Seasonic SS-300FS Active PFC PSU

Table of Contents

This well-established company enters the North American market with a line of quality power supply units that boasts an intelligent fan controller they call S2FC. Our review of Seasonic’s 300W ATX model. It’s pretty quiet. 🙂

June 5, 2002 — by Mike Chin

Products: Seasonic SS300-FS 300W Power Supply Units with
Active PFC
Date: June 4, 2002
Manufacturer: Sea Sonic Electronics
Supplier: Sea Sonic Electronics

Seasonic SS-300FS has the distinction of being the first product sample voluntarily
submitted for review in Silent PC Review. Until now, it is we who have
approached them. As a new site with no track record, sometimes we have
had to beg, cajole and plead for review samples. We don’t mind; this is to be
expected, and such experience may be good for humility, for the soul. Naturally,
we are pleased that Seasonic approached us. It feels like progress.

There are good reasons why Sea
Sonic Electronics
submitted their product for review in Silent PC Review:

  • We have the right audience. The SS-300FS is claimed to be a quiet product.
    This is exactly the kind of product we want to review and that our community
    wants to hear about.
  • They need the exposure. Seasonic is only just beginning to establish a presence
    in North America, which a big chunk of our audience. A review that focuses
    especially on their product’s claimed strength would help their marketing.

The fact is Seasonic was formed before the birth of the PC, all the back in 1975. Their headquarters are now in Taiwan, where production facilities have the capacity to churn out 850,000 power supply units a month. Yup, no missed decimal points there: 850 thousand. This company is no baby.


The review sample came in a nice colorful box suitable for retail display.
Not that we really care but it does show a certain level of attention
to detail. The MSRP is US$65, fairly standard for a quality brand PSU of this
power rating. A binder of information about Sea Sonic Electronics, 5 pages of
detailed technical specifications and 7 pages explaining their S2FC
fan control circuit
was also provided. A nice touch for the reviewer. There
are no instructions inside the box, just the SS-300FS PSU in plastic bubble-wrap.

The front panel sports the usual AC jack, power switch and a chassis cutout
grill for fan outflow. This last feature is never impressive. We all know that
a wire frame grill causes much less wind turbulence and noise, and likely adds
only pennies to the cost. Perhaps this review will show Seasonic that it matters
to some consumers.

A 110V-to-220V switch is notable by its absence; the unit automatically adjusts
for any AC voltage from 90V to 264V. This may not be a significant feature for
users, who are unlikely to transport their PCs between 110V and 220V zones.
However, it may help to save on production and distribution costs.

Cables are plentiful: aside from the usual ATX , legacy, and 12V
(P4) connectors, there are 7 4-pin Molex connectors for IDE devices and 2 for
floppy drives. Cables are on the long side, but not as long as the Enermax PSU:
27″ stretched for the IDE connector cables.

Specifications are available at Seasonic’s web site in PDF

The efficiency rating is average: over 65% at full load on 110 VAC. DC output specifications are consistent with those for most
quality 300W PSU.

DC Output

Ripple & Noise

Output Load Current

Combined Power














Under the cover, the SS-300FS has two reasonable sized heatsinks, and a tidy
layout with a clean printed circuit board. It sports an ADDA brand fan, model
AD0812HS-A70GL, rated at 0.25A, 12VDC. Adda has a reputation for making good
quality fans. Trying this one quickly in free air revealed a fairly smooth even sound unmarred by ticking, clicking or other annoyances. At 12V, it is quite loud. At 5V, fairly quiet but with a slightly buzzy or whirring quality — evidence of some bearing noise. According to ADDA’s
, it is rated for 38 CFM, 34 dBA and spins at a maximum of 3,000
RPM. The fan is thermally-controlled by a thermistor clamped to one of the 2


Before moving on to what the unit does when powered up, I want to tell you
something about temperature controlled fans in other PSUs. Even though Silent
PC Review has yet to publish an “official” review on a thermistor
fan controlled PSU, I have described their behavior in the forums and in the
articles on the P4 system and PSU experiments. They usually start out quiet,
but the fan(s) quickly speed(s) up to become quite noisy — even without a load,
outside a case, without any external heat to raise PSU temperature. The thermistor
that controls the fan speed always seems to be positioned near hot components
within the PSU that cause the thermistor to heat up and speed the fan up. This
is the behavior I have seen with two different models of Enermax 350W PSUs,
a SH 300W PSU, and a Zalman 300W PSU. Judging from comments
by a members in our forums, the new Antec True Power PSU appears to suffer the
same affliction.

