SPCR Power Supply Test Rig, v.4 (and v.4.1)

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Acoustics cannot really be separated from electrical or thermal performance because factors such as efficiency and cooling efficacy directly affect fan speed, which is the main cause of noise. However, the artificial separation helps to keep us organized about the tests themselves. We use thermometers, multimeters, a Bruel & Kjaer (B&K) model 2203 sound level meter (SLM), our digital audio recording system (see test equipment list, pages 5-6), and very careful listening to conduct the following tests.

9. Fan SPL and Fan Controller
The fan and the fan controller circuit are arguably the most critical factors in power supply noise. (With the exception of fanless PSUs, of course.) While the quality and speed rating of the fan sets the maximum and minimum noise limits, the thermal fan speed controller dictates how fast or slow the fans runs at various loads and temperatures. At every power level, we monitor the voltage fed to the fan by the fan controller l, measure the sound pressure level (SPL) of the fan with our SLM, and the in / out temperature of the airflow through the PSU. As the primary noise maker in the PSU, the fan naturally gets subject to a lot of scrutiny. We track down its origin whenever possible and report the manufacturer's specifications (for better or worse), as well as any previous experience or information we may have about the fan.

10. Temperature
This was alluded to in point 9, because the internal temperature of the power supply is a key factor in determining the output voltage of the fan control circuit. We monitor the temperature of the air flowing into the PSU and exhausting out of it. The temperature rise tells us something about the PSU's cooling efficicacy, which is affected by AC/DC conversion efficiency, cooling fin design and fan speed. We monitor the air temperature in the immediate vicinity of the PSU test rig and ensure that it falls within 21~23°C. From now on, no PSU tests will be conducted outside of this temperature range; the fan curves can be affected by higher or lower temperatures.

11. Electronic Component Buzzing
Most people know what we mean, and surprisingly to some, the most effective way of detecting such noise is to listen. Often, it is not measurable with a SLM. Such noise most often comes from capacitors or inductors, and can range from a simple buzzing to a complex mix of several components buzzing, whining, screaming and humming. Once we know what we are listening for, we can isolate it so that it can be recorded, and the waveform shown in a 3D FFT display. Most of the time, this is unecessary. We report on any direct electronic component noise, and the conditions in which the sound is apparent. Often it is only evident under specific loads.


Some questions will be on some readers' minds:

How will the new 120mm exhaust fan be used?
At up to 400W output load, the fan is set to 7V, which gives us about 20cfm in free air and 17 dBA@1m. It's run this slowly to best simulate a quiet PC. Its noise is irrelevant, as the fan its turned off during recording or SPL measurement. At 400W load, the fan voltage will be turned up to 9V, where we obtain about 28cfm. This switch is perfectly legitimate and relevant. Anyone with a real system drawing 400W or more would want to turn the exhaust fan up to this speed at least. The noise from the fan still remains at just 20 dBA@1m. Once we reach 600W load, the fan is turned up to full speed, which provides about 40cfm in free air. The measured SPL is about 22 dBA@1m. Again, this is to replicate the most realistic real-use scenario; no one would have a >600W DC power draw system with just one slow 120mm fan blowing at 9V. Any fan-cooled power supply would completely mask the miniscule noise contribution of the Nexus 120 fan even at 12V. (More typically in such a high power rig, there would be many more fans, including very high speed video card cooling fans, with a total rated airflow of well over 100cfm.)

What will be the effect of changing the 8~10cfm 80mm exhaust fan to a 120mm fan with twice the airflow?
The most likely effect is that some power supplies may fare slightly better for noise in the new setup compared to the old. The fan-cooled power supplies with good fan controllers have generally tended to start speeding up the fan above ~150W output load. With the same power supplies, it is possible that the fan ramp-up speed might go up a bit higher, perhaps to 200W. The overall fan speed rise curve may be less steep than with the previous fan setup. We will go back and check a few of our most popular models to see if there's any significant change. In general, however, we do not believe that any change will be dramatic, nor will it change current "rankings" in the Recommended Quiet PSU tables.

What are the potential effects of brownouts or voltage sags?
Here is a worst case scenario: Power supplies in some electronic equipment will fall out of regulation. Errors, due to erratic power supply performance, may creep into computer operations. Marginally performing electronic or electric devices will cease operation. Electrical interference increases and may affect computer and communication operations. Spikes generated by electrical machinery also greatly increase. Air conditioners, refrigerators and other motorized devices will generate local spikes. Industrial machinery (often miles away) may create additional spikes which may find their way on to the electrical distribution network, and into your equipment.

The vast majority of low cost UPS units will continuously cycle between power line and internal battery operation. UPS batteries will soon discharge, unable to generate additional back up power. If the system has not been shut-down, the UPS and computer may snap back on when power rises slightly, only to shut down again when the power line voltage falls back into the brown-out zone. Such "ON-OFF" operational cycling is unhealthy for UPS, batteries and connected electronics.

More typically, milder cases of brownouts or voltage sags are often the cause of otherwise unexplained computer instabilities, and the premature failure of components.

For those who are interested in the details of our test gear, please check the remaining two pages.

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