Seasonic Power Supply Units Roundup. Part 2

Today we are going to talk about three new models in the flagship Seasonic X Gold series, and about three more PSUs for compact FlexATX, SFX and TFX system cases.

by Oleg Artamonov
08/30/2011 | 08:41 AM

I’ve got as many as six power supply units from Seasonic for this review and they hail from two different worlds. On one hand, there are the expensive top-class ATX units of the X-Gold series with wattage ratings from 560 to 760 watts. And on the other hand, there are compact products for mini-ITX or microATX system cases that are described by the FlexATX, TFX and SFX form-factors and have a wattage rating of no higher than 300 watts.


Unfortunately, compact PSUs are not very popular among enthusiasts. I think they do not really deserve such an attitude because you can build a rather advanced gaming configuration even in a tiny cubic case with a 300W power supply. Yet the fact is that such PSUs are mostly limited to office machines built in slim low-profile enclosures.

Therefore I will not discuss the products in alphabetic order as I usually do. Instead, I will first talk about the more exciting X-Gold series and their junior cousins will follow later on.

Testing Methodology

Click the following link for a description of our testing methodology and equipment and a brief explanation of what the specified and tested parameters of power supplies mean: X-bit Labs Presents: Power Supply Units Testing Methodology. If you feel overwhelmed with the numbers and terms this review abounds in, refer to the Methodology.

You can also go to our Cases/PSU section to check out reviews of all other PSU models we have tested in our labs.

We will mark the actual power consumption of three system configurations (discussed in our article PC Power Consumption: How Many Watts Do We Need?) in the cross-load diagrams. This will help you see if the tested PSU can meet the requirements of a real-life PC.

Testing Participants

Seasonic X Gold: X-560 (SS-560KM), X-660 (SS-660KM) and X-760 (SS-760KM)

So, the focus of this review is on the X-Gold series from Seasonic, namely the X-560, X-660 and X-760 models. The series consists of six models in total. Besides the three mentioned one, there is a similar 850-watt unit and two fanless products with ratings of 400 and 460 watts. We reviewed one of the fanless X-Gold PSUs earlier (it’s called Seasonic X-400 Fanless SS-400FL) and are going to test the 850W one in the near future.

Curiously enough, the X-Gold series was in fact introduced twice. Last year we tested the Seasonic X-750 (SS-750KM) but soon after its release the company suddenly decided to restart the series. I don’t know the reasons but the fact is that the midrange X-Gold products have grown by 10 watts in their wattage ratings, so the SS-750KM is now transformed into SS-760KM. I don’t think this means something important. Seasonic must have just formally adjusted its product nomenclature for the newer products to be easily distinguishable from the first-wave ones.

The X-Gold power supplies come in black-and-gold boxes with the wattage rating of the particular model being explicitly indicated on its packaging.

The numerous advantages of the product are indicated on the back of its box, namely:

The accessories included into the box are not original but sufficient.

There are reusable and single-use cable straps, four screws, a sticker, and a multilingual user guide.

The PSU is parceled into a black velvety pouch with golden letters “Seasonic” to emphasize the high class of the product you’ve just bought.

Interestingly, the new PSUs of the X-Gold series have changed a little since the old X-750 even externally. The vent grid of the latter used to protrude a little above the surface of the case but now they are at the same level (the area around the grid is sunken in exactly to the same extent as the grid protrudes). This should improve the compatibility of the Seasonic PSUs with various system cases.

The overall design has remained intact, though. The PSU has a matte black case. It’s neat and lacks any decorations except for the Seasonic sticker whereas competing products can now sport colored plastic and metal details that can change a PSU’s appearance quite dramatically.

Well, I don’t think that the exterior design of a product you are likely to enjoy viewing for a maximum of half an hour (from the moment you unpack it to the moment you put it into your system case) is really important.

There are two rows of connectors for detachable cables on the back of the case. The X-Gold doesn't have fixed cables. Even the mainboard power cable is detachable. The connectors are not color-coded and are right next to each other, so you may want to connect the cables before putting the PSU into your system case, especially if the latter isn't roomy.

The design of the connectors has changed but slightly since the first version of the X series. A plastic decorative cover plate is missing and the labels are now printed on the metal rather than on a sticker above (the sticker looked rather unserious, I must confess). A 4-pin connector for a CPU cable is missing, too; there is only one 8-pin connector left.

