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Cooler Master Real Power 550

It’s a step up from the 450W model we reviewed a while ago, and it has the same range of features, high efficiency, and compliance with both ATX12V 2.xx as well as EPS12V. This means, for instance, three 12V lines. Is the Cooler Master Real Power RS-550-ACLY quiet enough for SPCR?

July 18, 2005 by Devon

*POSTSCRIPT added Oct. 22, 2005*

Cooler Master Real Power 550 RS-550-ACLY ATX12V
2.01 / EPS12V 2.1 550W Power Supply

Market Price


The two major markets in the computer industry, desktop and server, have traditionally
been quite separate. For the most part, they have different requirements, use
different products, and are marketed separately. The only crossover
between the two is where high-end desktop or workstation products meet entry-level server products.
Some power supplies fall into this category; the similarity of the ATX12V (desktop) and
EPS12V (server) power supply specifications makes it possible for a PSU
to conform to both.

Most manufacturers seem to prefer to keep their products to one market or the
other, or to “unofficially” support EPS12V by supplying the 8-pin
EPS connector but not complying with other EPS12V requirements. However, this
is not true of Cooler Master, which has chosen to support both standards in
the Real Power 550. The previously
reviewed Cooler Master Real Power 450
did not support

The most obvious differences between the 550W and 450W version relate to the
550’s support for EPS12V: The +12V output is split across three separate rails,
and a workstation power connector is included for use with high-end workstations.
There is a also a 6-pin PCI-e plug for use with power hungry video cards.

A fairly compact but colorful box provides a good idea of the Real Power’s features
as well as the usual marketing fluff.

The connectors are listed on the outside of the box, so you can tell at
a glance whether it has what you need.
The length of the cables would have been nice as well…

Inside the accessory box: Power supply, AC cable, manual in eight languages,
24 to 20 pin ATX adaptor, screws, and something called a “Human Computer

A considerable amount of effort has been put into the presentation of the retail
package. For once, the printing on the retail box is actually useful: It shows
exactly how many and which connectors are provided. Inside the box, the individual
pieces are packed in a form fitting recycled cardboard tray, a step above the
usual bubble-wrapped packing job.

The manual is also impressively thick at first glance, although in reality
it’s no longer than usual. Its 64 page length comes from the fact that it is repeated in eight

There is also an external power meter that can be installed in a spare 3.5″
bay. It appears to be identical to the one that came with the Real Power 450W,
and comes with both a black and a silver faceplate. Our test of that device
showed that it approximates AC power draw, although the rough markings do not
permit precise readings.

Feature Highlights of the Cooler Master Real Power
550 RS-550-ACLY

Compliance with the newest Intel standard ATX12V v2.01 and
the newest SSI standard EPS12V v2.1
No longer the newest of
either standard, but changes to the newer revisions are minor.
Super silent operation with intelligent fan speed control (<23dBA) We measured the 450W
model at 23 dBA/1m. Can the 550W match this number?
Power consumption management by human computer interface (HCI) I’d call it monitoring,
not management. Use Cool ‘n’ Quiet if you really want power management.
PCIe connector satisfy high end graphic card requirement Almost mandatory on power
supplies of this size.
Support +12V1, +12V2
and + 12V3
outputs for higher power usage
In compliance with the
EPS12V requirements.
Green power design to meet energy star and blue angel requirement A guaranteed maximum
power draw in standby, but these certifications do not address efficiency
at idle or load.
More than 75% efficiency
at typical load operation
A conservative claim.
Most modern PSUs can do better than this, including Cooler Master’s 450W
Higher reliability (MTBF
> 400,000 hours
Eight times the EPS12V
recommended MTBF.
/ Short / Full protection
All the usuals plus over
temperature protection.
Active power factor correction
(PF > 0.99)
Required by EPS12V, but
uncommon on ATX12V power supplies.
Real power capacity
satisfy high-end system operation
More than enough room
for any desktop system.

