Large Power supply for the modular high end audio amplifier

Project presentation

Abstract

The power supply presented here fits my high power amplifier module and is also part of my modular audio amplifier system. For lower supply voltage and lower power designs, I would recommend a smaller power supply.

Design presentation

Design considerations

The form factor of this power supply fits my modular audio amplifier system. Actually it is physically too large to fit the enclosure together with all other modules and it is also electrically way over-sized for the modular amplifier system. It was designed for the high power amplifier module and is sized accordingly. Focus is on versatility and all the small details that improve performance compared to the most basic and commonly seen power supplies.

Interface

I don't like screw terminals since I've seen the torque of the screw damaging the solder joints too often. Also, a screw may become loose or even lost and rioting inside the enclosure. My preference is cage clamp terminals. Easy to operate, good contact, accepts any kind of wire end, DIY friendly, and reliable as long as the spring does not fatigue or break. The ones I've chosen come in 5 mm pitch and can be swapped for some other kind of connectors instead, like 6.3x0.8mm Faston connectors (although I don't like them). I added plenty of those terminals for versatility. The transformer terminals accept a lot of different configurations, providing clean point to point wiring, avoiding any messy loose wires around. Transformer secondary windings can be connected in parallel or serial. The output offers many terminals in order to connect either multiple modules or to accept segregated ground and supply connections from the amplifier modules.

Rectifier diodes

The rectifier arrangement is clearly unusual as one of the bridge rectifiers appears to be superfluous. However, for highly powerful applications, this arrangement has slight advantages, discussed controversially. Since both Nelson Pass and Bob Cordell agree that the dual bridge rectifier arrangement has advantages over the single bridge, this is what I implemented here.

Basically any >400V and >40A rated diode should work well as a simple rectifier. My preference for the rectifiers are fast and soft recovery diodes. Nonetheless, diode turn off causes substantial noise emission. This is dealt with snubbers later on.

My design accepts two terminal TO-247 packages and there are plenty of them on the market. Reasonable parts start from 2€. Diodes can be either mounted to a small heat sink or to the metal chassis. However, in case of chassis mounted diodes, the mounting holes would not be on the 20mm grid system.

Snubbers

I added plenty of R-C snubbers in order to mitigate RF emissions from rectifiers switching due to resonance with the transformers secondary windings inductance. Those parts don't cost a fortune, but help to avoid unnecessary noise emission. The snubbers should be tuned for best performance, but the values chosen may be a good starting point and are better than nothing in any case. Some more expensive China power supplies may have small capacitors across the diodes, which does not completely solve the problem, but shifts it to a lower frequency range instead - at least a partial improvement. Many commercial designs I have seen so far, don't feature any snubbers at the rectifiers. Obviously it is possible to pass EMI compliance without snubbers. For commercial products built in huge quantities, shaving off a few cents is pretty common. However, I strive for best performance with high end audio performance in mind and don't want any unnecessary radiation inside my amplifier that can be tamed with part worth just a few cent.

Resonance
Capacitor parallel
Snubber parallel
Capacitors

The capacitors are the single most expensive parts of the whole assembly in any case. I selected a long life high quality capacitor series for the amplifier modules. The capacitors have 40mm diameter and four terminals with a pitch circle of 22.5mm. For terminals ensure that the pins can support the huge mass of the capacitors. I just like to built things durable.

Schematic

Assembly

PCB Design

The PCB is designed using two layers. This is the lowest layer count that makes any sense. Some DIYers enjoy etching their own PCBs. I've done that 25 years ago and I'm done with it. The performance improvement of good PCB design is significant and for such a high cost assembly, it does not make any sense to save on one of the least expensive components and in turn accept performance degradation.

The placement and routing is pretty straightforward. The only thing that stands out is the fancy meandering between the storage capacitors. The idea behind is that the current is forced to pass the capacitors in a defined sequence. Also, the resistance of the copper between the capacitors forms a low-pass circuit. As little as 3*10-3 Ω prior to a 10000µF capacitor forms a low-pass filter with roughly 5kHz corner frequency. Standard copper thickness is perfectly adequate as a bit of resistance is welcomed.

The routing to the output terminals follows the "T" approach, which forms a kind of star point. Actually two of them - one for the positive supply and one for the negative. The sequence of connection should be that more noisy grounds and supplies are connected more central within the terminal array.

Cost

Here is the cost breakdown of the large power supply module:

Part Quantity Cost p.p. [€] Cost total [€]
Storage capacitors 8 13.00 104.00
Rectifier diodes 8 2.00 16.00
Heat sinks 8 2.50 20.00
Cage clamp terminals 36 0.25 9.00
Small capacitors 14 0.50 7.00
Resistors 14 0.15 2.00
PCB 1 10.00 10.00
168€ incl. VAT

The calculation clearly shows that the DIY module built using quality parts, costs two to three times as much as the Chinese modules made of fake components. In case the rectifiers are mounted to the chassis, 20€ could be saved without compromising performance. A stripped down module could be roughly 10€ cheaper, but with some impact on performance. For me the point of DIY is not to design something that just covers the bare basics, but to include all the small details omitted in many commercial designs for profit maximization.

Summary

This is a massive power supply targeted towards high power applications in the range of 800W to 1kW. For lower power applications, the medium size power supply for the modular audio amplifier would be more suitable.

Links and references

Here are some links for further reading or entertainment:

  • Texas Instruments AN-1849 shows a basic power supply with some very basic, but maybe useful additional control circuitry and a terrible PCB layout example not to be copied in any case.
  • Rectifier snubbers explained in depth
  • DIY Buck-Boost PFC designed without dedicated controller IC. This method is basically the best way to draw a lot of power from the mains. Downside is very high complexity