New PSU for Crossfire?

[WarWolf]

I currently have a SeaSonic M12II 620w running a XFX HD7950 and i5-3570k.

I'm considering picking up a second 7950 in the near future. Problem is, my PSU only has one 6pin and one 6+2pin, and from the looks of it 620w would be cutting it close with overclocking.

My questions.
1. For the problem of the connectors, I've seen molex to 6/8pin adapters but are they recommended?
2. Worth the upgrade? Newegg currently has an OCZ Fatal1ty 750W for $90 shipped, with an additional $30 mail-in-rebate. I have a friend interested in my current one if I upgrade, so in the end a <~$10 upgrade with little/no hassle. It's on the Recommended PSU's list, cheap upgrade, and allows me to go crossfire soon. Best option, or is there a better alternative?

 

[EarthDog]

1. They are fine.
2. If its on the list, go for it.

 

[dostov]

I didn't know that OCZ was still making PSUs under their own name. I thought they bought PC P&C and were only making PSUs under that name.

Anywho..... I've had an OCZ PSU for years now. Solid products. Go for it!

 

[WarWolf]

Bought. Ah, new toy on the way. Thanks.

 

[Knufire]

3570K is a 77W CPU. This includes the iGPU, which isn't being used, so it's morelike 65W stock. OCed, say 95W.

The 7950 has a TDP of 200W. AMD has the +20% power limit, so 240W each for the cards.

Everything else in the build, I usually just use 40W assuming you have a "typical" gaming system.

240+240+95+40= 615W

Thing is, this is on the 12V rail so you gotta look at the label to determine if your PSU can actually run it.

Looking at the the label for the M12II 620W on Newegg, you actually have 2 12V rails, each with 24A on it. 12V*24A= 288W. So one rail might have the CPU and one GPU on it, while the other rail might have the 2nd GPU and peripherals on it. Either way, no matter how the PSU has allocated what connectors to what rails, you definitely can't run two high end GPUs on it, not even close. Even two midrange GPUs would be bad because you'd either need the CPU+GPU on one rail, which would be overloaded, or two GPUs on a rail, which would overload it.

 

[Knufire]

Double post so it bumps the thread for the OPs attention.


Looking at the label for the OCZ Fatal1ty 750W, I see 4?! 12V rails with 18A each. 12V*18A = 216W. So now you have 4 216W rails to split everything on. You're PROBABLY fine with one rail for one GPU, so the PSU should work, but OCed to the max + running a benchmark is gonna put some heavy stress on that PSU and overload the rail a tiny bit.

Moral of the story, PSUs with only one 12V rail are so much easier to manage. And I probably would have sprung for something different than that OCZ. If you can still cancel the order, I would.

 

[WarWolf]

Thanks. Exactly why I come here for comments. Canceled.

So it's best to find one with only a single 12V rail?

 

[Bobnova]

Don't buy into the single rail myth.
Remember that GPUs get up to 75w through the PCIe slot, which is on a different rail.
Also remember that the PSU won't trigger OCP right at the 18a cap, likely more like 24a.

If you're going to buy a new PSU with MIR, go for the CoolerMaster V850, it's $110 after MIR and instant savings at newegg. Epic epic PSU.

 

[EarthDog]

@Knu - That is saying that there is actually limits on the rails as not all that say multi rail are true multi rail. Many PSUs say multi rail but each rail can handle the full 12v output making it not a true multi rail PSU. It depends on if it has OCP on the rails (from what I understand... BobN?) really. Your point is spot on, just thought I would add that. In regards to rails, looking at the sticker may not hold the whole truth. That specific PSU, no idea.


EDIT: In fact, that PSU is a single rail...

To which I reply: calm down, people. Neither the manual nor the cabling made any indication of what went to what rail, so I popped the cover off. This unit does not have two 12V rails at all - it's a single 12V design. There is no multirail overcurrent protection in there. That'll make load testing easier.

http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=185

errr crap, its little brother is...but that should help put a visual to my post at least.

 

[dostov]

Don't buy into the single rail myth.
Remember that GPUs get up to 75w through the PCIe slot, which is on a different rail.
Also remember that the PSU won't trigger OCP right at the 18a cap, likely more like 24a.

That.

You may also wanna browse the Classifieds section. There's some nice PSUs on the cheap right now.

 

[Bobnova]

It really is a multi-rail unit, but OPP doesn't work. http://www.hardwaresecrets.com/article/OCZ-Fatal1ty-750-W-Power-Supply-Review/1019/8

I don't like the Sirfa platform it's built on, I've yet to look at a PSU on that platform that didn't have switching transients off the chart.

Why it got a gold rating when it burnt up failing to OPP is beyond me.

 

[Knufire]

Remember that GPUs get up to 75w through the PCIe slot, which is on a different rail.

Completely forgot about the PCIe power.


Also look at the XFX Pro 750W: http://www.newegg.com/Product/Produ...-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=

After the promo code and the rebate, it's only 55. Can't seem to find a review on that specific one though, maybe Bobnova knows exactly what platform it's from? I know a lot of XFX's PSUs are based off the Seasonic M12II/S12II but I'm not 100% sure.

 

[EarthDog]

It really is a multi-rail unit, but OPP doesn't work. http://www.hardwaresecrets.com/article/OCZ-Fatal1ty-750-W-Power-Supply-Review/1019/8

I don't like the Sirfa platform it's built on, I've yet to look at a PSU on that platform that didn't have switching transients off the chart.

Why it got a gold rating when it burnt up failing to OPP is beyond me.

Sorry. I meant the Seasonic in the OP.