The Real Power is aptly named: It is rated more realistically — make
that honestly — than most other power supplies on the market.
Both the peak and the continuous output capacities are specified. The majority
of power supplies list only their peak capacity, but cannot deliver their rated
capacity indefinitely.

The peak ratings of the Real Power are in line with the ratings of other power
supplies of this capacity: 30A apiece on the +3.3V and +5V rails and close to
20A on each of the +12V rails. In fact, the additional +12V3 rail puts the combined
+12V rating at a whopping (and completely unnecessary) 48A. If the Real Power
was rated in the way that most other power supplies are rated, it would be sold
as a 650W power supply. However, Cooler Master’s decision to rate it properly
drops its capacity down to “only” 550W.

The more reasonable continuous ratings put the combined +12V amperage at 30A,
which is still a very hefty rating. It is almost impossible to think of a combination
of components that actually requires this amount of power.

SPECIFICATIONS: Cooler Master Real Power 550 RS-550-ACLY

AC Input

90-264V @ 47-63 Hz

DC Output








Peak Output Current








Continuous Output Current








Continuous Output Capacity


Peak Output Capacity


The trio of +12V rails are specified by the EPS12V form factor, and deserve
a little explanation for those who are only familiar with ATX12V. Like ATX12V,
the CPU is isolated from the rest of the system on its own +12V rail (+12V1).
In addition, the motherboard and workstation connector (+12V2) are separated
from the rest of the peripherals (+12V3), which usually means the drives. This
allows a large number of drives to be spun up simultaneously without starving
either the CPU(s) or a powerful video card for power.

It’s not entirely clear how this three-rail EPS12V design is adapted to meet
the ATX12V specifications. Neither the 4-pin Auxiliary ATX connector nor the
PCIe connector are a part of EPS12V, so the specific rails for these connectors
are unknown. However, so long as these connectors are kept on separate rails
there should be no problems. The most logical and most likely configuration
would be to put the Auxiliary ATX connector on +12V1 and the PCIe connector
on +12V2. This would keep the two connectors isolated from each other while
still keeping the weaker +12V3 line free to power any drives.


From the outside, the Real Power appears quite sleek thanks to its smooth black
finish. Those who like bling should appreciate the bright blue LED fan; the
rest of us will probably want to either swap the fan entirely or find a way
of disabling the LEDs. This is not something you’d want to put in a system you
have to sleep next to.

The smooth exterior doesn’t have any internal vents. Any air drawn in
by the fan must exit through the rear of the power supply.

A standard 120mm design with a clear LED fan.

There is little to comment on based on an exterior inspection. The Real Power
looks like most other power supplies on the market. It uses the 120mm fan design
that has taken over the industry in the past couple of years, and its rear exhaust
is quite unrestricted. One things is missing from the rear panel: No voltage
selector switch is needed because
the PSU features auto range input of 100 to 240VAC.

The rear grill is stamped in the hexagonal pattern that has become in
vogue for its low impedance.

The “Human Computer Interface” power meter takes the form of a plastic
box that can be screwed into a spare floppy bay. It is backlit with a blue LED
when the power supply is on. A thin red needle allows the approximate power
level to be read from a scale marked in 100W increments. The scale is too small
to make readings within 25W or so, but it does give a rough idea of how much
power is being used. A portion of the scale is marked in red to indicate that
the power supply is operating above its capacity, but realistically speaking
a reading in the red is more likely to be an indication of a broken power meter
than an overloaded system. 550W is a lot of power.

The “Human Computer Interface” comes with black and silver faceplates.


The interior is almost identical to the 450W version. Only a different model
number on the main transformer and a few differently placed parts indicate that these
are not identical PSUs. Even the airflow deflector that directs air through
the middle of the power supply is in the same position.

Two aluminum heatsinks serve the dual role of heat dissipation and directing
airflow towards the exhaust.

The interior of the 450W version. The basic layout is identical; the differences
are in the quality of the components used.

