Corsair RM750 750W Power Supply Review

We have all read about Corsair power supplies and it is quite likely that many [H]'ers have one in their computer case right now. Today we are looking at Corsair's newest RM series power supply rated at 750 watts. The RM is built, "fully modular and optimized for silence and high efficiency." Let's see how it handles our grueling power tests.

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Conclusions

The Corsair RM750 is the first product we have seen from Corsair’s newest line of power supplies the RM series. This particular line of power supplies is being billed as one of the quietest power supplies available. Certainly, in the past, Corsair has produced some quiet units, some high quality units and even managed to balance these items in a nice harmony. Today, however, we see Corsair leaning on one facet of this units design more so it seems than we have ever done so before. Has this singled minded focus produced the quietest power supply available? Or did this drive to be so very quiet blind the process and result in a power supply that misses the mark altogether? Let’s move on now and see.


HardOCP’s testing methodology is intended to very much push power supplies to advertised wattage ratings in temperatures that will represent some of the hottest computer enthusiast cases. So if a unit passes all our testing it is definitely not something to take lightly. In fact we expect more power supplies to fail our testing than make it through unscathed.

Build Quality

Given Corsair’s overall reputation, we immediately assume that the RM750 would be another high quality product. Overall, that seems to be true today but there are a few things about this unit that make us stop and scratch our head a bit. The exterior of this unit falls directly in step with the current design trend of Corsair power supplies save for the now blunted corners on the unit (for whatever reason). The cables are all the FlexForce style once more with the sleeving being complete. Moving to the interior, the interior build quality is very good with the biggest detractions being the lack of heatsink on the 12v MOSFETs and the lower quality Ltec and Taicon capacitors on the secondary side, including those on the modular interface which are really going to be starved for airflow (not a good plan). The primary side capacitor is still a Nippon Chemi-con but that isn’t saying much. We also see that the unit uses a Corsair branded "custom" rifle bearing fan to try and cool this unit. This is the first time we have seen this fan, which is interesting, so the jury is still out on the quality of these until we see some more from it. When we look at the integration we see that it is generally good and clean so it seems that Chicony did a good job with what it was given to work with by Corsair's "custom" designed power supply. We also see that this unit is backed by a 5 year warranty and that the documentation that comes with this unit is worse than useless. Oh well, hopefully you don’t ever actually need to know anything about your unit when you buy it. It should work, right?

Load Testing

Overall, the Corsair RM750 was an across the board disappointment as it failed to complete our load testing today. However, if we ignore the fact that the unit failed and look at the tests it did complete we still don’t see a great power supply. For instance, in the voltage regulation, the 12v rail saw a total drop of 0.18v and the 5v rail saw a total drop of 0.11v during our testing. Among comparable units like the Seasonic G-750, XFX Black Edition 750W, XFX Core Edition 750W, and Corsair TX750 V2 this unit manages to bring up the rear by only managing to end mixed compared to one unit (the Corsair TX750M). The RM750 does a bit better when it comes down the efficiency. The efficiency on this unit ranged from 85.87% to 89.56% at 120v and 84.28% to 87.90% at 100v with the 80 Plus results being 87.53%-90.59%-86.93% efficient at 20%-50%-100% load using 80 Plus’ load testing parameters. Given that, we see that the RM750 does at least post numbers that make this units 80 Plus rating likely on production units.

Moving over to the Transient Load Testing results for the RM750, we find that the results are passing for the test that the unit would complete. In that test, we saw a peak 12v change of ~350mV when loaded and a peak 5v change of ~100mV when directly loaded. However, that one test result, if we are to assume that was as bad as it was going to get, put this unit behind the Corsair TX750M, the XFX Black Edition 750W, and generally the Seasonic G-750 but, in a bit of a reversal of fortune, ahead of the Core Edition 750W and the Corsair TX750 V2.

DC Output Quality

The DC Output Quality of the Corsair RM750 is easily passing, and good, for the tests it would complete. Overall, the 12v rail peaked at ~40mV of ripple/noise while the 5v rail peaked at ~20mV and the 3.3v rail peaked at ~15mV. Unfortunately, these relative results put this unit in the same boat as we saw in our load tests. That is to say, it trails the XFX Black Edition 750W, XFX Core Edition 750W, and the Corsair TX750 V2 while only being comparable to the TX750M. Some days passing is just that, passing, and not really excelling. Today is one of those days.

Noise

Over the years, we have seen a number of Corsair power supplies, including a number of 750W models, and they have generally been quiet. The RM750, however, is the one unit where quiet operation has seemed to be put forward over all else and, as we know already, something did not work right in that gamble. When you bet big you can win big. However, when you bet big you can also lose big, and with the failure to perform all the load tests today the RM750 clearly lost big. Now, when this unit was not losing big the results for this unit were really good. Indeed, since the fan would not run through 50% load at 45C (or through about 75% load at room temperature) it was literally silent at least آ½ of the time. Once the fan did begin to run, the unit was still very quiet and never was noticeable in our load tests. However, while this would seem to make the unit ideal for quiet cooling environments, it really isn’t as we have learned from some back and forth with Corsair this unit is going to rely on a good bit of airflow through it to function properly because of the way the fan controller was designed and tested. Not exactly ideal if you are using passive components in order to come up with an extremely quiet machine and you are counting on the fan in the power supply to look after the power supply appropriately.

