Universal Filament Regulator

The Universal Filament Regulator is a power supply that features ultra-low output noise. It is intended for powering heaters and filaments in vacuum tube amplifiers but it also serves in other applications where a low-noise power supply is needed. Such applications include high-end DACs and Raspberry Pi media streamers.

A regulated filament supply results in a dramatic reduction in mains hum in tube amps. The result is complete silence between tracks and a much cleaner tone. This is especially true when used in combination with the 21st Century Maida Regulator.

Technical Documentation

Design Documentation

• Description
• Specifications
• Performance Graphs

• This fifth revision of the Universal Filament Regulator takes an AC input from a transformer and provides DC output voltages of 1.5, 2.5, 5.0, and 6.3 V selectable by a jumper. It also features a custom output voltage option where builders can add a 1206-size SMD resistor and achieve any output voltage from 1.5 to 12.6 V.

  The Regulator can provide up to 3 A of output current. Its built-in current limiter ensures a gentle startup suitable even for fragile DHT filaments. The four-layer circuit board with 2 oz (70 µm) copper thickness requires no heat sink and will fit just about anywhere with its diminutive size (76.5 x 45.0 x 24.0 mm).

  The Universal Filament Regulator is a high-efficiency, switching power supply. The switching frequency is 2.1 MHz – well above the audio band. Furthermore, it uses a dual random spread spectrum (DRSS) switching frequency dithering scheme to further minimize switching noise and EMI. It offers the same sonic benefits as linear regulators, including constant current regulators, without any of the bulk.
  

  The recommended AC input voltages for the various output voltage options are tabulated below.

  DC Output Voltage	AC Input Voltage (RMS)
  1.5 V	5 – 10 V
  2.5 V	6 – 10 V
  5.0 V	9 – 15 V
  6.3 V	10 – 15 V
  12.6 V	13 – 15 V

• The specifications for the Universal Filament Regulator Rev. 5.0 are tabulated below.

  Parameter	Value	Notes
  Absolute max. input voltage	17.6 V	RMS, AC
  Absolute max. continuous output current	3.0 A
  Output current limiter	3.5 A	Typical
  Output voltage options	1.5, 2.5, 5.0, 6.3 V + Custom	Jumper selectable
  Output voltage tolerance	±2 %
  Output current, 1.5-2.5 V	0 – 3.0 A	Ta ≤ 50 ºC
  Output current, 5.0-6.3 V	0 – 2.7 A	Ta ≤ 50 ºC
  Output current, 12.6 V	0 – 2.4 A	Ta ≤ 50 ºC, Custom Vout
  Minimum load current	0 A
  Output noise	1.7 mV, RMS	800 kHz BW
  Output impedance	7 mΩ	DC
  Line Regulation	76 dB	DC
  Load Regulation	43 dB	DC
  Switching frequency	2.1 MHz	Dual random spread-spectrum
  Dimensions & weight	76.5 x 45.0 x 24.0 mm 50 g

• The output impedance of the Universal Filament Regulator Rev. 5.0 is shown below.

  

  As seen in the graph, the output impedance is very low throughout the audio range. This means the output voltage of the Regulator will vary by only a minuscule amount as the load current changes.

  The output noise spectrum of the Regulator is shown below. Note that even the tallest peaks are still in the µV range. The total noise sums up to 1.7 mV (RMS) across an 800 kHz measurement bandwidth.
  

  

  And finally the step response of the Regulator. The measurement below shows the change in output voltage as the load current is changed from 1 A to 2 A. As seen, the regulator responds to the changing load very quickly and without tendency to overshoot.