Thinking of buying a diual-rail PSU? Think again...

Richdog · 1 Mar 2006 at 21:04

http://www.theinquirer.net/?article=29940

Single rail PSU solves RD480 Crossfire problems

Xtremesystems, Sampsa, ATI and support guys confirms it

By Fuad Abazovic: Monday 27 February 2006, 21:32

WE have a solution for problems that we have had with A8R-MVP motherboard and two graphic cards. You can check the original post here. We spoke with many people today and they all indicate that it has to be the power problem.
ATI key motherboard designer John Bruno told us that a single rail PSU is better than a multi rail PSU. He reported that ATI had some issues with multi rail PSUs while all of the single rail 500 W + PSUs were running fine. It turns out that both OCZ Power stream 600 W PSU and Akasa 650W PSU are using more than one power rails.

One of the readers an administrator of a motherboard company forum recommended ENERMAX EG651P-VE FM(24P) PSU with 36A and a single12 V rail. Our colleagues at Bit tech managed to test dozen X1900 cards including Crossfire with a single rail 520W OCZ PSU. You can check the article here.

We got a nice link where one of the top overclocker, our good friend Sampsa had the same problem with 600W multi rail PSU and A8R-MVP board and two X1900 cards in Crossfire. He solved its troubles by switching to a single rail 560W PSU. You can check the post here.

This specific problem reminded us on the same problem that we had with two Geforce 7800 GTX 512 cards. Those two cards in SLI were crushing the machine while two 7800 GTX 256 MB cards worked flawlessly. You can check it out here and we have been working for two weeks with Nvidia to solve the problem but failed to make it work. Someone simply could suggest that we should try single rail PSU 500+ W and everything should work. SLI works with two 7800 GTX 512 after all.

Hurray, now we know that Nvidia and ATI should warm people that they need single rail PSU not just any 600 W PSU. That really makes the difference. We are speechless why no one warned the users about this issue.

Despite we have two 600+ W PSU we are back at the spot where we need to get a new PSU for the lab to test those power hungry rigs. Asus A8R-32 MVP Deluxe board powered with RD580 chipset can even work with multi rail OCZ 600 W or Akasa 650W PSU as we guess it is distributing the power better than the previous one. A mystery solved, but beware if you are buying top motherboard SLI or Crossfire and you want to plug two power greedy cards inside. You need a lot of quality single rail power, not just any power.

This is a VERY significant report imo, and one that confirms the fears and opinions I have had for a long while now. Dual-rail PSU's just don't have what it takes to effectively new, power-hungry uber-rigs, confirmed by some of the biggest names in OC'ing.

To me this says if you're thinking of buying a new PSU now or in the near future and don't want to worry about something as dramatic as not being able to power your rig then get a beefy single-core... avoid the duals.

EDIT - That's just my take on it, others may have different opinions of course...

Richdog · 2 Mar 2006 at 11:04

I was talking to a guy called JonnyGURU last night on my home forum, he's a well-known PSU reviewer and expert and he was somewhat disgruntled with the article The Inq posted. He said:

jonnyGURU said:
What stinks about that article is that they don't go into any detail about how different power supplies split loads up differently in different PSU's.

Most PSU's put both of the PCI-e's on the 12V1 with all of the drives, lights, fans, etc.

Others, like the Enermax I thought, put one PCI-e on each rail.

According to madmat at TechGage, the X2 actually puts the PCI-e's on 12V2 with the CPU.

And some power supplies, like the NeoHE, actually give you a third rail just for PCI-e. The Silverstone Strider ST60F gives PCI-e their own rail out of the four 12V rails.

The article makes blanket statements that are going to make everybody knee jerk into assuming that all dual 12V rail power supplies are bad for SLI/Crossfire configurations without looking into the how and the why.

It makes me wonder if they even know how multiple 12V rails work.

So basically what that means to me is that some multi-rail GPU's can do it no problem, but seeing as manufacturers are churning out multi-rail PSU's at a rate of knots how can you be assured that the dual-rail you are getting cuts the mustard?

Sure, that £120 dual-rail with 25+ on each rail made by a top brand may be alright with the latest gear but what about the cheaper, more "budget conscious" models of £80 and lower?

