"Upgrading" computers with linear regulated power supplies

Introduction

Computers have become a popular platform for audio use. Lately there has been a lot of interest by audiophiles to "upgrade" their media computer by replacing the switch mode power supply (hereafter abbreviated as SMPS) with a linear regulated power supply unit (LPSU), because the latter tend to have lower noise characteristics.

Before going into whether such a modification makes any sense, it is useful to understand the pros and cons of each power supply type, and how they may or may not apply to computer motherboards.

SMPS vs. LPSU

Computers are usually powered by SMPS because this type of power supply is much more energy efficient, with low thermal losses. This is important because a computer motherboard can require a substantial amount of power. Linear regulated power supplies, by contrast, are quite inefficient. A fair amount of the energy is lost in heat dissipation. The benefit of a LPSU is of course lower noise, but this depends greatly on the specific LPSU and its implementation.

Computer motherboards can require anything as small as a 50W power supply, or something up to 1000W, depending on the type and processors onboard. Peripherals such as disk drives and other add-ons add additional load to the power supply. While 50W is not a difficult load for the proper LPSU, several hundred watts is a problem due to the aforementioned heat issue.

Multi-voltage

Laptop computers and some small form factor single-board computers (such as the Raspberry Pi) are usually powered by a single voltage. Most desktop and server computer motherboards (such as the popular *ATX varieties) require several supply voltages (+5V, +12V and -12V) that are easily handled by an off-the-shelf SMPS of the appropriate power rating and form factor. However, when it comes to replacing such a multi-voltage unit with a LPSU, things get a bit more complicated. There is no off-the-shelf LPSU in any standard ATX form factor, so many hobbyists have to build such a power supply from scratch or in the form of kit pieces or modules. A large multi-secondary power transformer, or several separate transformers would be needed to feed separate linear regulators for the different output voltages. The high thermal loss means large pass transistors (or MOSFETs) mounted on substantial heatsinks, in addition to fan cooling. High current linear regulator circuitry are also complex in nature, on sizable circuit boards or modules. All these adds up to something that probably would not fit in the space normally occupied by the SMPS.

The fundamental question

Is a lower noise power supply beneficial for a computer? This is the fundamental question that must be answered. It turns out that the only things that may benefit from a low noise power supply in a media computer is the master clock that feeds the digital-to-analog converter (DAC), the DAC itself, and any analog audio circuitry thereafter, usually in the form of operational amplifiers (opamps). Having a low noise supply for the DAC-related circuitry theoretically minimizes jitter (and this depends on the design and implementation of the DAC), and prevents power supply noise from polluting analog audio performance.

For the rest of the computer, whether it is the processors, memory, support chipsets, expansion bus, disk drive interface, networking, serial/parallel I/O, USB or other circuitry, a low noise power supply makes absolutely no difference.

One might think that using a LPSU on the whole computer can't hurt, and it might produce some tangible improvement even if much of the circuitry doesn't benefit from it. While that seems valid on the surface, but the reality is a bit different. Different portions of a computer motherboard requires different supply voltages, some that are not provided by the ATX power supply's standard +5V, +12V and -12V. So a computer motherboard utilize onboard DC-to-DC convertors to create the supply voltages needed. These converters are by nature switching devices, they also produce supply voltages that are noisy. It is highly likely that the DAC and opamps are powered by a DC-to-DC converter. Thus, it doesn't matter how quiet your LPSU is, by the time the supply voltage gets to the master clock, DAC and opamps, they are re-polluted by noisy DC-to-DC converters, and the LPSU "upgrade" accomplished nothing!

The solution

The best solution is to use a high quality external USB DAC with its own LPSU. This avoids the wasteful use of a big LPSU on the computer, and targets a smaller LPSU where it has real meaningful benefits. It also bypasses the computer motherboard's onboard audio, which is usually substandard in quality anyway.

by Ti Kan, last revision: 11/27/20 02:14:38