From Tape Op: No. 25 • Sept/Oct 2001

A do-it-yourself tube mic & de-mystifying the Neumann U-47

The Head Amplifier

The head amplifier in a condenser microphone takes the audio signal from the microphone capsule, amplifies it, and sends it down the cable at a suitable signal-level and impedance. To facilitate the discussion, Figure 2 is the schematic for a U-47 microphone and its power supply, reproduced from a copy of the original Neumann prints.

Figure 2- U-47 (above)
and power supply (below)

The backplate of the capsule is connected to a 60-volt DC supply and either one or both diaphragms (selected by a switch to allow a choice of either a cardioid or omni pattern) are connected to the tube grid. When power is applied, the capsule charges through the resistor going from the tube grid to ground, until it is fully charged. Once the capsule is “charged”, any variation in the capacitance of the capsule will cause a current flow through the resistor between the tube grid and ground, which will be amplified by the tube, fed to the output transformer, and sent down the cable.

A few pesky facts of life dictated the design of the U-47 and other condenser microphones, by the way…

First, the impedance of a capsule like the Neumann M-7 is many tens of megohms at audio frequencies, which demands that the “head amplifier” be very close to the microphone capsule. Second, the tube’s output impedance is too high for feeding a long cable, so a transformer is needed to give an output impedance of roughly 200 ohms. Finally, the microphone capsule and the tube (both its heater and B+ supply) need very clean DC supplies to operate, which entails the use of a separate, dedicated power supply.

The Power Supply

The operating voltages needed for the U-47 microphone are provided by the dedicated power supply, which was designed so that when the mic is drawing its normal plate and heater current (40 mA) the output will be 105 vDC. The arrangement with the VF-14’s heater being strung off the B+ supply through the 1780-ohm resistor is unique to the U-47. The heater rating of the VF-14, 55 volts at 50mA, allows for this shortcut, which I suspect was a cost cutting measure.

So, there it is, the U-47 microphone, stripped of its mystery. What we have is a well designed combination of the three main elements in a tube condenser mic - a condenser capsule, a vacuum-tube amplification stage and a power supply - all designed to do the jobs they need to do…