Seasonic X-760

The interior design of the PSU has changed more (since I tested a 750W model in my earlier review, I show you a photo of its closest 760W cousin). First, the PCB is different. Although the circuit design has remained the same, some components have changed their positions inside the case. The capacitors and transformer have moved a little whereas the main PWM controller chip now resides on a small upright daughter card. These movements do not tell us much about any possible improvements in the operation of the PSU because Seasonic may have optimized the component layout for some other reasons, for example to make it easier to assemble the PSU or to utilize more affordable or available components. On the other hand, the X-760 is obviously not just a rebranded X-750 but a truly new version in its own right.

Seasonic X-760

The heatsinks are the most notable difference as they have become larger. We can see simple fins where the X-750 used to have smooth metal. And where the X-750 had simple fins, we now have a rather complex heatsink cut lengthwise and crosswise.

All of this is meant to improve the cooling of the PSU (of all of its components since every heatsink has become larger). Although I didn’t observe any overheat with the X-750, you can’t have too much cooling.

Seasonic X-560

Interestingly, the junior model of the series doesn’t differ much from its senior cousin except that it has fewer capacitors at the output. The heatsinks are the same. It must be simpler for Seasonic to order and install identical components into all the models of the series rather than to try to save on the lower-wattage ones.

Even with the changes I’ve described above, the heatsinks of the Seasonic X-Gold products look toy-like compared to many other PSUs. Those slim shiny bars that split up into "fingers" at the top (you can see them in the right part of the PSU next to a large power transformer) are installed on the input rectifier which is usually equipped with a large and massive heatsink. This is all due to the high efficiency which is itself due to the up-to-date circuit design of these PSUs. For example, they use a synchronous rectifier based on field transistors instead of a regular output rectifier based on diodes which dissipate quite a lot of power. The synchronous rectifier is more complex (the diodes work by themselves whereas the transistors have to be managed with a special chip) but also far more economical. The resistance of a modern field transistor is no higher than a hundredth of an Ohm when it's open, so it doesn't dissipate much power even when working with high currents.

There is a card with output connectors and voltage converters at the back panel of the PSU. Its component layout has also changed a little compared to the similar card installed in the X-750 although the main components have remained the same. There are two switching converters and two synchronous rectifiers on that card.

Chokes and smoothing capacitors are hidden between the card and the side panel of the case. I guess Seasonic engineers had a difficult time trying to fit all the required components into this small volume. There are polymer capacitors on the card which is especially important as they do not get any air flow from the fan, as opposed to the capacitors on the main PCB. On the other hand, these capacitors in the +5V and +3.3V rails have a much lower load in modern computer systems than the capacitors of the +12V rail.

Each of the three X-Gold models is cooled with a Sanyo Denki San Ace 120 fan (9S1212F404, 120x120x25 mm, 2200 RPM, 2-pin connection). If the fan fails or otherwise ceases to satisfy you after the warranty period, you will be able to easily find a replacement. The fan of the X-750 had a 4-pin connection, by the way.

Besides the smooth, aerodynamic shape of the impeller we can note the circle of hollows around its center. In two out of the three fans some of the hollows were filled with some brown glue-like substance. This indicates that each impeller was individually balanced by adding drops of glue to shift the center of mass as necessary. With such balancing the Sanyo Denki fans should ensure low vibration, excellent acoustic characteristics and a long life of the bearing (by reducing the stress caused by the beating of the impeller).

Each of the three models can deliver almost all of their full output power across the single +12V rail. This rail is not split up into multiple lines. The X-760 doesn't seem to be greatly different from the X-750 except that the -12V rail is weaker (0.5 instead of 1 ampere) but a modern computer has almost no consumers of that voltage (except for COM port drivers which are going to leave mainboards very soon, too). The total output power is 10 watts higher as is indicated by the model name.

Each model comes with the following cables:

Nothing seems to be missing here. The HDD power cables are now only 55 or 35 centimeters from the PSU case to the first connector whereas they used to be 55, 45 or 35 centimeters long to the first connector with the X-750, but the total number of connectors is the same.

The cables are packed into a cute pouch.

