U47 Powert supply

Neumann U-47 Condenser Microphone Power Supply (for up to 6 microphones)

Highlighted features

  • Precisely regulated +105V DC 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.
  • 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 a hard time competing with the "deep" and "warm" sound from the original U-47. 

An original functioning power supply can be hard to find. And if you find one - it probably has dried out 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. So 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.

Fig.1 Prototype testing with a incadescent bulb as a load.

  IMPORTANT WARNING  

The power supply and its 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!


Fig.2 Complete schematic of the power U-47 supply.

About the circuit

The raw AC-voltage needed as input for this circuit comes from a transformer, preferably a toroidal type (due to the small stray magnetic field). This tharnsormer would havea an 110..120VAC secondary, and suitable primarie(s) according to your region voltage (100/115/230VAC). The transformer power rating should be chosen accordingly, qbout 8 Watt per microphone you indend to drive. Feeding 6 microphones thus requires a 50 Watt transformer. 

The transformer output is followed by plain vanilla full wave rectifier (D1-D4, use >= 1A and at least 200V type. Using discrete diodes any diode in the 1N4003...1N4007 would do.  The rectifier output is connector 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 to achieve the small output ripple (<1mV) 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 a N-channel high voltage power MOSFET transistor M1 (IRF450). This transistor handles the pre-regulation and handles 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 allower 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 +105VDC at the output.


Important notes

  • ​The power MOSFET must be mounted on a heatsink and must be properly isolated (Silpad or similar). Especially if the chassis is used as the heatsink. Even with a separate heatsink its a good idea to isolate the pass transistor.
  • Mounting the LM117 regulator on top of the pass transistor provides an additional safety and overtemperature protection (the pass transistor heats up the regulator that goes into a protection mode if these run to hot).
  • 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 use, especially if the mains voltage is contaminated with noise (AC dimmers etc.).
  • Do not use thermally melted glue for assembly of any components! The stuff visible in Fig5 is epoxy.




To be continued...


Fig.3 Detail of "piggyback" mounting of the LM11/ regulator on top of the MOSFET.

Fig.4 Detail of the assembled prototype.

Fig.5 Layout of the prototype. Note the placement of the main parts. An mains differential filter is strongly recommended, as shown here.

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