Favoured by artists like Elvis Presley, Frank Sinatra and The Beatles, the venerable Neumann U47 studio microphone ushered in a new era of music recording and broadcast. Pictured here is the studio condenser microphone that I converted from transistorized to tube operation.
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It is modeled after the U47 in a popular DIY modification designed by studio engineer, Dave Royer, who adapted the original 1950s Neumann schematics to a modern-day refresh known as the Royer Mod 1.
Having just replaced a blown 5840 tube in my mic, it was a great opportunity to show some photo details. I managed to source a “new” subminiature military-grade pentode amplifier tube with a date code of 1963. So it’s a brand new tube that’s been chilling out in storage since the beginning of the cold war.
The project starts off with an inexpensive condenser microphone, the MXL-2001, with an FET front end. The modification consists of two circuit boards and an external power supply box.
I etched the boards using the toner transfer method. This is the first time I’ve ever done double-sided boards, and they turned out quite well. The top side has large areas of ground plane that really help to reduce the noise floor.
The small board replaces the preamplifier electronics in the head of the microphone itself. The power supply unit is a voltage tripler circuit that generates the B+ and filament voltages for the tube. It is fed by a 24-volt external transformer.
The microphone comes apart revealing the two original FET boards and the transformer capsule that will be replaced with the new parts.
There are several impedance matching microphone transformers available for this application. The higher the quality, obviously the better the sound. In the original modification, Dave Royer used a Jensen DB/E transformer for about $60, and there is also a Cinema Magnetics device with the same specifications.
The subminiature tube has long flexible wire legs, so they need to be soldered into place. If you absolutely need a socket, the tube will fit into a standard transistor socket, oddly enough.
The microphone must be converted to a five-pin XLR as opposed to the original standard three pin. The two extra pins will carry the 100 volt B+ and six volt filament supplies.
Once everything is wired and reassembled, connections are checked and double checked and the power is turned on. It’s best to let the tube warm up for about a half-hour to begin with.
Attached are some photos of the kit consisting of the microphone, the five-pin XLR cable, the external power supply and an outboard 24-volt AC transformer.
The new mic sounds superb considering the relatively low cost of the modification. It is extremely sensitive and has booming low-end performance, suitable for voice overs.
I find the sound to be pleasantly detailed, but natural, without any exaggerated sibilance. The bass is deep and warm and mellow, perfect for voice and not at all muddy.
Here we are in the studio, recording a new episode of Condo Talk for Nebulix Internet Radio. The microphone performed flawlessly.