The junior model didn’t work well with my APC SmartUPS SC 620. The UPS would shut down when switching to the batteries even at a load of 300 watts. The other two models could work with my UPS at loads up to 340 W when powered by the batteries and over 400 W when powered by the mains.

These PSUs are not very confident working with near-zero loads (take note of that part of the diagrams built for the X-560 and X-660) but have no stability related issues otherwise. None of the voltages violates the permissible limits even at highly misbalanced loads, i.e. when there is near-maximum load on one rail and near-zero load on another rail.

These PSUs have virtually no output voltage ripple. People often ask me what power supply is best for a computer which must deliver high audio quality via analog output without an external DAC. Although I don't think that high-frequency voltage ripple of 20 millivolts more or less can affect the sound card (the mainboard is going to have a stronger effect on the latter, for example), you may want to consider these Seasonic products for that purpose.

The fans did not rotate at all at loads up to 150 watts and remained silent for quite a long time after that threshold. It is impossible to hear a high-quality 120mm fan rotating at less than 600 RPM from inside a system case. The Seasonic X-Gold PSUs become somewhat audible at loads over 500 watts but the X-560 does not really get noisy at all. The X-660 and X-760 can be called noisy at loads of 600 watts and higher. That's just an excellent performance.

Compared to the previously tested X-750, the X-760 is somewhat quieter at loads above 400 watts. This may be due to the larger heatsinks of the X-760 or to random variation in their component specs. By the way, the X-760 uses a 2-pin fan connection whereas the X-750 has a 4-pin one.

The efficiency is expectedly high. Each model is over 90% efficient, the 760W model being the only one to touch the 80% mark with one end of its graph. Each of them is 90% or more efficient at full load.

The standby source is okay, too. At the maximum load of 3 amperes its voltage was never lower than 4.9 volts, the allowable minimum being 4.75 volts.

Summing up this part of the review, I can say that Seasonic has come up with excellent power supplies which are quiet, stable and reliable. It’s not clear what reasons the company had for producing the second revision of the X-Gold series. Although there are conspicuous changes, none of them is critical, and I haven’t heard about any problems with the first revision. Seasonic seems to have just corrected any small shortcomings that had been noticed over the previous year and made the good power supply even better.

Seasonic SS-250SU

The first of the compact PSUs I am going to discuss in this review follows the FlexATX form-factor which was introduced not long ago, in 2007. Although the namesake mainboard form-factor developed back in 1999 and representing something in between microATX and mini-ITX (FlexATX has two expansion slots and is compatible with microATX in terms of the mounting holes) never really took off, FlexATX power supplies can be found in compact system cases such as the rather popular Cooler Master Elite 100. Moreover, this PSU from Seasonic has a wattage rating of 250 watts whereas the native PSU of the Elite 100, only 150 watts. Of course, it’s hard to think of components that could be fitted into the tiny Elite 100 and would require over 150 watts, but what if you've really got them?

The SS-250SU is a small box you can easily grasp with one hand. It measures 150 x 81.5 x 40.6 centimeters. There are even cutouts in the top and bottom panels of the case to accommodate its 40mm fan, so compact this power supply is.

As you might expect, the interior is filled far more densely than any ATX power supply. There is no empty spot on the PCB even the size of a square centimeter. A soft heat-conductive electrical-insulation pad is laid on top of the L-shaped heatsinks to transfer the heat to the top panel of the case. Of course, the steel panel with a sticker is not much of a heatsink, but every means of cooling is welcome in a PSU with such a high component density.

The PSU is cooled with a 40mm fan (Superred CHA4012DB-M) that has a max speed of as high as 8900 RPM. Hopefully, it won't rotate at its maximum. The fan is 20mm thick (the native PSU of the abovementioned Cooler Master Elite 100 has a thickness of 15 millimeters), so we can expect it to be rather efficient.

The output cables are obviously designed for OEMs. The mainboard power cable is quite normal but the rest of the cables can be called modular. There is a 10cm tail going out of the PSU which ends in a 12-pin connector a bunch of cables is attached to. Thus, the SS-250SU can be instantaneously tailored by Seasonic to the particular client's needs.

My sample of the PSU has the following cables and connectors:

The only problem I can see is the lack of SATA connectors. There is not much space for adapters in those system cases this PSU is designed for.