The Seasonic SS-300FS is different. This PSU starts out quiet upon turn on
— and stays that way indefinitely without a load, outside a case. It
is a very pleasant and unique distinction among thermistor fan controlled PSUs
I have seen. I later measured the startup fan voltage to be 4.34 VDC

The PSU was started outside a case without connection to a motherboard. Running
a jumper wire between the green wire contact and any black wire contact in the
main ATX connector to the motherboard tricks the PSU into starting up. There
is no load, which is not ideal for many PSUs, but this is a convenient way to
listen to its minimum noise without fussing with motherboards. Later, I tried
the PSU outside a case while connected to a motherboard, with the same results.


The 7-page documentation about the S2FC fan control circuit
explained its quiet behavior. It also explained more clearly for me the
reason for the increasing fan speed behavior of other thermally controlled fan
PSUs. Much of the following is my interpretation and rewording of the printed
material Seasonic sent; the graphs come from Seasonic’s web site.

Seasonic reasons that an ideal fan control in a PSU should achieve sufficient
air flow for cooling with minimal fan noise. Thermal control of PSU fans is
not a new idea, but the concept of tailoring the response of the fan controller
to closely follow the real cooling needs of the PSU appears unique.

At low power levels, almost any power supply can be run with minimal airflow
from any 80mm fan because little heat is generated. That minimal airflow is
more than enough for adequate cooling as the power load is increased, until
a certain critical point is reached when the heat generated demands higher airflow
to cool the PSU components adequately. At this point, the fan speed must be
increased for effective cooling. But until this critical point is reached,
any increase in fan speed is not needed for adequate cooling.
It only
results in greater noise and wear on the fan.

Seasonic says its S2FC fan control circuit ensures that the fan remains at
minimal speed until extra airflow is really needed for cooling, thereby ensuring
the lowest noise over the widest range of operating conditions. Once the critical
power load point is exceeded, the curve becomes exponential, apparently to match
thermal characteristics of the PSU and to keep fan RPM (and noise) lower until

The above graph (courtesy of Seasonic’s
web site
) shows the behavior of 3 thermal fan controller circuits under
load for a 250W PSU. The vertical scale shows the fan voltage, the horizontal
scale shows system load in watts, which presumably has a direct correlative

The red curve represents a linear thermal fan controller. As system
load and heat increases, fan speed increases from the 5V minimum, which is the
usual safe consistent start voltage for 12 VDC fans, linearly to 12V at full
load. This circuit does not take into account any of the cooling effects of
heatsinks, and natural convection at low power loads. In this example for the
250W model, 75W is the load at which the fan must provide additional cooling;
the linear fan control already has increased the fan voltage to ~8.5V, with
accompanying increase in noise. It is my belief that all the other thermally
controlled fan PSUs I’ve had my hands on follow this simple linear curve. This
is why, running idle in a cool room with minimal load, they all get louder for
no good reason.

The green curve represents a thermal fan controller with exponential
characteristics. As system load and heat increases, fan speed increases from
minimum to 12V at full load following an exponential curve. Throughout most
of the operating range, an exponential thermal fan controller will keep the
fan spinning slower and quieter than the linear thermal fan controller. However,
because this circuit still ignores the fact that no additional cooling airflow
is required well past 75W, the fan is allowed to speed up much sooner than necessary,
thereby causing unnecessary wear and tear as well as unnecessary additional

The blue curve represents Seasonic’s S2FC thermal fan controller. It
starts as a straight horizontal line turn curves up exponentially. As system
load and heat increases, fan speed remains at the minimum (startup) 4.35V level.
The fan is not sped up at all until the thermistor reaches a temperature where
additional cooling is needed. For the SS300, this point is reached at ~100W
(at 25° C). When the fan does begin to speed up, it follows an exponential
curve. This means that the increase to maximum fan speed occurs faster the closer
the PSU gets to maximum rated output. The area under the curve is much smaller
than for the other curves; the difference in area represents the theoretical
reduction in noise and wear and tear provided by the Seasonic.

