Linear Low Noise Analog Power Supplies – DIY

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Updated test results – 09/28/2021, replaced original 9VAC transformer with 14VAC transformer.

As a young teenage electronics enthusiast and licensed UK radio amateur (G8GBL), and all the way up to my early forties, buying any piece of pre-built equipment; stereo amplifiers, head amps, turntables, tuners, speakers, receivers or transmitters etc. was often beyond my budget. So I learnt to design and build them. So having just purchased an analog power supply, and now having lots of time on my hands, I decided to wind the hands on the clock back in time and build my next power supply.

Analog power supplies, even low noise audiophile quality, are not complicated, and can readily be built from many established designs with relatively low cost components. That is assuming that you have the aptitude, desire and resources to do so. What I needed was an adjustable 5-12 volt, 1.0 amp, low noise analog power supply. Unfortunately I only had a transformer that would just support 9volts at 1.0amp so I used that for the original project. I will not go into the technical design and its operation, but I will provide an overview of the component functions.

Remember that I built this PSU as a fun exercise and around my available component stock. Given a ‘clean sheet’ the design approach would have been rather different, especially if very low noise was required. Also the readily available foil boards are NOT ideal for high currents or foil track layouts. An optimally designed PCB would have provided lower noise and improved the voltage drop figures.

Rear view

Top View (The toroid was replaced with a dual 7 volt winding to give 14VAC)

Schematic Design:

Component Functions:

1. IEC AC socket and filter (fused) – removes incoming AC power noise.
2. T1 – Toroid transformer – provides the required AC voltage and current:
   • Very low strayfield
   • High efficiency
   • Low standby current
   • Extremely low level of radiated magnetic field
   • Very low induced noise (hum)
   • Very low iron loss
3. BR1 – Bridge rectifier (4 diodes) – converts AC to DC (high speed or Schottky diodes will reduce switching noise)
4. C1 thro C4 – absorbs peak inverse voltage and reduces bridge rectifier diode noise
5. C5/6 & C7/8 – main DC smoothing capacitors to reduce AC ripple and noise – L1 creates a Pi filter.
6. C9/C10 – reference DC smoothing capacitors to reduce AC ripple and noise – L2 creates a Pi filter.
7. C11/C12 – final DC storage capacitors to provide low impedance and clean, noise free DC – L3 creates a Pi filter
8. C13 thro C16 – LF noise decoupling
9. C16 thro C24 – HF noise filtering and decoupling
10. C25 – op-amp HF compensation
11. L1/L2/L3 – DC smoothing Pi filter inductors
12. L4/L5 – display HF noise filter inductors
13. REG1 – output voltage regulator and current limiter (ultra low noise linear power supplies may use multiple LT3042/45 regulators)
14. REG2 – reference circuit voltage regulator (a TL431 or LT6657 may be used to generate the required reference voltage to drive REG1)
15. D1/D2 – reverse voltage protection for the regulators
16. AMP1 – operational amplifier (unity gain) presenting a low impedance current sink and reference voltage for the output voltage regulator REG1
17. VR1 – potentiometer to set reference voltage range
18. VR2 – potentiometer to set the DC output voltage
19. SW – on/off display switch
20. The aluminum case helps reduce RFI noise and provides the required heat sink for the regulator REG1

If currents much above 1.0 amp are required there are several approaches to modifying this design:

1. Parallel multiple REG1 regulators
2. Add a high power bipolar junction transistor (BJT) or field effect transistor (FET) to offload the higher current demand from REG1
3. Use a high power BJT or FET instead of regulator REG1 – a different design approach that also creates a lower drop out (LDO) regulator.

The transformer and possibly some components would also need to be upgraded to meet changes in voltage and current requirements.

Bill Of Materials (BOM):

Italicized components indicate already available in my workshop.

• Case – Amazon
• PCB – Amazon
• Display (with bezel) – Amazon
• Transformer by Talema – Digikey (2x7VAC & 1.785 amps) – In series for 14VAC.
• DC connector (cable)- 16280-2PG-318 – Digikey
• DC connector (panel) – 17280-2SG-300 – Digikey
• IEC filtered AC power connector (double fused) – Schaffner FN9260S-2-06-10 – Digikey 
• AC power switch – Uxcell SPST 2 Position ON/OFF Rocker Switch AC 125V/12A – Amazon
• Electrolytic capacitor C5 thro C10 – NRSZ, 2200uF, 25VDC 
• Electrolytic capacitor  C13 thro C16- MUSE, 47uF, 63VDC
• Capacitors C1 thro C4 & C16 thro C24 – polyester 10nF, 63VDC
• Capacitor C25- polyester 220pF , 63VDC
• Bridge BR1 – KBU8G – 8 amps, 280 VAC-RMS 
• Inductors L1 thro L5 – DIY high current, ferrite core
• Regulators REG1/2 – LM340T5 – 1.5amps, 5VDC
• Regulator insulators & heatsink – Amazon
• Diodes – 1N4005 – 1.0 amp, 600VDC
• Op-amp – NE5534 (5532 or 741)
• Resistors R1 thro R3  – carbon film 0.125 watt
• Potentiometers -VR1/VR2 – 1K Lin, Bournes trimmers
• Rear power LED
• PCB connectors – various
• Various mounting and assembly hardware
• Case feet (stick on)
• Various assembly cabling
• IEC power cable
• DC power cable with ferrite bead and DC connector

Test Results – (At the case terminal & display OFF)

These updated results are using a 14VAC (2x7VAC) transformer instead of the original 9VAC transformer.

All measurements: Leader LMV-185A AC Millivoltmeter BW: 5Hz-1MHz, Tektronix T935A oscilloscope BW: DC-35MHz.

• Voltage range: adjustable 5 to 12 VDC
• Max current: 1.0 amp @ 5 to 12 VDC
• Noise+ripple level @ 12 volts:
  • 0.035mV (35.0µV) RMS @ 1.0 amp: -111dB relative to 12 volts. 
  • 0.005mV (5.0µV) RMS @ 0.5 amp: -128dB relative to 12 volts

• Voltage drop at case connector @ 12volts (copper foil DC resistance restricted):
  • 1.0 amp: 30mV
  • 0.5 amp: 15mV

Noise+ripple comparison to the two commercial analog low noise supplies supplies in this post:

• 25VA supply (modified): 0.14mV (140.0µV) RMS @ 0.5/1 amp: -99dB relative to 12 volts.
• 100VA supply: 0.07mV (70.0µV) RMS @ 0.5/1 amp: -107dB relative to 15 volts.
• Noise & ripple with displays on:
  • 25VA – 0.2mV (200.0µV) RMS: -96dB
  • 100VA – 0.9mV (900.0µV) RMS: -85dB

Conclusion

Despite the less than optimal layout and track foil size this basic PSU design significantly improves upon the low noise performance of the two ‘commercial’ supplies tested earlier. Lower noise figures would be obtained using an optimally designed printed circuit board and larger values of C5 thro C8.

NOTES:

• All of the LED voltage displays digital micro-controllers generate very significant electrical noise. Even after filtering the displays power feed, this low level noise could still be measured at the main supply terminals, so a rear switch was installed to turn off the display. (This switch has since been added to my other two commercial supplies)
• Noise in ALL three power supplies is predominantly 120Hz power supply ripple.
• This style of regulator (REG1/2) is not low drop out (LDO). It requires its input voltage to be at least 2.5 volts higher than its output voltage in order to operate.

Ultimately this DIY low noise supply was used to replace the Little Green Computer DC supply due to its significantly higher noise.

Electronic Component Suppliers:

• Newark Electronics
• Mouser
• Digikey