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I decided to modify a PCI riser cable, the type of riser is:

http://www.aliexpress.com/item/Free-Shipping-PCI-Express-1X-to-16X-Adapter-PCI-E-Riser-Card-Flexible-Extension-USB3-0/1677878410.html?spm=2114.01010208.3.105.kPH12c&ws_ab_test=searchweb201556_0,searchweb201602_1_10037_10017_407_10033_406_10032_10040,searchweb201603_8&btsid=38dba5c6-133a-461e-9c9b-8bf58a67dd77 (http://www.aliexpress.com/item/Free-Shipping-PCI-Express-1X-to-16X-Adapter-PCI-E-Riser-Card-Flexible-Extension-USB3-0/1677878410.html?spm=2114.01010208.3.105.kPH12c&ws_ab_test=searchweb201556_0,searchweb201602_1_10037_10017_407_10033_406_10032_10040,searchweb201603_8&btsid=38dba5c6-133a-461e-9c9b-8bf58a67dd77)

The power is only 12v input power from a server power supply, there is no 5v output from the server psu. The riser requires 5v input in order to regulate the voltage down to 3.3v used by the PCIe signal bus.

So this unholy creation came to life:

https://i.imgur.com/3NSOXXE.jpg

https://i.imgur.com/vr2MzyS.jpg

https://i.imgur.com/hoaWe3I.png

The GPU is now only powered by a 12v server psu, the step down buck converter takes the 12v. Much to my surprise, it actually works. Currently testing 370 and 380x with it. There might be issues with grounding, to be sure one should share the ground on the 12v output side for multiple power supplies.

You shouldn't have a problem as long as all the grounds are common, done this kind of stuff a lot.

I've done the same thing with my GPU rigs - the one thing to keep in mind (at least with the R9 390 cards) is that it consumes about 1a (at 5v) per card, so my 6 card rigs all consume about 6a of 5V. 

I can't really tell how big that stepdown converter is, but you could be running a bit much through it - these are the ones I use (10a):

http://amzn.to/29TTntS

I originally used this one (20a):

http://amzn.to/29mNvqW

But after putting power monitoring on it, I realized there really wasn't any point.  Both are prime which is something I'm always after.

These are also handy if you're powering the MB/CPU via 12v as well (and assuming your breakout is a bunch of 6-pin PCIE:

http://amzn.to/29mNJ1g

And these to power a bunch of 12v fans:

http://amzn.to/29mPAD6

Hope that helps!

Great to get information from others.

The step-down buck converter uses the LM2596, it has an output rating of 3 A. The electromechanical specification of the PCIe bus states that the maximum burst current at 3.3 V should be 2700 mA and max nominal current around 2000 mA. The intention was to have one step down converter per riser for modularity and the step-down converter is less than a dollar per unit.

http://www.aliexpress.com/item/10-PCS-DC-DC-Step-Down-Converter-Module-LM2596-DC-4-0-40-to-1-3/1975111111.html?spm=2114.01010208.3.2.yAEyBa&ws_ab_test=searchweb201556_0,searchweb201602_1_10037_10017_407_10033_406_10032_10040,searchweb201603_8&btsid=3513d8de-8fc2-4050-b7dd-6f5295aa688f (http://www.aliexpress.com/item/10-PCS-DC-DC-Step-Down-Converter-Module-LM2596-DC-4-0-40-to-1-3/1975111111.html?spm=2114.01010208.3.2.yAEyBa&ws_ab_test=searchweb201556_0,searchweb201602_1_10037_10017_407_10033_406_10032_10040,searchweb201603_8&btsid=3513d8de-8fc2-4050-b7dd-6f5295aa688f)

I like the idea of having one big converter to power all risers.

Yeah, here's what my test unit looks like - I put amp monitors on my systems because I like to have a simple way to get a heads up on monitoring loads, etc.

http://www.analogx.com/images/gpu/dsc00072s.jpg

This only has 4 cards in it at the moment, so you can see the load is a bit over 1A each, but that fluctuates.  The 12v side is for the 12v side of the molex connectors, as I was interested to see what the draw was like.   This one obviously has the 20a version, as this is my test box.

Awesome, great to see. I like the clean setup :)

I agree very nice setup, thanks for showing us, makes me miss doing stuff like that.

Looks pretty gnarly. Good job.

Thanks for all the comments on my GPU miner build - I've refined the design a bit, the one in the photo is my prototype one, so it has things the other ones don't.  The newer ones are working really well (not that the old one was that bad, mind you) - great airflow plus it's easier to assemble now.  They all are powered with the DPS-2000BB, and up until now I mod those to include two 60mm fans, but with my new design I've re-positioned it so that it's cooled by two of the 120mm fans.  The position of the cards is also relative to the fans, so they get the majority of the airflow over the front and back of each card, not obstructed by the card itself.  I put them on those black bakers racks:

http://amzn.to/29PTCZ3

Pretty hard to beat - you can lock them together also, but the rack does a better job and I put the PDU's on the top, so it makes for a nice individual setup.  The new ones also have the PSU outlet off the side, so the front and back are largely free flowing (apart from the bar that the GPU's screw into to hold them in place).

Once I'm satisfied with the overall design, I'll start a thread with a bunch of pictures for those who are interested. 

I'd be interested in more pictures! :)

Here's a teaser shot for you of my latest version:

http://www.analogx.com/images/gpu/dsc00087s.jpg

It still has a couple of minor tweaks to do, I've got some custom cables coming to clean up that rats nest off of the breakout, and I'll probably use a thinner acrylic and a bit longer for the divider between the top and bottom sections.  A couple notable changes that work REALLY well are that the DPS-2000BB is cooled by the 2 case fans next to it - works even better than my last one where I modded the PSU to have 2x 60mm fans dedicated to it.  The PSU cord also comes out the side, just to clean up the back - on the next one I'll take the ethernet out the side also, so the only thing out the back will be the keyboard and monitor, when I need to hook them up for some reason. 

Overall the case is rock solid and the airflow is great, plus it isn't nearly as loud as you'd think.

Very nice, where did you order the slotted profiles?

It's all 8020 (8020.net), I love their stuff - not cheap, but top quality and very flexible.  For me I'm less concerned with the specific rig cost, and more with building a robust long-term platform.