The characteristics of Seasonic’s S2FC thermal fan controller
should make for a PSU that is quieter throughout the operating range yet remains
well cooled. A particularly salient point is the 100W speedup point of the 300W

NOTE: Although Seasonic refers to power levels, the truth is that there is no direct power monitoring within the fan controller. The thermistor is the only monitoring device in the fan
circuit. However, the circuit’s calibration is such that the fan speed up point
corresponds to the temperature reached by the thermistor when the PSU is delivering
about 100W in an ambient temperature of 25° C. This explains the next
graph provided by Seasonic, which shows how the speedup point shifts depending
on temperature. So keeping the case cooler, whether with cooling fans or a cool
room, should have the effect of keeping the PSU fan low to a higher power point.

Those who have read my article on a Super
Simple Power Meter
may recall that the highest measurements I obtained on
total power dissipation of various systems was 120W maximum peak for a fully
loaded AMD XP1700+ system. The long term maximum for that same computer was
110W. This result was obtained with the CPU at 100% utilization. Two points
relevant to these results:

  1. My simple power meter may not be highly accurate for complex AC loads.
    One apparently knowledgeable reader said it is likely to read too high, perhaps
    as much 20% too high. Until tested against a better power meter, the accuracy of my meter is in doubt. I have to give some credence to the idea that my measurements are inaccurately high, which means even less power is drawn.
  2. Most applications that most people work with keep the PC at close to idle, with
    occasional instances of 100% CPU usage, again not for prolonged periods. In
    other words, the load on the PSU in a desktop PC is typically dynamic, with low averages
    accompanied by short burst of high power activity.

All of this suggest that the fan of the SS-300FS will not often speed up past
its startup voltage in a typical PC used in a typical way. It ought to be pretty quiet most of the time.


Testing Platform

The SS-300FS was installed in one of my main systems, which I would describe
as a stable, very quiet, low airflow PC running Windows 98SE, fully updated:

Case Landmark ATX-202 18″ tower
CPU Pentium 4 – 1.6A (overclocked to 2.0 GHz)
Motherboard Gigabyte GA-81RXP
Video Card Matrox G400 Max (dual mode, driving two 19″ monitors)
Hard Drives Seagate Barracuda IV – 20 G
Seagate Barracuda IV – 40 G
Both drives in cage at bottom of case, behind inflow case
Floppy Drive Generic
DVD Drive Toshiba SD-M1502
CD-Writer Creative RW121032E
Network Card Intel Pro/100VE – Built-in to motherboard
Sound Card Creative SB PCI128 – Built-in to motherboard
Fans 1 Panaflo 80mm “L” @5V over stock Intel heatsink
1 Panaflo 80mm “L” @5V over video card / NB heatsink
1 Panaflo 80mm “L” @4V lower front case fan

Test Instrumentation and Environmental Conditions

CPU temperature
Motherboard Monitor 5 reading CPU diode
PSU temperature
Veriteq Spectrum 1000 with probe lodged in PSU
Fan voltage
Heath / Zenith SM-2320 Multimeter across fan terminals
System power
Calculated with Super
Simple Power Meter
Heath AD-1308 Real Time Spectrum Analyzer
Ambient temp 23° C
Ambient noise 32 dBA (“A” weighted, approximate)

Monitor 5
needs no introduction for most PC enthusiasts. It enables monitoring
of temperatures and voltages off motherboard sensors. With the diode being read,
there is little question of inaccuracy here; it is usually accurate within 1°
C. The Veriteq Spectrum
will be familiar to anyone who has read my PSU and hard drive articles
on this site: it is a highly accurate data logger that samples temperatures
via its probe. The sampling frequency can be varied from every 10 seconds to
once a day. The Heath is an ordinary multimeter that has proven to be fairly

The Heath AD-1308 is a portable half-octave Real Time Spectrum Analyzer with
SPL meter functions. Below 40 dBA, its accuracy is limited to 3 dB increments,
down to 23 dBA. Some 15 years old, this LED-based unit has long since been
displaced by digital devices with better interfaces to PCs. However, it is one
I have access to and doesn’t cost hundreds of dollars to buy. (Thanks, Tommy!)
The “A” weighting was used, as recommended by numerous acousticians.

