New Level of Gold: Three Seasonic Power Supply Units Reviewed

We are already well familiar with two generations of Seasonic X Series power supply units, which are 80 PLUS Gold certified. But the manufacturer doesn’t stop at that and today we are going to talk about the most powerful product in this series as well as a couple of refreshed PSUs of slightly lower capacity.

by Dmitry Vasiliev
04/28/2013 | 04:11 AM

We first met Seasonic's Gold-certified X series PSUs back in 2010. A year later we tested their second generation which sported a substantially modified hardware platform. And now we’ve got Seasonic’s new X series PSUs with two platforms we have not tested previously. There are two models in the same wattage range as before (they are now referred to as KM3 instead of KM) and one model from the high-wattage XM subseries which goes from 1000 watts upwards.


The KM3 subseries is represented in our review with wattage ratings of 650 and 850 watts while the XM-indexed subseries is represented with the junior of the two currently available models which has a wattage rating of 1050 watts.

Testing Methodology

The following article offers a detailed 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 SS-650KM3 (X-650) and Seasonic SS-850KM3 (X-850)

We were somewhat confused when we first saw these products. We thought we had tested such PSUs already…

However, the model names were different from what we’d tested and the picture of the interior design was unlike anything we'd seen before.

The overall design of the packaging has not changed since the previous models of the series, though.

The packaging used to be individual for each model but is unified now for the entire series. The wattage and specs of the particular model are indicated on stickers.

The accessories haven’t changed, either. We can see a user manual, a set of single-use and reusable cable straps, mounting screws, a mains cord, and a Seasonic sticker for your computer case.

The PSU itself is traditionally packed into a velvety pouch.

The modular cables come in a dual-section bag made of some synthetic fabric.

Exterior Design

There are but few external differences we can note in comparison with the previous models.

The most notable one is the cooling system switch you can see next to the modular connectors. We've already seen it in Seasonic's Platinum-certified PSUs.

We can also see that the modular connectors take more space and the manufacturer had to remove any vents from that panel.

There are no differences from the previous models in this view.

Circuit Design

The SS-650KM3 (X-650) and SS-850KM3 (X-850), shown in the top and bottom photos respectively, are almost identical in their interior design excepting some differences in component ratings.

It is easy to see that the lower-wattage model has blue Hitachi AIC capacitors at the input whereas its higher-wattage cousin has brown United Chemi-Con capacitors there.

The overall circuit design has remained the same: active PFC and DC-DC converters located on the card with modular connectors. These PSUs are similar to their predecessors in smaller details, too.

The component layout is completely different, however. Even the card with DC-DC converters and modular connectors is not the same we saw in the previous models of the series, just because the configuration of connectors is different.

There is now a large aluminum heatsink instead of tiny individual ones on the field-effect transistors of the synchronous rectifier. The design and position of the daughter cards have changed as well. The input capacitors are closer to the corner of the case and one long heatsink is replaced with three small ones.

These PSUs employ an SRC/LLC+SR controller Champion Micro CM6901.

The standby source is based on an Infineon ICE2QR4765 chip.

There are electrolytic capacitors from United Chemi-Con at the output. They enjoy an excellent reputation.

All in all, the new features of Seasonic's latest X series PSUs are comparable to those we witnessed on the transition from their first to second generation: the internal layout is completely overhauled although there are no fundamental innovations. We’ll see shortly if these changes have affected the electrical parameters.

Cables and Connectors

The all-modular X-650 is equipped with the following connectors:

Included with the X-650 are:

The X-850 has the same cable system plus one more cable with two 6+2-pin connectors for graphics cards.

The selection of cables and connectors is quite sufficient and versatile. The single graphics card cables of the new PSUs are handier than the dual cables of their predecessors. The number of infrequently used PATA power connectors has been reduced from eight to five while the number of SATA power connectors has increased from eight to ten (but the connectors are placed with smaller spacing, which may be inconvenient in some cases).

Overall, the cable system has become more balanced and up-to-date compared to the PSUs' predecessors.


The SS-650KM3 (X-650) and SS-850KM3 (X-850) are similar to their predecessors in their specs and meet today’s requirements by being able to deliver most of their output power via the +12V rail. The load capacity of the less important +5V and +3.3V rails is rather low at 125 watts.

As their predecessors, these PSUs comply with the 80 PLUS Gold specifications. The warranty period has been extended from five to seven years, so the SS-650KM3 (X-650) and SS-850KM3 (X-850) match Seasonic’s Platinum-certified products in this respect.

UPS Compatibility

Working together with our APC SmartUPS SC 620, the X-650 was stable at loads up to 412 watts when powered by the mains but could only switch to the UPS’s batteries at loads up to 302 watts. The numbers for the X-850 are 403 and 300 watts, respectively.

Cross-Load Stability

It’s impossible to find fault with the cross-load stability of these PSUs. The +12V voltage is always within 1% of the required level whereas the other voltages are within 2%.

Thus, the newest Gold-certified PSUs from Seasonic are superior in this test to their predecessors which couldn’t always keep their voltages within the 3% range as promised by the manufacturer.