Despite its low wattage rating and limited applications, the Seasonic SS-250SU is quite a modern product. It boasts 80+Bronze certification and can deliver over 200 watts across its +12V rail.

This PSU had no problems working with my APC SmartUPS SC 620, which is no wonder considering its 250-watt rating.

The cross-load diagram looks good. The PSU copes with misbalanced and near-zero loads.

The “+1” in the diagram marks the measured power consumption of a computer system with a Core 2 Duo and a Sapphire Radeon 4850 which corresponds to quite a high-performance gaming configuration of some three years ago. Considering that modern CPUs and midrange graphics cards consume the same amount of power, this indicates that you can build a fast enough computer with this power supply.

The output voltage ripple is very low even at the maximum load.

The PSU is over 80% efficient at loads up to 50 watts and over 85% efficient at loads above 65 watts. The 80+Bronze standard requires that the PSU be as efficient as 82% and higher at loads of 20% (i.e. 50 watts). My measurements produced a lower number, which might be due to measurement inaccuracies of our testbed at low loads.

The standby source does its job well, delivering up to 2 amperes without lowering the output voltage much.

Unfortunately, I could not measure the speed of the PSU fan. My tachometer could not catch a stable reflection from its tiny blades even after I had glued a piece of shiny film to one of them. Subjectively, the PSU gets noisy at loads above 150 watts but the noise is not very discomforting. It is a soft whisper of the air rather than the irritating hiss typical of such small fans.

Seasonic SS-300SFD

The next product is a variant of the SFX12V standard. There are as many as five such variants in existence today, and they are not even compatible with each other.

The SS-300SFD represents “SFX12V with a top-mount fan”. The variant owes its name to the 80x80x25mm fan which is large for such a compact PSU and sticks out of its case a little.

There are SFX12V products without the protruding fan. For example, the Silverstone Sugo SG06 is equipped with a FSP300-60GHS which features a 15mm-thick fan which fits within the case. In other words, there are two sub-variants of one of the five SFX12V variants, and these two are not even fully compatible. The SS-300SFD will not fit into some system cases where the height of the PSU is a critical factor.

The interior design is more alike to traditional ATX power supplies than that of the above-discussed SS-250SU. There is a heatsink with the power transistors of the converter and active PFC on the right. The diodes of the output rectifier are on the left. The power transformer is in the middle. Still, the component density is impressively high.

Quite a lot of components reside on the bottom side of the PCB. The quality of soldering is next to ideal, as you can note.

The PSU is cooled with an ADDA AD0812HB-A70GL fan (80x80x25 mm; 3010 RPM). The fan is perfectly standard and has a popular form-factor, so you can easily replace it if necessary.

This PSU is equipped with the following non-detachable cables:

Computers with gaming graphics cards are but seldom equipped with such PSUs, yet I know such examples, therefore it is regrettable that the SS-300SFD lacks 6-pin graphics card connectors. It would also be better to replace one of its two PATA power cables with a SATA one.

Working with my APC SmartUPS SC 620, this power supply was stable at loads up to 270 watts (also when switching to the batteries and back again). The UPS would become unstable at 280 watts and would shut down immediately at a load of 300 watts as soon as it tried to switch to the battery power. Thus, this PSU calls for a reserve of power on the side of your UPS.

The PSU can deliver almost all of its output power across the +12V rail but also supports high loads on the +5V and +3.3V rails as well as on the standby source.

The output voltages were not ideally stable (this PSU does not have dedicated voltage regulation, after all), yet I am quite satisfied with them. Our test configuration (including a rather fast Radeon HD 4850 graphics card) is in the green zone where the voltages deflect no more than 1% from their nominal values. The PSU copes with very low loads well, too.

The high-frequency ripple of the output voltages is much lower than the permissible maximums.

The low-frequency voltage ripple is very low.

The PSU features 80+ certification and meets it indeed: its efficiency is over 81% at loads from 50 to 300 watts.

The fan controller keeps the fan speed constant at 1250 RPM, which is almost silent, when the load (and temperature) of the PSU is low. When the load is higher than 130 watts, the speed of the fan increases linearly. The PSU becomes audible at 200-220 watts and noisy at over 250 watts.

The standby source is okay. Its voltage sags by a mere 0.1 volts at a load of 2 amperes.