A small plastic square deflects airflow towards the yellow-topped main

The fan in the 450W model was a weak point, and things look no better in the
550W model. The two fans seem quite similar. Both are dual ball bearing designs
that bear Cooler Master’s logo. The fan in the 550W model is rated for 0.45A
— more than the 450W model — which suggests that it will be faster
and louder.

Dual ball bearings and a 0.45A rating are signs of a high speed fan.


There are a total of eight cable sets, plus a thin cable that operates the
power meter and a 24 to 20-pin adaptor for the main ATX connector:

The longest cable is three feet long.

  • 18″ sleeved cable for the main 24-pin ATX connector
  • 19″ 4-pin auxiliary 12V connector
  • 19″ 8-pin auxiliary EPS connector for EPS12V (dual CPU) systems
  • 19″ 6-pin EPS Workstation connector (2 x +3.3V, 2 x +12V2, 2 x neutral)
  • 18″ 6-pin PCIe connector
  • 31″ cable with three 4-pin IDE drive connectors
  • 36″ cable with three 4-pin IDE drive connectors and
    one floppy drive power connector
  • 31″ cable with three SATA drive connectors
  • 23″ proprietary Human Computer Interface cable (+6″ on HCI itself)
  • 6″ 24 to 20-pin adaptor

The EPS Workstation connector is unusual, and deserves some explanation.
Like the PCIe connector, it is designed to supply auxiliary power to a powerful
VGA card. It even uses an identical plug, but the pin layout is not identical
so extreme care should be taken to ensure that it is not used in place of a
PCIe connector. Doing so could easily fry the card it is connected to and possibly
the power supply itself. For the record, the EPS connector is white and the
PCIe connector is black.

So, what use is a plug that provides auxiliary power to a VGA
card without any compatible VGA cards? The EPS Workstation connector is designed
to supply extra power to the motherboard, which then passes the
power on to the VGA card through the PCIe slot.

Care should also be taken in an SLI system with two cards that
require PCIe connectors. Because the Real Power only provides one of these connectors,
an adapter must be used for the second connector. However, the Molex connectors
that such adapters draw their power from draw from the less powerful +12V3 rail,
not the +12V2 rail that is designed to handle a heavy load. Although it’s hard
to imagine such a setup continuously drawing more current than
the 6A rating on the +12V3 rail, it does place it under a disproportionate load,
which could lead to odd voltage fluctuations.


For a fuller understanding of ATX power supplies, please read our article Power Supply Fundamentals & Recommended Units. Those who seek source materials can find Intel’s various PSU design guides, closely followed by PSU manufacturers, at Form Factors.

For a complete rundown of testing equipment and procedures, please refer to the

SPCR’s Revised PSU Testing System. It is a close simulation of a
moderate airflow mid-tower PC optimized for low noise.

In the test rig, the ambient temperature of the PSU varies proportionately with
its output load, which is exactly the way it is in a real PC
environment. But there is the added benefit of a precise high power load tester
which allows incremental load testing all the way to full power for any
non-industrial PC power supply. Both fan noise and voltage are measured at
various standard loads. It is, in general, a very demanding test, as the operating
ambient temperature of the PSU often reaches >40°C at full power.
This is impossible to achieve with an open test bench setup.

Great effort has been made to devise as realistic an operating
environment for the PSU as possible, but the thermal and noise results obtained
here still cannot be considered absolute. There are far too many variables in
PCs and far too many possible combinations of components for any single test
environment to provide infallible results. And there is always the bugaboo of
sample variance. These results are akin to a resume, a few detailed photographs,
and some short sound bites of someone you’ve never met. You’ll probably get
a reasonable overall representation of that person, but it is not quite the
same as an extended meeting in person.