*NOTE on Noise Measurements: The mic was positioned almost touching the back case panel, about an inch to the side of the PSU fan exhaust to avoid fan turbulence in the microphone itself. The dBA measurements obtained here cannot be compared to any other measurements due to the lack of adherence to a repeatable standard and an uncontrolled reflective environment. (Sorry,
I tried, but I will not have access to that anechoic chamber till September.)

No effort was made to change acoustics in my small office, which measures 12 by 10 feet, with an 8 foot ceiling. The PC sits on the floor, under table that supports the monitors.

At Startup

Subjectively, the PSU is quiet, but audible. Considerably more audible than
my modified power supplies, which run Panaflo 80 mm fans at 5V or less, and
are essentially inaudible most of the time (depending on ambient noise). It’s
an obviously unfair comparison, because the Panaflo is rated at 21 dBA at full
tilt, while the ADDA in the Seasonic is rated 13 dB higher! Still, that is my
ideal reference for a fanned PSU.

The following measurements were taken after Windows 98SE was running for 2
minutes, with no programs open:

CPU temperature 40° C
PSU temperature 35° C
Fan voltage 4.31 V
System power 72W
Noise* 41 dBA (A)

With Hard Use

Time to see what happens when the system is pushed. I opened up my most demanding
applications: Photoshop 6.5 and Adobe Framemaker 6.0, and opened large files
in both. For good measure, I turned on my mail program and opened MS Internet
Explorer to the default home. I then turned on the only real game on the system,
Command & Conquer: Red Alert 2 and proceeded to play it in
the main 19″ monitor. The CPU temperature climbed steadily and quickly to 55-56° C within a few minutes, but then remained unchanged after that. The following
readings were taken 15 minutes after the game was started.

CPU temperature 55° C
PSU temperature 40° C
Fan voltage 4.32 V
System power ~80W to 98W
Noise* 39-40 dBA (A)

As you can see, the voltage to the PSU fan did not change. The 0.01V change
recorded is well within error tolerances and does not represent a real change.
I certainly did not hear any difference in the noise between startup and at
this point.

I tried a slightly different tack: writing this article in Macromedia Dreamweaver
4 while leaving all the above programs on — including the game. Now I began
to see and hear some changes. I continued pushing my system for another 15 minutes.
Here are the results:

CPU temperature 57° C
PSU temperature 42° C
Fan voltage 4.38 V
System power 96-98W
Noise* 40-41 dBA (A)

The fan was only very slightly but definitely louder. The change hardly registered
at all on the sound meter; I believe it amounted to a maximum of 2 dB, perhaps
as little as 1 dB. After another 15 minutes, I established that the temperature
could not be pushed beyond these levels. Fan noise remained unchanged.

With CPU Stability Test

CPU Stability Test by Jouni Vuorio
is a useful tool to stress systems. I set it to CPU warming only after shutting
down all the other programs and allowing it temperatures to go back down to
those at startup. Here are the results over 45 minutes.

System power is not provided as it stayed at a constant 120W.

Time (min)
CPU (° C)
PSU (° C)
Fan (VDC)
Noise dBA* (A)

The measured noise difference between start and finish was about 4 decibels.
I have to say this is definitely an approximation, as environmental conditions
were not ideal. What I heard jibes with that number — the fan did get louder,
and this difference was substantially louder than before. I personally would
not be satisfied with the sound level of the PC at the end of this test. However,
running torture programs is not something the usual PC user is likely to do
often, and the noise that I heard was substantially less than I’ve experienced
in the past with other thermistor-controlled fan PSUs.

