Much-Improved Acoustics, With One Nagging Issue
Cooling A More Elegant Solution
AMD buttons the Radeon HD 7990 up into a dual-slot card that only needs two eight-pin auxiliary power connectors to drive it. With that said, the card jams right up against the PCI-SIG’s electromechanical specification for one x16 slot and two eight-pin connectors: 375 W.
Even still, it’s impressive that AMD created a two-slot flagship after we’d seen nothing but bulkier solutions from PowerColor, HIS, and Asus. The company achieved this by screening out the best of the best ASICs prior to shipping off its Tahiti GPUs. For a while now, it’s been setting aside the top few percent of the lowest-power, highest-frequency parts, building up an inventory specifically for this launch. And while AMD’s partners are locked at certain voltage levels for their Tahiti-based products (inherently affecting power), AMD was able to drop the voltage on its special chips to still get up to 1 GHz out of them inside of a 375 W power limit.
That’s not to say it’s easy to keep the Radeon HD 7990 cool (or even quiet, for that matter). Instead of the one axial fan Nvidia uses on its GeForce GTX 690, or even the centrifugal blower on Radeon HD 6990, AMD employs three axial fans on 7990. Their blades aren’t particularly thick, and the fans themselves aren’t reinforced for stability (you can rock them side-to-side with your fingers). But they spin slowly enough under typical gaming loads that they address the one thing that bothered me most about Radeon HD 6990: fan noise. In games, the 7990s fans are nearly imperceptible.
There is a price to be paid for this sort of design, though. Three fans, side-by-side, are only effective if they aren’t doing battle with each other. And that means channeling air vertically, rather than horizontally. At the end of the day, then, the slotted rear I/O panel that typically helps exhaust hot air from most graphics cards is almost non-functional. Instead, waste heat from both GPUs is jettisoned out the top of the card, right into your case. Consequently, you’ll want to be careful picking the right chassis for the Radeon HD 7990. AMD currently recommends two models: Antec’s Eleven Hundred with two 120 mm side-panel fans, and Cooler Master’s HAF-X, also with side-panel cooling. Enthusiasts who go a different route need to build with airflow in mind. Almost assuredly, small form-factor isn’t a viable option.
Physically, the Radeon HD 7990 measures the same 12-inches long as Radeon HD 6990. That’s an inch longer than GeForce GTX 690. Fortunately, both of its eight-pin power connectors are up on top of the board, so you don’t have to worry about leads extending another inch or two behind the already-long add-in. There’s also a metal plate on the back of the PCB. Given that the first fan’s blades protrude up above the plastic shroud a tad, you won’t want to put two 7990s right next to each other in a quad-CrossFire configuration.
Wait, What’s That Hum?
The Radeon HD 7990’s cooling fans spin quietly—something I was so happy AMD addressed. But another acoustic issue nagged at me. Previously, PowerColor sent in its AX7990 6GBD5-A2DHJ Devil13 for us to look at. But I was surprised at just how much noise the card’s inductors generated—like, I couldn’t believe an engineer would kick something like that out the door and expect someone to pay a grand for it.
To a lesser degree, the Radeon HD 7990 runs into something similar. AMD explained it to me as an artifact of oscillation between heavy and light workloads, where current draw spikes and dips, causing ceramic capacitors and the PCB itself to vibrate. The volume and tone of this phenomenon vary according to the task you’re performing, but it was noticeable enough during our real-world game testing with Bakersfield-based volunteers that several asked me to explain what was happening.
The solution is to turn on v-sync, capping the frame rate and preventing those highly variable loads. I don’t think it’s particularly ideal to have to use v-sync, but there it is. Igor in our German office created some video and performed frequency analysis that you’ll be looking at shortly. Decide for yourself if this is a deal-breaker.