Efficiency, Temperature And Noise
Our efficiency testing procedure is detailed here.
Using the results from the previous page, we plotted a chart showing the ST75F-PT's efficiency at low loads, and loads from 10 to 110 percent of its maximum-rated capacity.
The efficiency difference between 115V and 230V input is notable, and it increases significantly at higher loads. This is natural, of course, since amperage is reduced at 230V, cutting power losses. Compared to high-performance 750W Gold-rated PSUs we've tested, the ST75F-PT easily takes the lead. But it can't match Seasonic's Snow Silent, which is based on a top-notch Platinum-rated platform.
Efficiency At Low Loads
In the following tests, we determine the ST75F-PT's efficiency at loads significantly lower than 10 percent of its maximum capacity (the lowest load the 80 PLUS standard measures). The loads we dialed were 20, 40, 60 and 80W. This is important for representing when a PC is idle, with power-saving features turned on.
|Test #||12V||5V||3.3V||5VSB||DC/AC (Watts)||Efficiency||Fan Speed (RPM)||Fan Noise (dBA)||PF/AC Volts|
Under light loads, the PSU operates in passive mode with high efficiency. We are pleased to see that, even subjected to a 20W load, it scores above 70 percent. In the rest of the tests, it easily exceeds 80 percent.
The ATX specification states that 5VSB standby supply efficiency should be as high as possible, recommending 50 percent or higher efficiency with 100mA of load, 60 percent or higher with 250mA of load and 70 percent or higher with 1A or more of load.
We take four measurements: one each at 100, 250 and 1000mA, and one with the full load the 5VSB rail can handle.
|Test #||5VSB||DC/AC (Watts)||Efficiency||PF/AC Volts|
The 5VSB rail registers high efficiency, even in the first two tests. Sirfa does a good job here.
Power Consumption In Idle And Standby
In the table above, you'll find the power consumption and voltage values of all rails except -12V when the PSU is idle (powered on, but without any load on its rails), and the power consumption when the PSU is in standby mode (without any load, at 5VSB).
Phantom power is well below 0.5W with both 115V and 230V input.
Fan RPM, Delta Temperature And Output Noise
Our mixed noise testing is described in detail here.
The first chart below illustrates the cooling fan's speed (RPMs), and the delta between input and output temperature. The results were obtained at 33 °C (91.4 °F) to 48 °C (118.4 °F) ambient temperature.
The next chart shows the cooling fan's speed (RPMs) and output noise. We measured acoustics from one meter away, inside a small, custom-made anechoic chamber with internals completely covered in sound-proofing material (be quiet! Noise Absorber kit). Background noise inside the chamber was below 18 dB(A) during testing, and the results were obtained with the PSU operating at 33 °C (91.4 °F) to 48 °C (118.4 °F) ambient temperature.
The following graph illustrates the fan's output noise over the entire operating range of the PSU. The same conditions of the above graph apply to our measurements, though the ambient temperature was between at 28 °C (82.4 °F) to 30 °C (86 °F).
If you keep the operating temperature at normal levels, you will enjoy a quiet experience. At up to 460W, the ST75F-PT operates in passive mode, and after that its fan spins slowly.