It is interesting to note that 41-42° C appears to be the heatsink temperature
at which the fan voltage begins to rise above default. After the CPU hit 65°
C, the PSU temperatures continued to rise while the CPU temperature remained
unchanged. I believe this has to do with the buildup of hot air from the CPU
/ heatsink which can only exit through the PSU. Left long enough,
what would likely happen is that the temperature of the PSU would increase high
enough to a point where the fan would also be increased high enough to stabilize
the PSU temp, and then an equilibrium would be reached. As I feel any kind of
prolonged torture testing is an unrealistic load, I did not bother to see when
this would happen in the test system; the result would be very system dependent anyway.


The Seasonic SS-300FS is an impressively quiet power supply that remains at
the same quiet level throughout a wide range of conditions in a system that
while not heavily loaded, is somewhat more full featured than average systems.
When the fan did speed up, it did so only under extreme conditions of use or
during CPU stress testing, and the degree of noise increase witnessed and
measured was modest. The fan voltage never actually reached anywhere close to
12V; it remained under 5V throughout the testing.

There was never any question of stability being an issue: I experienced not
one crash or other computer misbehavior during the several days this PSU has
been in my system.

Compared to ALL other unmodified power supply units I have used, the Seasonic
SS-300FS is about the quietest. These other units include 2 models of Enermax,
SH, Zalman and many generic power supplies. Despite the slightly quieter startup noise level of the Zalman ST-300BLP, that unit’s rapid speedup makes it significantly more intrusive than the Seasonic, overall. As mentioned
before, this is the same behavior observed in the Enermax and SH models, as
described in detail in my previous PSU articles.

The PC Power & Cooling 275W model I have on hand may come closest among the unmodified competitors I have heard. It does not sport a thermistor, thereby avoiding
the distracting annoyance of a variable speed fan, and the fan is rated for
20 dBA in free air, 34 dBA in the PSU. I have not had the opportunity to examine
this unit carefully. A review will be forthcoming in the near future. Other models that looks promising include the Fortron available from and the Q-Technology models from

Seasonic’s S2FC fan controller circuit certainly is a big step in the right
direction for quieter power supply design, an innovative and intelligent application of existing technology. The only improvements we can suggest is a quieter fan and a proper wire fan grill.



  • Quiet
  • Not as quiet as my own modded PSUs
  • Innovative fan control
  • No track record in North America
  • Stable
  • Not widely available yet
  • Nice build quality
  • Universal AC input
  • Long established company

Conversations with Seasonic

When asked why a quieter fan like the 24 CFM Panaflo fan (21 dBA) was not chosen
for this unit, Seasonic expressed the need to ensure more than adequate airflow
at full rated power for safety reasons. Various agency approvals required for
PC power supplies dictates certain design criteria. The current fan does deliver
more airflow than is ultimately necessary, which Seasonic says is one way to
ensure longevity and reliability, but they are experimenting with quieter fans
to reduce the noise further. Considering that under a real load, the controller
never fed the fan even 5V, I’d say it’s a pretty conservatively rated design that would probably be perfectly safe with a quieter fan even with airflow.
I cannot really imagine how the full 300W capacity could be drawn by any desktop PC.

A promising tidbit mentioned was Seasonic’s current effort to increase efficiency at
full power from the current ~65% to approximately 75% on all the ATX models before the end of the year.
This would be a most welcome benefit. If the efficiency can be maintained at
lower power levels, the internal heat generated at 100W power delivery would
drop for 53W down to 33W. That is a most significant reduction in heat that
would lead to a dramatic reduction in cooling needs. We all know what means:
a quieter fan.

I am grateful to Seasonic for the review samples, their patience and their responsive assistance. Thank you.

As noted earlier, Seasonic is only just beginning to establish their presence
in North America. Currently, the following resellers have been established.
They do have a full range of models.

– The Computer Superstore

Case Depot

Silicon Acoustics

PS — Look here for a review addendum on their 400W model soon!

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