Preamp Power Supply

Preamp Power SupplyThe Preamp Power Supply provides a 12 V output and can source up to 650 mA. This Power Supply features ultra low output noise which makes it an ideal power supply for preamplifiers, headphone amplifiers, and phono stages. The ultra low noise 12 V output is also perfect for the Purifi 1ET400A Hypex NC500 Input Buffer and Purifi Hypex amplifier modules. Technical Documentation Design Documentation Description Features Specifications Circuit Topology

The Preamp Power Supply provides a ±12 V output and can source up to 650 mA. This Power Supply features ultra-low output noise which makes it an ideal power supply for preamplifiers, headphone amplifiers, and phono stages. The ultra-low noise ±12 V output is also perfect for the Purifi 1ET400A / Hypex NC500 Input Buffer and Purifi/Hypex amplifier modules.  

Technical Documentation

Design Documentation

• Description
• Features
• Specifications
• Circuit Topology
• Performance Graphs

• The input to the Preamp Power Supply connects directly to the mains, and is compatible with the mains voltages and frequencies used across the world. The Preamp Power Supply is internally fused. In addition to the internal fuse, it is recommended to use a fused IEC power inlet. Doing so reduces the mains wiring to two wires which are trivial to connect.

  In addition to the main ±12 V output, the Preamp Power Supply provides an always-on, +5 V auxiliary power supply. This supply is intended for use with micro controllers and supervisory circuits, and can supply up to 80 mA. 

  The Preamp Power Supply features a +12 V compatible trigger output. This output is short-circuit protected and is capable of supplying 100 mA. This output is intended for remote control of power amplifiers, though, it is equally useful as the power supply for a relay-based input selector.

  The power on/off control is provided by an on-board, low-voltage control circuit. The control circuitry supports the use of both momentary ("doorbell") switches and toggle switches. This allows the use of the many anti-vandal switches, that are popular in DIY audio circles, as a power switch for the Preamp Power Supply. 
  

  In addition to the power switch, the Preamp Power Supply can be controlled by an external 12 V trigger. This is an attractive option when the supply is used to power the small-signal circuits of a power amplifier such as the opamps in the Purifi and Hypex Class D amplifiers. The 12 V trigger is opto-isolated which prevents ground loops.

  The Preamp Power Supply is protected against overload. This includes over-current and over-temperature protection on all outputs.
  

  To indicate the ON and STANDBY states, the Preamp Power Supply provides two LED outputs. Each output is equipped with a dimmer, which allows you to adjust the LED brightness to suite your taste. The dimmers work by pulse-width modulating the LEDs at a frequency well above the audio band. Unlike other solutions, pulse-width modulation works even with white and blue LEDs without changing the colour of the light emitted by the LED. The LED outputs on the Preamp Power Supply are compatible with both individual LEDs and with bi-colour LEDs (both 2-pin and 3-pin types).
  

  The Preamp Power Supply module is made in Canada. The two-layer, gold plated PCB is made in Toronto and assembled in Calgary. 

• The Preamp Power Supply has the following features:
  

  • Fixed ±12 V @ ±650 mA output. 
    
  • Ultra-low output noise and output impedance.
  • Mains voltage input with support for worldwide mains voltages and frequencies.
    
  • Integrated power control circuitry which supports the use of low-voltage switches, such as the anti-vandal switches popular with DIYers.
  • Support for both momentary (doorbell) and toggle switches for power control.
  • Built-in over-current and over-temperature protection on all outputs. 
    
  • LED indication of ON and STANDBY states with support for bi-colour LEDs (both 2-pin and 3-pin types).
  • Dual LED dimmer.
  • 12 V compatible trigger output with 100 mA current limit for remote control of power amplifiers.
    
  • 12 V, opto-isolated trigger input.
  • 5 V @ 50 mA always-on auxiliary power supply.
  • Rising cage clamp output terminals supporting wires up to AWG 16 (1.5 mm²) in size.
  • Compact 55 × 108 mm footprint. Fits within a 1U chassis height, including safety clearances.
  • Two-layer, gold plated PCB.
  • Made in Canada.

• The specifications for the Preamp Power Supply are tabulated below. 

  Parameter	Value	Notes
  Output Voltage	±12 V	±2.5 %
  Output Current	±650 mA	(Note 1)
  Output Noise, Positive Rail	20 µV RMS	20 Hz – 20 kHz
  Output Noise, Negative Rail	13 µV RMS	20 Hz – 20 kHz
  Output Impedance, Positive Rail	< 20 mΩ	20 Hz – 80 kHz
  Output Impedance, Negative Rail	< 70 mΩ	20 Hz – 80 kHz
  Line Regulation, Positive Rail	-107 dB
  Line Regulation, Negative Rail	-110 dB
  Load Regulation, Positive Rail	-69.3 dB (0.03 %)
  Load Regulation, Negative Rail	-59.9 dB (0.10 %)
  Auxiliary Output Voltage	+5 V	±5 %
  Auxiliary Output Current	80 mA
  Trigger Output Voltage	+11.3 V	±5 %
  Trigger Output Current	100 mA
  Trigger Input Voltage Range	6 – 16 V
  Mains Input Voltage	85 – 264 VAC
  Mains Input Frequency	47 – 63 Hz
  Standby Power Consumption	0.3 W	120 VAC
  Mechanical Dimensions	108 × 54.6 × 27.6 mm	±0.5 mm (Note 2)
  Weight	95 g

  Note 1: For best performance, the main output should be loaded symmetrically by at least ±25 mA. The sum of the positive load current, trigger output current, and auxiliary output current should not exceed 670 mA.

  Note 2: A 6.4 mm safety clearance is needed from the longest component pin to the chassis. Thus, I recommend using 10 mm standoffs with the Preamp Power Supply.

• The block diagram of the Preamp Power Supply is shown below.

  The main ±12 V output is provided by a pair of ultra-low noise linear voltage regulators from Texas Instruments. The Power Control circuit controls the two linear regulators to allow for a smooth power-up and power-down.

  All outputs of the Preamp Power Supply are protected against over-current. All regulators and the current limiter are protected against over-temperature as well. If the main output is accidentally short-circuited, the power supply will simply power down. 

• The graphs below show the measurements of the prototype build of the Preamp Power Supply. The final version will likely perform slightly better.

  The output impedance of the ±12 V output is shown below.

  
  

  As seen in the graph the output impedance of the Preamp Power Supply is very low. It is near 1 mΩ mid-band for the positive output and below 70 mΩ within the frequency range of 20 Hz – 80 kHz. The positive output has lower output impedance as the positive regulator is a tad better than the negative.

  The graph below shows the output noise of the ±12 V output of the Preamp Power Supply.

  
  

  The output noise spectrum is completely free of rectifier and mains components. Only a minuscule amount of switching noise is present, and only on the negative output. The integrated noise measures 20 µV and 13 µV for the positive and negative outputs, respectively, within the frequency range of 20 Hz – 20 kHz.

  Transient Response

  While the performance under steady-state conditions are important, it is also important that the power supply responds gracefully to load transients. The plots below show the change in the positive and negative output, respectively, as the load current is increased from 10 mA to 600 mA in a fast step.

  As the graphs show, the Preamp Power Supply responds cleanly without any tendency to ringing or overshoot.