There are still a considerable number of the original, refurbished or "cloned" Neumann U-47 microphones in use - all around the world. Modern large-diaphragm microphones have still a hard time competing with the "deep" and "warm" sound from the original U-47.
An original and functioning power supply can be hard to find. And if you find one - it probably has dried electrolytic caps and can even be hazardous due to very old components and insulations. The original PSUs had some measurable output voltage ripple due to the purely passive (LC-filter) voltage regulation. There is a need for a modern and suitable power supply, especially if it can be built to perform better than the original, and with non-exotic components.
The power supply and its outputs and cabling contain lethal voltages. I cannot be kept responsible for any injuries or deaths caused by not following proper safety measures. Please note that high DC voltages are especially dangerous!
This project is not intended for beginners.
Fig.1 Prototype testing with a incadescent bulb as a load.
Fig.2 Complete schematic of the power U-47 supply. Pardon for the bad quality picture.
The raw AC voltage needed as input for this circuit comes from a transformer, a toroidal transformed is preferred due to the small stray magnetic field. This transformer must have a 110..120VAC secondary, and suitable primarie(s) - according to your regions voltage (100/115/230VAC). The transformer is essential to guarantee isolation, even if it would be tempting to run directly from a 115VAC mains.
The transformer power rating should be chosen accordingly, about 8 Watt per microphone. Feeding 6 microphones requires a 50 Watt transformer.
The transformer output is followed by plain vanilla full wave rectifier (D1-D4, use >= 1A and at least 200V types. Using discrete diodes any diode in the 1N4003...1N4007 would do. To prevent transformer secondary ringing at the cutoff during each mains cycle, it is recommended to connect a 100nF/250VAC polyester or polypropylene capacitor across the input of the rectifier bridge (NOT shown in the schematics).
The rectifier output is connected to the smoothing electrolytic capacitor C2. This capacitor is rated at 330uF/200V for 6 microphones, but the capacity can be reduced if drive fewer mics. A single microphone PSU could do with a 68uF/200V capacitor.
The trick how the small output ripple (<1mV) is achieved is to connect TWO regulators in series in a manner where the second regulator regulates the first regulator's reference! In this circuitry the first regulator is an N-channel high voltage power MOSFET transistor M1 (IRF450). This transistor handles the pre-regulation and handles the majority of the high voltage drop. The second regulator - that regulates the final voltage - and limits the maximum current - is the well known LM117 (or the cheaper equivalent LM317 with worse voltage precision).
The maximum allowed voltage across this regulator is 40 volts, but is limited in this circuit to about 12 volts. You can use the LM317 but you have to trim R67 to achieve a precise +105VDC at the output.
Fig.4 Detail of the assembled prototype.
Fig.3 Detail of "piggyback" mounting of the LM117 regulator "thermally" on top of the MOSFET. Make sure to isolate the regulator tab from the screw, using a plastic washer designed for this purpose.
Fig.5 Layout of the prototype. Note the placement of the main parts. An mains differential filter is strongly recommended, as shown here.
To be continued...
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