REAL SYSTEM POWER NEEDS: One very important point is that the while
our testing loads the PSU to full output (even >600W!) in order to verify
the manufacturer’s claims, real desktop PCs simply do not require anywhere near
this level of power. The most pertinent range of DC output power is between
about 65W and 250W, because it is the power range where most systems will be
working most of the time. To illustrate this point, we
recently conducted system tests to measure the maximum power draw that an actual
can draw under worst-case conditions.
Our most powerful P4-3.2
Gaming rig drew ~180W DC from the power supply under full load — well within
the capabilities of any modern power supply. Please follow the link provided
above to see the details. It is true that very elaborate systems with SLI could
draw as much as another 150W, but the total still remains well under 400W in
extrapolations of our real world measurements.

high fidelity sound recording system
was used to create MP3 sound files
of this PSU. As with the setup for recording fans, the position of the mic was
3″ from the exhaust vent at a 45° angle, outside the airflow turbulence area.
The photo below shows the setup (a different PSU is being recorded). All other
noise sources in the room were turned off while making the sound recordings.

Ambient conditions during testing were 23°C and 20 dBA, with input of 120 VAC
/ 60 Hz measured at the AC outlet.

DC Output (W)
AC Input (W)
Intake Temp (°C)
PSU Exhaust (°C)
Fan Voltage
Noise (dBA/1m)
Power Factor

NOTE: The ambient room temperature during testing
varies a few degrees from review to review.
Please take this into account when comparing PSU test data.


1. VOLTAGE REGULATION was within ±3% throughout testing except
for the full load test, where the +3.3V line was just over 3% low. For a desktop
machine, this is not a problem; ATX12V allows up to 5% variance within its specification.
However, EPS12V specifies that the +3.3V line can sag by a maximum of only 3%,
which was missed by 0.01V at full load. This variance is so small that it is
within the bounds of error for our test system, so it’s impossible to say for
certain whether the power supply was out of spec — only that it got very
close. Keep in mind that this variance occurred at a load of 550W, which is
near impossible to achieve.

  • +12V: 12.07 to 12.26
  • +5V: 4.82 to 5.03
  • +3.3V: 3.19 to 3.34

2. EFFICIENCY was low-to-average in the important <150W range, although
it poked up above 80% across the higher wattages. This performance is typical
of a high capacity unit. Those looking to buy for a low power system should
look elsewhere, but a high-end gaming rig or workstation will be well served
by this power supply.


While the 450W version of this power supply appeared to display the AC power
draw, this model did not appear to be the same. The displayed amount of power
was ~50W lower than the DC load throughout testing. Obviously, this is not a
precise piece of equipment, but it appears to be quite poorly calibrated. At
best, it is useful for checking the relative load on the power supply and for
showing off to friends, but it is not suitable for collecting objective data.


The active power factor correction in the Real Power 550 kept the power factor
consistently close to the ideal 1.0 value. This is much better than most ATX12V
power supplies, which often lack PFC of any kind, at least in North America
where power factor correction is not required by law. EPS12V requires active
power correction, so the Real Power is no better than any other EPS12V power
supplies in this regard.

5. FAN, FAN CONTROLLER and NOISE: The test environment is live, so
readings are higher than would be obtained in an anechoic chamber readings,
due to reflections and reinforcement of sound waves off the walls, ceiling and

The high speed dual ball bearing fan shows its colors as soon as the unit is
powered up: Even at the minimum voltage of 5.5V it’s a noisy beast. The noise
character is rough and clattery with a pure tone that increases in pitch and
volume when the fan speed increases. Bearing noise could be heard whenever the
power supply was on.

The starting voltage of 5.5V is a little higher than most other power supplies,
which also contributes to the loudness of the unit under low load. Even with
the high airflow provided by the fan, however, it still began to increase the
voltage around the 150W level. There is a reasonable amount of hysteresis built
into the fan controller, so changes in fan speed are smooth and fairly slow.

Cooler Master has been very conservative with the cooling of this power supply.
The high speed fan, and the higher than average fan voltages guarantee that
internal cooling will not be a problem for this power supply. This is reflected
in the high MTBF rating of 400,000 hours. Cooler Master is proud of this rating;
link the engineer’s test report
from the product page.