Output Voltage Ripple

The newest PSUs from Seasonic were superior to their predecessors in the previous test but things aren't so bright here.

The high-frequency voltage ripple is within the permissible limits, yet stronger than with the previous PSUs of this series. By the way, the SS-650KM3 (X-650) and SS-850KM3 (X-850) perform in the same way here, so we only show you the graph of the higher-wattage model.

The output voltage ripple at the double mains frequency is rather weak with the X-650.

The low-frequency voltage ripple of the X-850 is weak on the +3.3V and +5V rails but strong on the +12V rail. It is always within the norm, though.

Temperature and Noise

As the previous Gold-certified PSUs from Seasonic, the SS-650KM3 (X-650) and SS-850KM3 (X-850) use a Sanyo Denki San Ace 120 fan (part number: 9S1212F404; rated speed: 2200 RPM). Such fans are perfectly balanced to minimize their operating noise.

The new PSUs feature a switch to select the operating mode of the fan between Normal and Hybrid. In the Normal mode the fan works always, starting up at low speed and then accelerating at higher loads. In the Hybrid mode the fan is idle at low loads and only works constantly at high loads.

So, in the Normal mode the fan of the X-650 starts up at a speed of 830 RPM and begins to accelerate at a load of 550 watts only. The acceleration is rapid, adding more than 400 RPM to the fan’s speed over a load range of 100 watts, yet even the resulting 1300 RPM doesn’t sound uncomfortable thanks to the high quality of the fan.

In the Hybrid mode the fan is idle until a load of 400 watts and then begins to turn on occasionally. It is only at loads above 500 watts that it works constantly. Its top speed is the same 1300 RPM as in the Normal mode.

The X-850 is expectedly louder at high loads just because its wattage is higher.

In the Normal mode the initial speed of the fan is somewhat lower compared to the 650W model and the fan accelerates at a somewhat higher load. Anyway, the X-850 is noisier than its cousin at full load as its fan rotates at 1700 RPM (which is comparable to its predecessor's result, by the way).

As for the Hybrid mode, the X-850 behaves like the X-650. The fan accelerates sooner than in the Normal mode but not so rapidly, reaching the same top speed in the end.

The two PSUs are very quiet at medium loads (even absolutely silent in the Hybrid mode). It is only at high loads (above 600 watts) that their fans remind you of their existence but we doubt that the PSU is going to be the computer’s main source of noise then.

Efficiency and Power Factor

At the reference loads of 20%, 50% and 100% the X-650 was 90.6%, 92.9% and 90.5% efficient. Its peak efficiency of 93.4% was observed at a load of 311 watts. We can also note that its efficiency was higher than 90% at any load above 120 watts.

The power factor is just a little lower than the promised 99% at high loads.

At the reference loads of 20%, 50% and 100% the X-850 was 91.8%, 93.3% and 89.8% efficient. Its peak efficiency of 93.5% was observed at a load of 435 watts. We can also note that its efficiency was higher than 90% in a load range of 100 to 830 watts.

The power factor was 99% at high loads.

Overall, the PSUs are a little more efficient compared to their predecessors.

Standby Source

The standby voltage of each PSU is normal.


Seasonic SS-1050XM (X-1050)

Although Seasonic’s XM subseries came out earlier than the above-discussed KM3, it is still being produced, so we’ll follow our traditional course of reviewing PSUs from lower to higher wattages.

We won’t dwell upon their packaging and accessories as they are identical to those of the KM3 PSUs and their predecessors.

We can only note that the packaging is individual for each model in the series: the wattage rating is printed right on the box.

Exterior Design

External differences from the lower-wattage X series products are not limited to the larger dimensions. Instead of a conventional punched-out fan grid we can see a cutout in the PSU case which is covered with a gold-colored plastic piece the fan is attached to. The plastic plate easily bends in if pressed with a finger. It’s got a honeycomb grid of vent openings which helps identify the PSU as a Seasonic product but is no good in terms of acoustic comfort. It is going to produce more aerodynamic noise at high speeds of the fan.

There are almost no free space on the panel with modular connectors since the increased wattage implies broader expansion opportunities.

Even the back view is different from the other X series models. The case is definitely longer here while the Power switch is larger.

As a matter of fact, the exterior design of this PSU, from the plastic detail with fan to the large Power switch and modular connectors, is identical to the Seasonic SS-1000XP. However, the overall style is unmistakably that of the X series.

Circuit Design

The interior design is similar to the Platinum-certified Seasonic we tested earlier. Compared to the Gold-certified models, the SS-1050XM (X-1050) seems to use elements of both newer (see above in this review) and older products on a somewhat larger main PCB.

The bottom and left of the photo correspond to the new KM3 series: the layout of the heatsinks with a daughter card in between, the filtering chokes, and the input capacitors.

The leftovers from the older KM series PSUs can be spotted in the top and right part of the photo: the low-voltage section with small individual heatsinks, and the mains connector.

Thus, the overall circuit design is comparable to the other X series products: active PFC and dedicated voltage regulation based on DC-DC converters.

Like the above-discussed PSUs, the SS-1050XM (X-1050) uses an Infineon ICE2QR4765 chip for its standby source. A Rubycon capacitor can be seen nearby. The lower-wattage PSUs didn’t have components of that brand.

