When the power supply was working in pair with an APC SmartUPS SC 620, the UPS overload indicator would turn on at a load of over 350W irrespective of the power source (electric mains or batteries). The switching-over to the batteries was performed without problems.
The first problem with the Neo HE was that it only started up at a second attempt. Our testbed wakes the PSU up as an ordinary PC does, i.e. by sending a low-level signal to the PS_ON contact (it’s the green wire, usually) of the PSU connector. The Neo HE didn’t react at all to my first press on the Power button, but would start up normally on a second press. This didn’t depend on whether the load on the PSU was zero or other at the moment of my trying to turn it on.
The second problem was about the pulsation of the output voltages. It looks normal at first sight:
At a load of 550W the output voltage ripple was 38 millivolts on the +5V rail, 31 millivolts on the +12V rail and 32 millivolts on the +3.3V rail, but the amplitude would occasionally jump up so high that I began to suspect some problem with our testbed. However, there were no problems with the High Power HPC560-A12S and the OCZ OCZGXS700 that were tested right before and after the Neo HE, so I threw my suspicions away. The testbed couldn’t have had something against products from the Antec brand only. Moreover, the surges of voltage didn’t vanish at lower loads, although their amplitude and duration diminished proportionally (the voltage ripple fitted into the requirements of the ATX standard at a load of 250W and lower). The oscillogram with a 1ms/div time base, recorded under a load of 400W, shows one such surge. It lasted for about 6 milliseconds and had an amplitude of about 100 millivolts on the +5V rail (to remind you: the allowable maximum is 50 millivolts).
It would go beyond the scope of this review to analyze the circuit design of the PSU to find the reason for the described phenomenon. So, I only have to say that the stability of the revision A3.1 Antec Neo HE 550 calls for improvement. Perhaps I was testing a defective sample, but recalling the forum discussions concerning incompatibility of some mainboards with the revisions lower than A4, I’m inclined to think that that is a common problem, not a single instance.
The output voltages proved to be very stable irrespective of the load on the power supply. The Neo HE features a superbly implemented independent regulation of all the three voltages.
The PSU is cooled with a single 80mm Adda AD0812HB-A71GL fan whose speed is adjusted linearly depending on the temperature.
The PSU is really quiet. The fan speed is just a little higher than 2500rpm even at full load. In a load range typical of a modern computer (i.e. below 300W), the PSU is altogether silent.
Of course, such a quiet cooling with a single 80mm fan is made possible not only by the large heatsinks but also by the excellent efficiency, which is 86% at the maximum. However, the Neo HE doesn’t meet the requirements of the 80 Plus program that’s becoming popular nowadays because its efficiency sinks down under low loads.
The Neo HE 550 left me doubtful. In most of its parameters it is a superb power supply (which is not a surprise considering the reputation of its actual manufacturer Seasonic): excellent efficiency, quiet operation, very stable voltages, all the necessary connectors (but you must make sure you connect the graphics card cables correctly). But the surges in the pulsations on the PSU output and the PSU’s waking up only at a second attempt are somewhat alarming facts. Perhaps these defects are corrected in revision A4 (and users file less complaints about it), but I can’t say it for certain until I test it. Anyway, if you are going to buy this model, make sure you buy at least its fourth revision.