I really don't care what the argument for dual-rails is, i'm staying single all the way...

Richdog · 2 Mar 2006 at 12:59

What are the combined rails of the Tagan 580 then?

Richdog · 2 Mar 2006 at 14:36

MikeTimbers said:
TG580-U15 can combine its rails by using the back of PSU switch. 20Ax2 or 35A combined.

http://www.tagan.de/pages/products/easyconS/TG-580-U15_Tagan_UK_mini.pdf

So it functions as a true single rail, exactly as any regular single rail would? 35A is impressive, enough to make me buy one...

Major_hangover said:
with the exception of a few highly overclocked set ups.

That's a big enough exception for me!

Richdog · 2 Mar 2006 at 16:30

MikeTimbers said:
TG580-U15 can combine its rails by using the back of PSU switch. 20Ax2 or 35A combined..

Website specs state combined 12v is only 30A.

Richdog · 3 Mar 2006 at 07:45

lay-z-boy said:
The manual and the psu state 35a here.
.

I think that's the max possible figure, not the average.

Richdog · 3 Mar 2006 at 11:21

Agr3sive said:
Stick 2 in a machine with all the extra bits that a high end user has and there still using power supplys that havent really progressed at all in the last 3years, the average supply is still in the 400-500w range with only a slight amp increase, for these cards to shine we need to see big (700-1000w units with around 50amps) power supplys hitting the market.

Nah, with high-end crossfire + dual-core systems with an average amount of components/devices all you need is 34A on a single rail and it will cope fine.

Quad SLI then yes you are then probably looking at ~40A needed but to me that's a non-issue as I would never be silly enough to buy it.

------------------------------

Regarding the Tagan combined rails, the official Tagan specs .pdf says 32A combined http://www.tagan.com/pages/products/2forceS/TG-580-U22_Tagan_UK_mini.pdf

Think i'll wait till they (hopefully) bring out a 630w (or whatever) model...

Richdog · 3 Mar 2006 at 12:33

Agr3sive said:
They need more than that mate. Even ATI say that 38A is the minimum they reccomend for a crossfire rig, thats before you oc the cards to the max and introduce a heavy oc'ed dual core into the equation which needs more power still.

http://www.ati.com/products/RadeonX1900/specs.html

I have a mate that has dual X1900XT's and a 3.0ghz FX-60 running on a PCP&C 510 (he's a crazy bencher) with no problems whatsoever. The quality of the PSU counts a hell of a lot.

Dutch Guy said:
What I don't understand is this bit from this review:

With that quote, i'm not sure they were taking uber-rigs with lots of devices into account.

Intel stated a dual-rail specification requirement and have since backed away from it slowly.

Richdog · 3 Mar 2006 at 15:16

Agr3sive said:
If ATI say you need 38amps to run crossfire on a average rig, then to run it on a rig of someone like Sampsa whose machine is voltage modded in every known form, running g-cards at 1000/2000 and pushing cpu's to there limit you probly need a psu that can supply around 50a.

The guy I told you about, Dynasty, has a system like that, volt-modded to **** and phased. He is in the top 10 on the orb. There's no way you need close to 50A for a rig like that, and as I said, he runs it on a PCP&C just fine.

Here's a quote from a good thread about PSU requirements http://www.hardforum.com/showthread.php?t=1017620&page=2

Now we get into dual PCI express video cards. Let's say you're using SLI or ATI's Crossfire. One would think that two video card means twice the power requirement, but that's actually not true for the same reason as your performance not doubling when you have two video cards. The additional power requirement for the second video card is still fairly significant, though.

* Add 4A if you have a pair of video cards and neither of them have their own power connector.

* Add 6A if you have a pair of video cards with a power connector for each.

As you see, not as much as you;d think. Volt-mods also add fairly little to the overall loads in the grand scheme of things.

I would imagine that if you hooked up one of those rigs to a pcp+c 1000w which slams out 66a with a peak of 70a, then there would be no problems using a multi rail psu.

One problem with massive PSU's like the 1kw is that to prevent cross-loading issues you really need to be utilising a whole lot of its power in order for there not to be problems. You would need an uber-rig of uber-rigs to justify one, and even then it would likely be excessive.