MP3 Sound Recordings of Cooler Master Real Power 550

Master Real Power 550 RS-550-ACLY @ <90W (29 dBA/1m)

Master Real Power 550 RS-550-ACLY @ 150W (32 dBA/1m)

There was no need to make recordings at higher power levels; it’s simply too loud.

Sound Recordings of PSU Comparatives

Tornado 400 @ 65W (19 dBA/1m)

S12-430 @ 150W (19 dBA/1m)

Noisetaker 600W (2.0) @ 150W (27 dBA/1m)

RX-520XPW @ 200W (30 dBA/1m)

92mm case fan @ 5V (17 dBA/1m) Reference


These recordings were made with a high
resolution studio quality digital recording system. The microphone was 3″ from
the edge of the fan frame at a 45° angle, facing the intake side of the fan to
avoid direct wind noise. The ambient noise during all recordings was 18 dBA or

To set the volume to a realistic level (similar to the original), try playing the Nexus 92 fan reference recording and setting the volume so that it is barely audible. Then don’t reset the volume and play the other sound files. Of course, tone controls or other effects should all be turned off or set to neutral. For full details on how to calibrate your sound system to get the most
valid listening comparison, please see the yellow text box entitled Listen to
the Fans
on page four of the article
SPCR’s Test / Sound Lab: A Short Tour.


There is much to like about the Real Power 550. Cooler Master’s decision to
rate it according to its continuous output spec is commendable. The added support for EPS12V
also speaks well of the design, as it has more stringent
requirements than ATX12V spec. The three +12V rails should make for
an exceptionally stable power supply.

Confusion could arise from the similarity between
the Workstation and PCIe connectors. Some documentation, ideally a warning sticker
on the connector itself would be a good idea to prevent newbies from just plugging
in whatever fits.

The 400,000 hour MTBF is also an impressive (some would say unrealistically high) number.
For those of you who are counting, 400,000 hours = ~45 years. This seems a stretch. Most other manufacturers
rate their power supplies for 80,000~100,000 hours. Personally, I would have preferred Cooler Master to sacrifice some reliability
for a slower, quieter fan. The stock fan makes the Real Power 550 a model to
avoid if noise is any concern at all. While the 450W version was hardly whisper
quiet, it was much, much better than the 550W model.

The Real Power 550 is a well-built product, but Cooler Master has not paid enough attention
to aural ergonomics to meet SPCR standards. If performance is all you care about, then you
should be well satisfied by this power supply, but if you are at all concerned
with the noise in your system, there are plenty of quieter alternatives
out there.

* * *

Much thanks to Lucy at
Cooler Master
for the opportunity to examine this power supply.

POSTCRIPT: Efficiency Correction
October 22, 2005

Recently, we discovered that our power supply testing equipment and methodology were providing erroneously high efficiency results. In general, the biggest errors occurred at higher
output load points above 300W. At lower output levels, the efficiency error
was often no more than one or two percentage points. No other tested parameters were significantly affected.

Through a fairly arduous process of discovery, analysis and old fashioned problem solving, we modified our testing equipment and methodology to improve the accuracy of the efficiency results and described it all in the article SPCR’s PSU Test Platform V.3. As part of this revision, we re-tested most of the power supplies on our Recommended PSU List. In most cases, the same sample was used in the second test.

The corrected and original efficiency results for all the re-tested PSUs are shown in in the article, Corrected Efficiency Results for Recommended Power Supplies. The relative efficiency of the tested power supplies has not changed.
If the tested PSUs are ranked by efficiency, the rankings remain the same whether we use the original results or the new results.

data is also being added to relevant reviews as postscripts like this one.

CORRECTED EFFICIENCY: Cooler Master Real Power 550

Target Output








Actual Output




In this case, our original efficiency calculations were 3~4% too low through to about 200W output. Above that, the original results were too high, and the error kept increasing with rising output power till it reached over 7 percentage points off at maximum load. The new figures show much better efficiency at the more important <200W level, which suggests a quieter, slower fan or smarter fan controller would work well in the PSU for typical systems.

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