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Are switchers a bit of a turn off?

Idea: Replace Granite Digital (GD) PS' with one "dirty" ATX PSU. That is cMP² now has 2 ATX PSUs. This "dirty" PSU powers all "dirty" peripherals as before (as was done by the GD PS') and the CPU.

cics’s post confirm yet again that PCs dedicated to music replay respond well even to modest improvements in the supply and distribution of power to their components.

The topic has been discussed at length on AA with suggestions ranging from the absurdly simple (smoothing the P4 line) to the intimidatingly complex (an all-battery system with power-on sequencing and elaborate soundcard mods).

However, all but the most rudimentary of a hierarchy of designs (see www.audioasylum.com/forums/pcaudio/messages/53957.html ) call for some competence in electronics and access to specialised tools. Many would-be users have neither.

It is a merit of the core cMP^2 design that it uses stock components and can be built by those with few constructional skills. However, although the proposed upgrade using two ATX PSUs maintains that low-skill approach (and gets switchers and mains power out of the case), it doesn’t follow that it is the best solution even for those without access to core skills.

In my opinion and with all due respect, even before plugging in a stray CD-ROM drive (or a few power resistors?) to keep switchers switching, it’s entry-level at best. In fact, it’s a bit of a kludge.

I’ll try to explain why I think that but would first like to clarify a couple of basic points.

** A 4-pin, 12-volt connector (the P4 or Pentium 4 line) was added to the ATX spec in February 2000 to power the CPU. If manufacturers now use it to power other components, they are arguably wandering off spec. Despite what it says in the manuals, I doubt that P4 powers anything on Gigabyte motherboards other than the processor and perhaps its cooling fan. As a cMP^2 system typically has no fans, it’s reasonable to assume that it powers only the CPU.

** ATX spec revisions in 2003 inter alia acknowledged that nowadays most power is required on the 12 V rails: power on 3.3 V and 5 V rails was reduced and -5 V was scrapped. As www.silentpcreview.com explained in 2005:

The high reliance of current systems on the 12V is dramatic compared to even just a couple of years ago and the evolution of the ATX12V spec reflects this change. Almost any system assembled from current components will draw the vast majority of current from 12V, in some cases, as much as 90% at load. (http://www.silentpcreview.com/article28-page2.html )
In short, a modern motherboard is essentially a 12-volt device even if it needs power-sequencing circuitry and a couple of ancilliary lines.

(For anyone sad enough, like me, to find these arcane topics mildly interesting, Intel specs are the official source but material on Wiki and elsewhere is more readable.)

++++

1. I have difficulty with the concept of the CPU as a “dirty” device that can be powered with impunity from a noisy source. First, sound quality is widely recognised as being improved at least as much by smoothing P4 as P24, arguably more. Second, there is no evidence to hand that a polluted P4 line will not pollute the supply to other, perhaps more noise-sensitive, devices. Separate SMPSs may to an extent mask that but it does not follow that they are the optimal solution.

2. The power demand of an under-clocked, small-format motherboard with a 45nm CPU is modest – typically under 25 watts. Allowing for accidental reversion to default BIOS settings, a 3-amp, 12-volt linear supply is likely to be adequate and a 5-amp one certainly is.

There may well be SMPS designs with better noise specs than off-the-shelf linear units but not ATX ones. The experience of several builders of cMP^2 and other systems is that a half-way decent linear supply outperforms even the best ATX designs in this context.

3. A Pico-PSU designed for use with a 12-volt regulated supply (i.e. not for car batteries) uses switching only for ancilliary lines. Even without additional smoothing (à la Greg), they are reported as bettering conventional ATX units. ( See: htttp://db.audioasylum.com/cgi/m.mpl?forum=pcaudio&n=47156 .)

A 12-volt “laboratory”-style PSU driving the P4 line directly and some P24 lines via a Pico PSU requires virtually no constructional skills to implement, costs much the same as two decent SMPSs and will comfortably outperform them. (Just about any 5-volt linear supply will perform as well as a Granite Digital switcher, perhaps better, for the “dirty” kit.)

Of course, one ideal would be to get a PCB laid out for John Swenson’s low RFI, low ripple design. Offers?

Just a few thoughts - sorry to go on for so long . . .

Dave


Edits: 11/16/09 11/16/09
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