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Overclocked Radeon 7950 - realistic power needs?

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Allan Speers
Overclocked Radeon 7950 - realistic power needs?
on Sep 6, 2016 at 6:14:29 am

I need a new graphics card for my 2012 Mac Pro asap. I've decided to go with a 7950.

I was initially puzzled by the fact that the Saphire Mac Edition only runs at 800 Mhz, when even the lowly "reference" Windows version runs at 850. Several others run at 950 and even 1,000 Mhz without overclocking.

I was also puzzled by why the Mac version had only one fan, which theoretically should be more noisy.

Now I think I understand, but it raises some important questions:
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As we all know, (I think) a Mac Pro can only supply 225 watts max to the GPU. (75 watts from the PCIe slot, and 75 from each 6-pin cable.) I now know that a 900 Mhz HD7950 can pull as much as 250 watts when really stressed, and maybe even more. The 1,000 Mhz variants can pull almost 350 watts.

But that stress test was done by Tom's, using a bit coin server (whatever that is.) What I am trying to figure out is:

1: What % of the card's maximum power consumption should be expected when using FCPx (rendering, or whatever) and only 1 monitor? I know it's not 100%. I'd kinda' like to have that extra clock speed, and I'd rather NOT add a second psu just to run one single card. (I may have to, of course.)

2: Do I really need that extra clock speed? I don't play any games, and I don't run 4K video. I guess (you tell me) that want I really NEED is just the extra GPU / open CL power for rendering. In which case, the Saphire Mac edition would be enough?
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I'm thinking one option would be to get a faster card, (flashed for Mac & with the resistor mod) and then UNDER-clock it to 800 Mhz, then run my worst-case FPCx scenarios. Monitor the card's draw, and adjust the clock accordingly. - but I'd like to have the answers firstr, if possible.
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As for the fan: I assume that 1 fan is enough for 800 Mhz, but more importantly, that design throws all the heat out the back, which is important considering the Mac Pros dearth of case fans. There are a few higher-clocked "Windows" 7950's with that type of cooling (MSI, etc) so maybe those are the best candidates overall?


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Allan Speers
Re: Overclocked Radeon 7950 - realistic power needs?
on Sep 6, 2016 at 6:59:22 am

Here's a related & kind of obvious question:

If the Mac's psu is 975 watts, then given normal "safety margin" and aging component concerns, that still gives around usable 700 watts. The other three PCIe slots require 225 watts total, and the cpu is going to pull MAYBE 150 watts on a bad day. Ram & SSD's might pull 50w, but probably much less.

That leaves a nice safe 275 - 300 watts for the GPU. That would run a maxed 900 Mhz card, maybe even a 1,000 Mhz card under "normal" FCPx usage. Is there some reason one cannot (somehow) put more wattage onto those 6-pin connectors?


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Mike Fitzsimmons
Re: Overclocked Radeon 7950 - realistic power needs?
on Sep 6, 2016 at 12:55:06 pm

Hey all..

Can't speak to all the specific wattages, etc., but I'm running an AMD R9 280X 3GB in a 2012 12 core without issue.

Quiet and fast. Can't ask for much more.

Good luck!


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Allan Speers
Re: Overclocked Radeon 7950 - realistic power needs?
on Sep 6, 2016 at 5:32:49 pm

Thanks, Mike. That helps me a little, because according to TOM's :
( http://www.tomshardware.com/reviews/radeon-r9-280x-r9-270x-r7-260x,3635-18....
The 280 pulls just about the same power, in there highest stress-test (a bitcoin server) as a 7950 running at 950 Mhz. Both cards require over 250 watts. I assume then that your FCPx setup does not stress the card as much as that test.

So: What IS your setup?

Also: The 280 is not on the official FCPx approved list. What was your reason for choosing it? Have you seen any test comparing it to the 7950, using FCPx or Motion?


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Allan Speers
Re: Overclocked Radeon 7950 - realistic power needs?
on Sep 6, 2016 at 6:34:35 pm

Wait - I just read that the 280x uses a 6 and an 8-pin connector, with the latter typically providing 150 watts. How did you rig this on a Mac Pro?

If you somehow combined two 75w sources into an 8-pin adapter, then that would explain it, but if so I'd sure like to know how!


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Allan Speers
Re: Overclocked Radeon 7950 - realistic power needs?
on Sep 6, 2016 at 7:12:11 pm

I also just found out that there's a way to restrict a card's maximum power consumption (to 225 watts, for instance) via some kind of firmware update. (I don't know the details) That would obviously restrict some areas of performance. No idea how it would affect FCPx or Motion.

Could your card have this mod?

(straying off-topic a little, but still important)


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