U47 Power supply

Neumann U-47 Condenser Microphone Power Supply

 Highlighted features

  • Precisely regulated +105VDC voltage
  • Less than 200uV output ripple & noise
  • Capable of driving simultaneously 6 U-47 microphones (50W of power)
  • Foldback current limiting - withstands constant short circuit on all outputs.
  • Uses standard and easily available components.
  • Soft turn on
  • Low parts count

Background

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.

  IMPORTANT WARNING  

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.

About the circuit

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.

Important notes

  • ​The power MOSFET must be mounted on a heatsink and must be properly and carefully isolated from the heatsink (Silpad or similar). The MOSFET drain (the body) sits at the input voltage potential. If you use the chassis as the heatsink (or have a separate heatsink that is NOT isolated from the metal chassis) then also protective earthing is MANDATORY. It's a good idea to isolate the pass transistor even when using a separate isolated heatsink.

  • Mounting the LM117 (or LM317) regulator on top of the pass transistor provides additional safety and overtemperature protection (the pass transistor heats up the regulator that will go into overtemperature protection - if these run too hot together). The regulator metal lug MUST be isolated from the screw, especially if the screw connects mechanically to the chassis.

  • The output voltage must be verified and Rxx adjusted accordingly to achieve +105VDC. Do NOT use a trimpot to guarantee stability and low noise!

  • A common mode mains filter is recommended for serious studio use, especially if the mains voltage is contaminated with noise (AC dimmers etc.).

  • Do not use thermally melted glue for the assembly of any components! The stuff visible in Fig5. used for parts mounting is durable two-component epoxy.

  • Make sure that no mains-voltage carrying wires can come in contact with the secondary side wiring if they would become loose.




To be continued...

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