U 47

Common name: Neumann U 47

Production Status: Discontinued / Vintage

The U 47 is without doubt the most important professional microphone in the post-war pro-audio industry. It was the first microphone with a switchable polar pattern response. The M7 capsule, derived from the CMV 3, has two moving diaphragms producing a cardioid polar pattern, when only one is active. With both membranes paralleled the transducer has an omnidirectional polar pattern response. The U 47 microphone is much smaller than the CMV 3. The first U 47 version, manufactured from 1947 until approximately 1957 is longer than the latest ones, so it is known as a “long body” U 47. The microphone amplifier is engineered around a much smaller vacuum tube than the large glass tube used for the CMV 3 microphone. At the time Germany was just coming out from the war and steel tubes were in plentiful supply. In particular a radio tube manufactured by Telefunken was available in large quantities. This tube, named VF 14, is a pentode having a very high nominal heater voltage (60 V DC), with a special selection and a particular powering, was perfect to develop the new microphone quickly. With a reduced heather voltage (36 V DC) and a very low plate voltage (34 V DC) the VF 14, working as triode, provides a very quiet amplification and long service life. To achieve this the available tubes were selected for noise and microphonics. They can be identified by the “M” (Microphone) marking. The microphone is powered by a single voltage and the heater is then powered across a large wire wound drop resistor. This resistor, located at the base of the housing, near the connector, dissipates about 3 Watts, and produces considerable heath. This could be a reason for the microphone longevity, because this heat generates a very low humidity environment for the capsule. Initially two large capacitors with a parallelepiped shape were placed between the microphone’s base and the output transformer. In the mid-50’s the Bosch brand developed smaller sized cylindrical capacitors so Neumann was able to reduce the microphone length and the  “short body” U 47 was born. As several long mechanical assemblies had been already produced, around 1956 many long body U 47’s had the smaller capacitors inside. At first the capsule head grille was chrome finished, but it was later on changed to the same body nickel finishing as the microphone’s body. For this reasons some the older U 47’s are referred to as “long body-chrome top”, then ” long body-satin top” and finally “short body” with satin nickel finished head grilles. Some “mixed” pieces (e.g. short body – chrome top) sometimes come up as a result of  heads having been exchanged between different microphones. It is worth noting that some “long body” microphones with the chrome top later became “satin” because, with time, the complete KK 47 head assembly were replaced with current genuine spare parts that were all satin. The “long body” U 47’s has the serial number plate on the cylindrical tube housing, while, on “short body” ones, it is located at the cone shaped base, near the socket. The capsule were originally M7 with PVC diaphragms. Later on, between 1959 and 1960 the M7 production ended,  and was replaced by the new K 47 with MCF made membranes. The output transformer changed three times. As Neumann had become the only purchaser of the VF 14 tube, it was no longer possible for Telefunken to manufacture such an old-technology tube, so they asked Neumann to make a last, free quantity, order before ending its production. In the end the U 47 (formally named U 47 N) were factory equipped with a small RCA branded 13CW4 nuvistor. A substitution nuvistor replacement kit was made available to keep all the U 47’s in service. At first the U 47 N had a very large metal grey Neumann brand logo that later became smaller, black and grey. When the U 47 was born Neumann was a small factory so Telefunken was appointed worldwide distributor and the microphones were branded with the TFK logo on the body. In the U.S. it was nicknamed “The Telly”. Such a high quality microphone was so revolutionary that RCA Victor secretly used them instead of their own RCA Corporation made ones. Elvis Presley and other important artists that officially endorsed RCA microphones, often used Telly’s behind closed doors. RCA Victor started to test German made microphones around 1954, at the beginning of the “Living Stereo” era. In 1955 Savoy records released the MG 12006 Kenny Clarke’s album titled “Telefunken Blues”. The cover shows a blond girl “sitting on” a U 47 microphone. Most of the U 47s produced are still in use today. Many words were, and are still, spent on the different sound quality of various versions, although the manufacturer never changed the name of the microphone. Some people prefer the older ones with PVC M7 capsules,  first series transformer and larger capacitors. Tone differences are often not only due to the version, but also to the “story” of that particular piece, its service history, original conditions and much more. For further details refer to the U 47 piece written by Stephen Paul.