High capacitance power supply with noise filter for the modular high end audio amplifier system

Project presentation

Abstract

The power supply presented here is part of my modular audio amplifier system. This is an improved version of the medium size low noise power supply for the modular high end audio system. The power supply presented here has higher capacitance, while retaining some of the noise filtering of the low noise power supply.

Design presentation

Design considerations

The design is mostly the same like the medium size power supply for the modular high end audio system and the medium size low noise power supply for the modular high end audio system, but with an additional storage capacitor per supply. When I added the huge toroid inductor to the low noise power supply, I wondered whether I could instead squeeze another large storage capacitor on the PCB. Finally I succeeded, but had to rearrange a lot of components in order to make it work. Once the huge inductor was gone, I immediately missed it and found a spot to squeeze in a smaller one. While the inductors used in this design are very small, they still form a low pass filter with roughly a few hundred Hertz cut off frequency. The cut off frequency of the low noise power supply with the guhe toriod inductor is roughly 130Hz. As always, compromised need to be made and I believe this is a good one.

Schematic

Assembly

Power and assembly options

The chassis is the main limiting factor for power dissipation. The chassis I chose for the modular amplifier system allows roughly 240W total dissipation.

In case the power supply is used for a low voltage class A design, this results in highest stress for the power supply. With say 30V rails and roughly 4A standing current in the OPS, the power supply needs to deliver 8A continuously when powering two channels. That is 6W dissipation per diode and results in a temperature rise of 150°C above ambient. This is clearly too much. In this case, using two of the supplies not only mitigates the dissipated power per rectifier, but also provides a more smooth supply and better channel separation. The sensible upper limit for continuous output current is at 4A to 5A.

In case of an amplifier biased into class AB, the idle current is only 1A per channel and the continuous dissipation therefore is well within the range the heat sinks can handle. Even continuous operation at high audio volume is tolerable since the average energy of music material is not very high. Amplifier in general are nut designed for sustained sine wave output at maximum volume.

Above schematic shows example values for high power class AB amplifiers with up to roughly 70V rails. Recommended parts would be:

  • Würth Elektronik TIS inductor 3.3µH 7.5A
  • Cornell Dubilier 15mF 80V (383LX153M080N082) capacitor

For lower power class AB amplifiers with up to roughly 55V rails, recommended parts would be:

  • Würth Elektronik TIS inductor 3.3µH 7.5A
  • Cornell Dubilier 22mF 63V (383LX223M063N082) capacitor

For class A operation using two power supply modules, one for each channel, recommended parts would be:

  • Würth Elektronik TIS inductor 10µH 5.1A
  • Cornell Dubilier 18mF 63V (383LX183M063N062) capacitor

Cost

Here is the cost breakdown of the module:

Part Quantity Cost p.p. [€] Cost total [€]
Storage capacitors 6 13.00 78.00
PCB 1 10.00 10.00
Rectifier diodes 8 1.00 8.00
Cage clamp terminals 24 0.25 6.00
Small capacitors 12 0.50 6.00
Inductors 2 1.00 2.00
Heat sinks 8 0.375 3.00
Resistors 14 0.15 2.00
115€ incl. VAT

Of course, the high capacitance power supply is more expensive and this is because it features more storage capacitors and those are the most expensive components on the PCB. However, the increase in capacitance results in an overall performance increase and the relative cost of all other components is lowered in comparison.

Summary

I find that this revision of the power supply is the best compromise in terms of size, performance and cost among all power supplies presented so far. Each of them is optimized for different applications and this one is balanced in the middle.