Paul's Thoughts:

The Corsair RM750 today is one of the more frustrating products we have had in to review in some time for a number of reasons. Starting at the top, the unit was delayed multiple times to fix issues and then at the end of the day the unit is still broken! Then there is the fact that IT IS broken all by itself, never what you want to see especially when you pay a premium for a Corsair branded unit. On that note, when paying a premium for a unit you generally like to see at least somewhere in the upper quartile components and, while not total trash, Taicon and Ltec are not there. This goes double for when we are looking at a unit that is going to be baking inside since the fan controller has been tuned so aggressively for quiet operation that it barely ever runs even if apparently it should. Finally, it isn’t even better than previous "lower tier" Corsair products we reviewed some time ago. Apparently Corsair was betting big on this game of Craps and came up snake eyes. But, on the bright side, Corsair did manage to differentiate this product from the other SEVEN Corsair 750W units on the market.....this is the one that doesn't work right? I am not sure that is what Corsair was going for though. At any rate, what Corsair should have been doing today was producing a power supply that did better than the best TX series units it currently produces as this unit is competing with those units and then drop those previous TX units. Not whatever this idea was that will likely kill off those better performing units due to the quiet when it works factor.

The Bottom Line

Overall, Corsair has been one of the extremely solid brands we have seen and its products have developed a rabid following among enthusiasts. Certainly, a number of its power supplies have been a part of that. The Corsair RM750, however, is not one of those. Barring the fact that this unit is an outright failure as it is today, this unit was just not good compared to the competition including its own internal competition. Sure, the voltage regulation, DC Output Quality, and Transient Load Test results were in specification for the tests it would complete, but the RM750 could not pull ahead of the competition in any category or even keep up with the competition in many aspects. If we couple that with some of the unknown and/or low end component selections in this unit, we quickly can see that, even if this unit had passed all of our load tests, there simply would be no reason to buy this unit as there better options out there. If an enthusiast wants a really quiet power supply, they probably are going to want one that at least is competitive with other products not the one that is bringing up the rear. So, if users are really considering this unit it would probably behoove them to consider some of this units competitors like the Seasonic G-750 or (if they must grab a Corsair unit) the TX750M or TX750 V2 before Corsair tries to kill those lines off in favor of this newer and lesser product. While rabbits apparently don’t lay Raisinets, Corsair has sure laid a turd with the RM750 in its current form.

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Discussion


Editor’s Note: We spent a lot of time in back and forth with Corsair and did receive two separate units for test which gave us the same disappointing results. Corsair did finally communicate this to us.

Corsair: Last week were able to duplicate your findings by restricting the airflow to the power supply and because of this have implemented rolling changes to the RM that will allow the fan to turn on sooner if such a condition occurs. As you know, the RM Series PSUs are backed by our 5 year warranty. If a user does experience an issue, we will have their PSU replaced, no questions asked and at no cost to them.

Editor’s Note: Corsair has publicly addressed this concern as linked below.

Corsair Blog: Potential thermal issue with RM750 and RM850 products

Corsair Forum: Potential thermal issue with RM750 and RM850

The Corsair RM series power supplies are designed to run without active cooling for up to 40% load at typical room temperatures (25آ°C, or 77آ°F) and are capable of continuously outputting 100% of output capability at ambient temperatures of up to 40آ°C (104آ°F). We have found that our power supplies are typically used in environments much lower than 40آ°C and tend to have some natural aspiration within the chassis that allows for some air movement, such as from a nearby graphics card or chassis fan, within the PSU housing, when temperatures exceed normal room temperatures. We have found that this normal aspiration can maintain Corsair’s Zero-RPM fan-less mode even at temperatures as high as 35آ°C (95آ°F) when only at 40% load for durations of up to 15 minutes.

Thanks to recent feedback provided by an early reviewer, we have discovered a PSU fan operation issue that can be triggered with the RM750 and RM850W under highly unusual operating conditions. For the issue to occur, the PSU must be run for more than 15 minutes at:

Ambient temperature 35آ°C (95آ°F) or higher

Zero ambient airflow, i.e. no airflow provided by system fans or by fans on other components

Under these conditions, impacted PSUs may shut down before the cooling fan starts spinning. This shutdown will cause no damage to the PSU or the system. As these conditions are extraordinarily harsh, we believe most users will not be impacted. If you feel you have a unit that has been impacted, we are offering advance replacement returns for the PSU, with Corsair paying all freight costs. Please click here for instructions on how to obtain a replacement unit, or to ask any questions.

Editor's Note II: Corsair has verified changes made to RM units that would address the shutdown issues we experienced. These changes would very likely earn the Corsair RM series at the very least a "HardOCP PASS."

The RM Series PSUs, like all Corsair PSUs, are designed and tested using methodologies that closely simulate the environment of an enthusiast/gaming PC under heavy, sustained workloads. Our testing has proven that the cooling system of the RM Series will engage and provide the necessary cooling during heavy real world use.

That said, we’re enthusiasts, too. We understand why Kyle and Paul came to the conclusions they did, and based on their feedback, we’ve implemented a design change that will allow our RM Series PSUs to go the extra mile and endure the extreme synthetic test conditions of the HardOCP test lab.

We have changed the value of the resistor between the thermistor and the fan controller so the fan turns on much sooner than the temperature that would trigger OTP. Therefore the fan should always turn on before OTP is triggered; preventing the PSU from shutting down before it is necessary.

This change has been implemented in all RM PSUs produced since mid October. While we honestly believe even extreme users will never run into the issue encountered during HardOCP’s synthetic test, those end users that want to know if their RM can endure HardOCP’s torture test can identify the updated PSUs by a lot code of 1341 or higher.