But as in the above-discussed models, the output capacitors are made by United Chemi-Con.

Cables and Connectors

The all-modular SS-1050XM (X-1050) is equipped with the following connectors:

Included with the PSU are:

The cable system resembles the previous generation of X series PSUs with its dual graphics card cables and large number of PATA power connectors. As for SATA, the X-1050 only has one such connector more than the lower-wattage models discussed above.

There is one significant difference from the earlier X series products, though. Their modular cables were mostly flat (excepting the 20+4-pin mainboard cable and the dual graphics card cables) but the top-wattage models feature more conventional round cables in nylon sleeves.

The selection of cables and connectors is quite sufficient and, considering that such PSUs are targeted at very advanced PC configurations, the dual cables are okay, too. Such PCs are likely to include a top-end graphics card with two power connectors, so there won’t be unused cables hanging inside the computer case.


Like every other modern product from Seasonic, the X-1050 can deliver most of its power via the +12V rail whereas the load capacity of the other rails is relatively low (150 watts isn't much for a 1050W PSU).

It sports 80 PLUS Gold certification.

As opposed to the newest products from Seasonic, it comes with a 5-year warranty. That’s long, but the 7-year warranty of the KM3 series is even better.

UPS Compatibility

Working together with our APC SmartUPS SC 620, this PSU was stable at loads up to 405 watts when powered by the mains but could only switch to the UPS’s batteries at loads up to 290 watts.

Cross-Load Stability

The X-1050 is somewhat worse in this test than the above-discussed models, its +12V voltage being more than 1% off the required level at low loads. When there's high load on the other rails, the +5V voltage is more than 2% off. On the other hand, this PSU is better than earlier X series products in this respect.

Output Voltage Ripple

The high-frequency voltage ripple is comparable to the new models we’ve tested above (it’s somewhat stronger on the +12V rail and weaker on the +5V rail) but worse compared to earlier Seasonic products.

The low-frequency ripple is rather strong on the +12V rail (as with the above-discussed PSUs) and on the +5V rail, but always remains within the permissible limits. It’s weak on the +3.3V rail.

Temperature and Noise

As the KM3 series discussed above, this model is cooled by a Sanyo Denki San Ace 120 fan (part number: 9S1212F404, rated speed: 2200 RPM) and offers a switch to choose between Normal and Hybrid operating modes.

In the Normal mode the fan behaves like in the above-discussed PSUs except that its initial speed is very high at almost 1100 RPM. The fan reaches its rated speed of 2200 RPM at a load of 900 watts.

It is only at loads below 700 watts that this PSU stays quiet.

The Hybrid mode is only good at extremely low loads. The fan turns on periodically even at loads of 100 watts and works constantly at 300 watts and higher. And its initial speed is over 1000 RPM, just like in the Normal mode.

There is a sudden increase in speed at 600 watts, the fan reaching its top speed by 900 watts - exactly as in the Normal mode.

Frankly speaking, the cooling system of this PSU left us disappointed. The initial speed of the fan is too high and the stretch of passive cooling in the Hybrid mode is too short. That’s not like other Seasonic PSUs, so we even suspected our sample to be defective. However, reports from users and other reviewers agree with our test results.

Efficiency and Power Factor

At the reference loads of 20%, 50% and 100% this PSU was 90.2%, 92% and 88.5% efficient. Its peak efficiency of 481 watts was observed at 93.1%.

The efficiency is somewhat lower compared to the above-discussed PSUs, which can be explained by the difference in wattage. Higher-wattage PSUs are generally less efficient than their lower-wattage counterparts. So, whatever the reason, Seasonic's newer KM3 series PSUs are more efficient.

The power factor is close to 99% at high loads.

Standby Source

The standby source copes with the specified 3A load easily.


Seasonic’s newest X series PSUs from the KM3 subseries carry on the glorious traditions of the earlier Gold-certified products from that brand. They are similar to their predecessors visually, come with numerous accessories, offer higher efficiency and better voltage stability, have a new operating mode for their fan and a more balanced cable system, and boast a longer warranty.

The only downside we could find in our tests was the increased level of output voltage ripple. On the other hand, this ripple is within the permissible limits, so it’s not a problem really.

Including top-wattage models, the XM subseries is not so all-around perfect. Yes, it is no worse than the earlier KM series PSUs in voltage stability or efficiency and even offers a richer selection of cables and connectors, but the KM3 subseries is better in these respects.

The key downside of the X-1050 is its rather noisy fan. Although the high-quality fan is more or less comfortable at its initial 1000 RPM, you can hardly be satisfied with it if you prefer silent computers. The Hybrid mode is not particularly useful because the PSU will only be cooled passively when idle considering its orientation at high-performance configurations with multiple graphics cards.

So, if you’re choosing between Gold-certified PSUs from Seasonic, you may want to think twice whether you really need a 1000-watt or higher PSU. Because you can instead prefer a lower-wattage model with better parameters. An 850W PSU would be quite enough for even very advanced configurations with an overclocked CPU and two graphics cards.