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What to look for in PSU intended for a CrossFireX system?


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#1 Elliot B.

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Posted 06 January 2014 - 16:08

Intending to buy a second 7870 and this requires me to get a new PSU.

I believe I need one with 2 of those GPU connectors and I'll make sure its 650W+.

Is there anything else I need to look out for?


#2 +snaphat (Myles Landwehr)

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Posted 06 January 2014 - 19:00

Make sure that it is rated for a continuous power draw at X Watts, and not just peak.



#3 Roger H.

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Posted 06 January 2014 - 19:31

Prolly need to check for extra 12v rails right to connect to the cards directly? Haven't bought PC video cards in a while so dunno if they still need those.

#4 Andre S.

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Posted 06 January 2014 - 19:34

Two PCI-E connectors per card, plenty of wattage. Apart from that, the usual advice: reputable brand, silent operation, modularity.



#5 Jason S.

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Posted 06 January 2014 - 19:36

i prefer modular, but i agree with Andre too



#6 Circaflex

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Posted 06 January 2014 - 19:47

Stick with known brands and read tons of reviews, this to me is the absolute best site to find PSU info

 

http://www.jonnyguru.com/

 

Whats your budget? Have you looked at any yet? If you can provide me with a budget and region im sure we can find something that will get the job done.



#7 +PeterUK

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Posted 07 January 2014 - 00:15

The only problem is your Asrock H77M will not run the second 7870 at a good speed being that your board will run one card at PCIe 3.0 x16 and the other at PCIe 2.0 x4.



#8 +snaphat (Myles Landwehr)

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Posted 07 January 2014 - 01:21

 

The only problem is your Asrock H77M will not run the second 7870 at a good speed being that your board will run one card at PCIe 3.0 x16 and the other at PCIe 2.0 x4.

 

 

^Good eye. Confirmed with the manual: 16x/4x.



#9 OP Elliot B.

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Posted 07 January 2014 - 07:32

 

The only problem is your Asrock H77M will not run the second 7870 at a good speed being that your board will run one card at PCIe 3.0 x16 and the other at PCIe 2.0 x4.

 

 

^Good eye. Confirmed with the manual: 16x/4x.

Not an issue.



#10 +snaphat (Myles Landwehr)

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Posted 07 January 2014 - 09:03

 

You may well be right, but that link doesn't provide evidence either way. It just has the following:

  • A guy who said it won't have issues without providing any evidence for the claim.
  • Another guy who looked at the performance numbers for single cards operating in different PCIe configurations and speculated that because the cards have similar FPS under single 16x and single 4x operation that 16x/4x crossfire solutions won't have issues. By the same logic even PCIe 2.0 16x/4x shouldn't have issues, but it is well known that it does (there were links to benchmarks showing this somewhere on this forum iirc).

Moreover, the article (that one of the guys posted in your link) doesn't make any statements about a dual PCIe 16x/4x configurations whatsoever. What it says that 2-way GPU is still viable for PCIe 2.0 8x/8x configurations because there is only around a ~2% difference in PCIe 3.0 8x vs PCIe 2.0 8x performance.

 

What I'm saying is that we are going to need actual PCIe 3.0 16x/4x benchmark results otherwise it is essentially impossible to tell whether the issue is a thing of the past or not.



#11 OP Elliot B.

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Posted 12 January 2014 - 12:54

You may well be right, but that link doesn't provide evidence either way. It just has the following:

  • A guy who said it won't have issues without providing any evidence for the claim.
  • Another guy who looked at the performance numbers for single cards operating in different PCIe configurations and speculated that because the cards have similar FPS under single 16x and single 4x operation that 16x/4x crossfire solutions won't have issues. By the same logic even PCIe 2.0 16x/4x shouldn't have issues, but it is well known that it does (there were links to benchmarks showing this somewhere on this forum iirc).

Moreover, the article (that one of the guys posted in your link) doesn't make any statements about a dual PCIe 16x/4x configurations whatsoever. What it says that 2-way GPU is still viable for PCIe 2.0 8x/8x configurations because there is only around a ~2% difference in PCIe 3.0 8x vs PCIe 2.0 8x performance.

 

What I'm saying is that we are going to need actual PCIe 3.0 16x/4x benchmark results otherwise it is essentially impossible to tell whether the issue is a thing of the past or not.

So now I'm confused :(

 

I can live with the two cards losing 5% by running at 4x but any more than 5% is a dealbreaker.



#12 Andre S.

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Posted 12 January 2014 - 18:10

The average difference on P67 between x8/x8 and x16/x4 was 10% according to Tom's Hardware



#13 +snaphat (Myles Landwehr)

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Posted 12 January 2014 - 19:28

The average difference on P67 between x8/x8 and x16/x4 was 10% according to Tom's Hardware

Problem is that was for PCIe 2.0 though, we  don't know if that pattern continues for PCIe 3.0

 

So now I'm confused :(

 

I can live with the two cards losing 5% by running at 4x but any more than 5% is a dealbreaker.

Yeah, I have no idea, I couldn't find any tests for 3.0. I don't think any exist to be honest. What I said was based off of the fact that I know PCIe 2.0 has performance loss (as Andre S.'s link shows).



#14 OP Elliot B.

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Posted 12 January 2014 - 19:31

Then I wish Tom's Hardware would "update" it :(