Seasonic SS-300TFX

The last compact power supply unit from Seasonic to be reviewed here is called SS-300TFX. As is indicated by its name, it complies with the TFX12V standard.

TFX power supplies have an elongated shape. They are thicker and more massive than the above-discussed FlexATX unit (SS-250SU). You can often see them in low-profile microATX and mini-ITX system cases such as the BL series from InWin or the slim enclosures from Foxconn and many others.

TFX-compliant PSUs are equipped with an 80mm fan which is positioned like the 120mm fan in an ATX power supply so that the air flow goes from above towards the PCB rather than along its surface. The TFX form-factor allows the fan to be flush with the case or protrude by no more than 5 millimeters. The SS-300TFX represents the latter option.

The SS-300TFX resembles the SS-250SU in its interior design but there is no need to invent heatsinks that transfer heat to the panels of the case. The air flow is quite enough for cooling. The fan is positioned right above the rear heatsink which is the hottest one, cooling the diodes of the output rectifier.

The PSU is cooled with an ADDA AD0812HB-C70 fan (80x80x20 mm, up to 3100 RPM; brushless bearing). It won’t be easy to replace it since such slim fans (the standard thickness is 25 millimeters) are less available in retail. Well, if your system case permits, you can try to improve this power supply by installing an 80x80x25mm fan that will protrude by 10 millimeters above the top panel.

The Seasonic SS-300TFX has 80+Bronze certification. It can deliver up to 252 watts across its 12V rail, its max output power being 300 watts.

The PSU is equipped with the following cables and connectors:

Working with my APC SmartUPS SC 620, this power supply was stable at loads up to 260 watts. At 280 watts the UPS would shut down when switching to the batteries. Thus, the UPS has to have a much higher wattage rating than the power consumption of the system you use the SS-300TFX for.

The cross-load diagram is quite a typical view for a PSU without dedicated voltage regulation, but the SS-300TFX is stable enough for its class. Its voltages go out of the permissible limits only when the load is highly misbalanced towards the +5V rail, our reference configuration hitting the bright green zone. Besides, the SS-300TFX can easily handle zero and very low loads.

The high-frequency output voltage ripple is very low even at the maximum load.

The same goes for the low-frequency voltage ripple.

The PSU is over 80% efficient at a load of 50 watts and higher and 85% efficient at 70 watts and higher.

The fan rotates at 900 RPM only when the PSU load is low. The PSU is virtually silent then. At a load of 90 watts and higher the speed grows up linearly, making the PSU audible at 170-180 watts and rather irritating at 230-240 watts. Anyway, the SS-300TFX is going to be very quiet considering that the typical power consumption of computers with such PSUs is no higher than 80-120 watts.

The standby source keeps the voltage very close to the required 5 volts.


The X-Gold series is a double success for Seasonic. A year ago I couldn’t find any fault with the X-750 model and now I can see nothing to complain about in the X-560, X-660 and X-760. Very quiet (and virtually silent at low and medium loads), stable and neatly assembled, these are among the best PSUs available today. Although many of Seasonic’s competitors already offer products with efficiency of over 90% or very quiet PSUs, the X-Gold series combines all of the best properties. I don’t call them the best because there are high-quality products from other renowned brands, but these Seasonic PSUs are surely in the top three. Therefore, we are proud to award Seasonic X-Gold series with our Recommended Buy title:

I noticed quite a lot of small differences between the new Seasonic X-760 and the old X-760 but none of them can be called critical. The two models are very close in their electrical parameters. So it seems that the new revision of the Gold series is not a correction of critical problems or a complete overhaul of the old design. It is just an improvement on an already good product. It means that you don't have to worry if you've got the first revision. And if you are shopping for a new PSU, you are going to be satisfied with a second-revision X-Gold unit.

As for the compact PSUs of the FlexATX, TFX and SFX form-factors, they show Seasonic’s traditionally high quality, good electrical parameters and stability. Unfortunately, the retail market of such PSUs is very small, so owners of compatible system cases have difficulty finding any suitable PSU rather than face the problem of choosing among competing models. The numerous compact PSU form-factors are the real problem here. There are a dozen form-factors of that kind, so it's hard and not always profitable for manufacturers and retailers to produce, store and distribute such a large product range.