Neuman U47
If I had a time machine one of the first things I’d do would be to go back and rescue the machines they used to make the VF14 thermionic valves.
It is not a complicated design which gives the Neumann U47 its desirable sound. It is in fact a very simple capacitor microphone design consisting of three capacitors and eight resistors, a transformer, a valve, and a capsule.
Many companies have tried and continue to try to reproduce the sound of the U47 but it is still widely considered that there is still nothing quite like an original. This is down to the specific components used which combine to give the mic its unique sound - particularly the transformer, the capsule and the valve.
Apart from the first 700 mics built most U47s used the BV08 (BV for “Bau-Vorschrift” or “construction regulation”)transformer. While it is possible to reverse-engineer dimensions, turns ratios, and wire gauge of the 4 chamber BV08 transformer it is more difficult to replicate the metal of the core material. So while there are very good modern reproductions of the BV08 you cannot guarantee that they will sound exactly like an original.
The capsule could be said to be the most important part of the mic. Mine has an original M7 made with a PVC membrane. Later U47s were made with the easier to produce and longer-lasting mylar K47, a capsule which @neumann.berlin still make today. The two capsules sound different but they are both of equal high quality.
Microtech Gefell the east german division of Neumann continue to make a version of the M7 capsule and while it continues to be a fantastic capsule, reports say that the original supply of PVC ran out at some point so the current M7s don’t sound quite like the ones produced in the 1950’s. Either way Gefell only supply their capsule for their own products and wouldn’t replace the capsule in a Neumann microphone.
Pretty much all original M7 capsules have been reskinned by now as the PVC tends to dry out and shrink but miraculously the capsule in my mic is all original and still sounds fantastic, there can’t be many left in the world like this.
The hardest part of the U47 to recreate is the valve. Built from 1946 the #VF14m ceased production in 1957 and with it the U47’s days were numbered. The tooling and materials used to make them have long since been lost and prices for these valves are now extremely high and even if you were to find one, you couldn’t guarantee that it will be reliable or quiet enough for use in a microphone.
Neumann have had great success in reissuing the U47FET and U67 microphones but the reason they’ll probably never reissue the U47 is because there will likely never be a supply of VF14 valves again - unless I can get my mits on a that time machine.