the U67 microphone, which was launched in 1960 and was able to work as an omni-directional, a cardioid or as a figure of eight.
However, the U47 was a firm favourite amongst recording artists, such as Frank Sinatra, because its response characteristics particularly suited the human voice. The reason why German engineers developed a microphone for best capturing the human voice will be obvious to students of history. It is still in use today.
A particular challenge facing the owners of such microphones today is finding a replacement valve, should the VF14 fail. No more VF14M valves will ever be found as Neumann exhausted these before 1960. Also, no exact substitute valve exists for the VF14, so any NOS (new old stock) VF14 tubes discovered today are almost certainly Neumann factory rejects, because they are either noisy, microphonic or suffer from a poor low frequency response. Fortunately, such ‘small-signal’ valves have a long life, but should one fail, some companies exist that make ‘equivalents’, but they are solid-state FET devices!
In 1949, Neumann produced the M49, which was the first microphone that could be remote switched.
Neumann’s work on the M49 coincided with a similar invention of the Central Laboratory of the Nordwestdeutscher Rundfunk in Hamburg. This invention had the front microphone diaphragm that had a fixed bias of 60Vdc in the opposite direction to the centre electrode and a back diaphragm that received a capsule bias voltage going from 0Vdc to 120Vdc, adjusted via a potentiometer. This made it possible to switch the microphone characteristic smoothly from omnidirectional, to cardioid and then to figure of eight. The resulting sound features a consistent smoothness that is highlighted by excellent lows and highs that can cut perfectly across recordings. Neumann acquired the patent for this principle and created the M49. Numerous microphone manufacturers followed, but all under license from Neumann.
The M49 was a favourite of recording studios in the 1950s and 60s and was the standard microphone used on smooth pop and soulful jazz recordings during the era. Despite being used predominantly on vocals, both in the studio and in the live setting, the mic was also used for instruments, especially brass and drum tracks. Miles Davis was said to have exclusively used this microphone on his best-selling album: “Kind of Blue”. For the vocals on Simon and Garfunkel’s “Bridge Over Troubled Water”, producer Roy Halee used a M49 to mic Art Garfunkel. Neumann M49s were also used for Bob Dylan’s first electric album, “Highway 61 Revisited”.
Other models of microphone were produced in 1951 by Neumann, most notably the M50, which featured a pressure capsule embedded in acrylic glass to give an outstanding omnidirectional pattern. Incidentally, there is also now a scarcity of the Telefunken AC701K valve (a low noise and low microphony version of the AC701), which is used in the M50.
By 1953, Neumann was meeting the demands of television companies for smaller models with a selection of condenser microphones only 21 mm in diameter. Later, stereo recording called for two microphones placed in such a manner that their time/amplitude response was coincident.
The ideal solution was to mount two capsules in one housing. So, in 1956, Neumann produced the SM2, which was to remain the only stereo microphone in the world for many years.
Of course, this marked the end of valve microphone production. However, many consider that valve microphones still produce the finest quality sound, and this is why these original valve-based Neumann microphones are still in use today. For example, Mike Valentine of the “Chasing The Dragon” record label owns several U47s. Mike specialises in audiophile recordings and, in particular, direct to disk vinyl. His U47s contain the correct Neumann-selected VF14M valves, but he’s not saying where he managed to source them.
All I can say is that they sound fantastic!
1. What is Neville Robert’s first name? (Always start with an easy one)
2. U47: was it
a) a WW2 submarine
b) a well-known 70’s pop group
c) a valve-based microphone.
3. Resolve the following differential equation:
My score? I got only one right. If you scored two out of three correctly, there is hope. If you got all three right, why are you wasting time hanging around with us dimwits?
Operating instructions for the U47 (strictly for audio engineers ) Post continues after.
My thanks to Neville Roberts for doing all the heavy lifting. (If you have a jazz-related or hi-fi related topic you are burning to write about, drop me a line). If you are an engineer who owns a Neumann U47 , congratulations! If you can add to the story about microphones used in recording modern jazz, like Roy DuNann’s use of the rival AKG C12, now’s the time.
“DuNann discovered high-output European condenser mics could put out more than a volt of signal when put in front of a loud jazz musician. Using matching transformers to bring the mics into a passive mixing network, he then hit each buss (left, right and echo sends) with line amps. You can listen to Sonny Rollins “Way Out West” to hear the results — super-clean and no distortion from loud drums or piano.”
Sonny’s 1957 Riverside album “Sound of Sonny ” found Rollins teamed up withanother Sonny, Sonny Clark, and gave the Neumann U47 pride of place on the Riverside cover. Upside down.
Here is another example of the U47 in action, Miles Davis “Musings of Miles”
So now you know. You can speak Microphone, chat authoritatively with sound engineers, debate the merits of this or that cardiac pentode (I made that up).
I’m no engineer, as you can tell, but something different happened to sound engineering quality when electric instruments came along, with their own amplified signal, making valve mics redundant. Maybe it had to do with the way sound from electronic instruments was captured. Just a thought.
Back to more records next post.