copyright 1997 by Eddie Ciletti

Unlike Ed Colverís artful technique of capturing classic "transducers" featured monthly in EQ MAgazine, this undercover edition chose the sensational approach, using a hidden Hi-8 video camera to expose the secret life of a classic mic.  Welcome to Microphile Undressed.  A mere student of microphone technology, I am a very lucky enthusiast, especially when an acquaintance brings two extremely mint condition Telefunken U-47s to my shop for evaluation. 

At one time, Telefunken distributed microphones made by Georg Neumann as well as those by AKG.  Some were "customized" versions tweaked to Telefunkenís specs, others were completely "stock."  In case you didnít know, the name is pronounced "noy-mahn," not "new-man," like that guy on the Seinfeld show. 

One of the first discoveries were the "tampering" indicators ó aluminum discs embossed with the Neumann "N" covering the access screws.  (See the inset on the lower right corner of the picture.)  Neither microphone had ever been opened.  Even the output connector on the power supply was the original, three-blade Tuchel.  Knowing that certain collectors focus on the degree of "originality," I was very careful to make only the necessary repairs in order to maintain value.  With the exception of switching to an XLR connector (on the power supply), I left the working mic untouched. 

The U-47 and the U-48 are identical microphones, the former offering Cardioid and Omni patterns while the latterís options were Cardioid and Bi-Directional (also known as Figure-of-Eight).  Both utilize the same, large, dual-diaphragm capsule.  Early capsules used PVC (poly-vinyl chloride) as the diaphragm material, coated with a molecularly thin layer of gold. 

Unfortunately, PVC didnít age well.  The most common failure was separation of the gold layer from the plastic resulting in a loss of output.  The ripples in the picture indicate that this particular diaphragm came unglued from the capsule.  Neumann eventually switched from PVC to Mylar, choosing to clamp the plastic with a retaining ring rather than use glue. 

Needless to say, even my own normally skilled hands were a bit more tremble-prone.  Unlike Phillips head screws which securely mate with their drivers, all of the U-47 screws required a flat bladed screwdriver which was more likely than not to wander off the head.  Since the cases were literally perfect, I couldnít afford to make a single scratch.

Even though one mic seemed to be in working order, odds are the its capsule would fail sooner than later. And so, being a symmetry freak, my first thought was to have both capsules rebuilt.  This, however, was not an option until the day before this article went to press.   Mark McKenna at The Bearville Studios turned me on to Tracey Korby (412-937-1349) who does capsule rebuilds ($600) as well as microphone repairs. 

Neumann (860-434-5220) does sell replacement capsules for about $900, but these are the more modern, mechanically clamped versions.  I wanted the mics to remain as close to the original as possible.  Another option is Stephen Paul Audio (818-905-9952) where a complete overhaul goes for about $1600. 

I eventually found a one-time alternate source for the M-7 capsule so that, with parts and labor, the job was completed for about $900.  A steady hand is even more crucial when changing the capsule wires.  One slip and the capsule would be toast!  These are not, in any way, user-serviceable items. 

Since these mics were rarely, if ever used, both VF-14 pentode vacuum tubes were in great shape. And yes (for those in the know) each had an "M" stamped on the case indicating the tube had been factory tested and selected for low noise.  A VF-14 is more rare than an honest politician. (None of the former were made after WWII and none of the latter ever existed!) 

Joe Leung at Gotham Service Labs (212-967-3120) tells me there are similar pin-compatible tubes ó the EF-14 and the UF-14 ó which are somewhat more available and less pricey (about $350).  This approach requires no mechanical modifications although a few circuit changes are necessary.  At one time, Neumann made a Nuvister "kit" which adapted a subminiature tube to fit in the original socket.  Nat Priest (212-343-0265) created his own version of this vacuum tube alternative for under $200.  Stephen Paul Audio also has a more modern, nine-pin upgrade. 

Any aged electronic device is likely to require some "maintenance" even if never plugged in.  This is especially true of electrolytic capacitors.  So far, however, no parts required changing but both mics are under observation for signs of potential failure. 

I did, however, increase the value of the polarizing and bias resistors from 100 meg and 60 meg, respectively, to 160 meg ohms each.  The original values created a gradual low-frequency roll-off and, based on a suggestion given by David Josephson, I made the changes while keeping the original parts.  Not only was David especially patient with my numerous emails, he also suggested a great primer on Microphones by Dr. Gerhart Bore. (the "e" should have an accent mark over it)  This book was distrubuted by Neumann, but may no longer be in print. 

Itís not difficult to see why the U-47 remains one of the popular vocal mics.  Warm, but not murky, present but not irritating, you can easily imagine itís impact on popular music back in the fifties.  It certainly  contributed to the "High Fidelity" sound of that era! 

PS  Thanks also to Russ Hamm and Phil Kapp at G Prime (212-765-3415).