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Nexus NX3500 120mm PSU

Dec 17, 2003 by Mike Chin

Product
Nexus NX3500 "Real Silent PSU" Special Edition
Manufacturer
Nexustek
Sample Supplier
EndPCNoise
Market Price
US$80

Since their entry into the marketplace at the beginning of this year, 120mm fan PSUs have proliferated. They're still not as common as the standard 80mm fan PSU, but they no longer have the power to turn geek heads in the same way as before. Nexustek or Nexus, as the Dutch brand is better known, has come up with some special PSUs in the past.

What have Nexus done this time with a 120mm fan 350W power supply? Our sponsor EndPCNoise sent us a sample of the Nexus NX3500 "Real Silent PSU" 350W Special Edition so we could find out.

Well, first of all, they've given it a dramatic look. Gone is the ordinary grey paint and black plastic fan. Instead, we have nice coat of flat or nearly-flat black paint and a bright orange fan you just can't miss.

Note that the fan grill is not recessed into the surface. The grill and the mounting screws protrude up from that panel a couple of millimeters. In very tight spaces or with a vary large CPU heatsink, this could pose a bit of a nuisance. The exhaust grill has a nice and open hexagonal pattern of holes. (And before anyone asks, I will NOT compare the grill side by side with the Seasonic Super Tornado! Suffice it to say that, surely, airflow impedance should not be an issue in either of these PSUs.)

The retail box follows the color / design scheme of their other PSU models. It is attractive, and descriptive in several languages. A power cord and 4 mounting screws are provided.

ELECTRICAL SPECIFICATIONS

AC Input
120/240 VAC, 50-60 Hz
DC Output
+3.3V
+5V
+12V
-12V
-5V
+5VSB
Load Regulation
-/+5%
-/+5%
-/+5%
-/+10%
-/+10%
-/+5%
Line Regulation
-/+1%
-/+1%
-/+1%
-/+2%
-/+2%
-/+1%
Min Load (A)
0.3
0.1
0
0
0
0
Normal Load (A)
14
12.7
4.5
0.4
0.15
1.0
Max Load* (A)
21.2 / 28
25.5 / 30
16
0.8
0.3
2.0
Max Power
220W
192W
9.6W
1.5W
10W
Max Power**
330W

*+3.3V & +5V total output not exceed 220Watt. When +3.3V load is 28A, the +5V maximum load is 25.5A. When +3.3V load is 21.2A the +5V maximum load is 30A.

**+3.3V & +5V & +12V total output not exceed 330Watt.

So the model designation notwithstanding, 330W is the real simultaneous output capacity of the unit. This is no surprise; it is the norm in the PSU power rating game. The 12V line current rating is a touch on the low side for Intel ATX12V-V1.3 standards, but the unit does not claim adherence to the V1.3 guide and is as good or better than the vast majority of PSUs of the same power rating. This unit is equipped with Passive PFC, a requirement in the EU.

MORE PHYSICAL DETAILS

The inside panel has hardly any vents at all. This ensures that the airflow is vented out the back to the outside of the case. The fan is hardwired to a controller circuit on a small green PCB that is screwed to the smaller of 2 heatsinks.

The heatsinks themselves are similar to those in most other 120mm fans, and they are short enough to keep from interfering with the big fan and looking a bit undersized as a result.

Here is the center of the fan itself. It is a Yate Loon Electronics D12SM-12, rated at 0.3A at 12V. That sounds like the potential to move a lot of air. The Yate Loon Electronics web site confirms that this fan is rated for 1650 RPM and 56 CFM at a noise level of 39 dBA (presumably at 1 meter). However, this would be with the fan in free air, without any impedance to airflow (and therefore no additional turbulence noise).

Connectors

There are a total of 6 wire sets, including a SATA drive connector:

  • 32" long cable with two 4-pin IDE drive connectors and one floppy drive power connector
  • 24" long cable with one 4-pin IDE drive connector and one floppy drive power connector
  • 24" long cable with two 4-pin IDE drive connectors
  • 21" long cable for main 20-pin ATX connector
  • 21" long cable for dual 12V (P4) connector
  • 21" long cable for 3.3V connector
  • 17" long cable with SATA drive connector

TEST METHODOLOGY

Parameters Tools
DC load on PSU DBS-2100 PSU load tester
Ambient temperature

Any number of thermometers
Fan / DC voltage regulation

Heath / Zenith SM-2320 multimeter
AC power

Kill-A-Watt Power Meter
Noise

B&K model 1613 SLM
The core PSU test tool on SilentPCReview's test bench is the DBS-2100 load tester, made (in Taiwan by D-RAM Computer Company) specifically for testing computer power supplies. The machine consists of a large bank of high power precision resistors along with an extensive selection of switches on the front panel calibrated in Amps (current) and grouped into the 5 voltage lines: +5, +12, -12V, +3.3, -5, +5SR. Leads from the PSU connect into the front panel. It is shown above with leads from a PSU plugged in.

To ensure the current safely delivered is distributed to as many short leads as possible, the DC output connector closest to the PSU on each set of leads is hooked up to the load tester. When pushing a PSU to its rated output, the wires can get very hot.

The PSU is tested at 5 DC output power levels:

  1. 65W: A very typical DC power draw by many system at low / modest load.
  2. 90W: Established previously as a typical max power draw of a mid-range desktop PC.
  3. 150W: Higher power machines usually don't draw much more than this.
  4. Maximum (350W) The usual 300W test was left off because it is so close to the max.

Care is taken to ensure that the load on each of the voltage lines does not exceed the ratings for the PSU. The PSU is left running 5~10 minutes at each power level before measurements are recorded.

The DBS-2100 is equipped with 4 exhaust fans on the back panel. A bypass switch toggles the fans on / off so that noise measurements can be made. The resistors get very hot under high loads.

Kill-A-Watt AC Power Meter is plugged into an AC outlet on the side of the DBS-2100 in the above picture. The AC power draw of the PSU is measured at each of the 4 power loads. The Kill-A-Watt is used to measure:

Efficiency (in AC-to-DC conversion) at each power level. This is the efficiency figure provided by PSU makers. It is obtained by dividing the DC power output (as set on DBS-2100 load) by the AC power consumption. Efficiency varies with load, and also temperature. PSUs seem to run more efficiently when warmer, up to a point. Generally, they are least efficient at low power and most efficient at 40~80% power load. The main advantage of high efficiency is that less power is wasted as heat -- this means a cooler PSU that requires less airflow to maintain safe operating temps (read: quieter.)

Power Factor (PF). This measurement can be read directly off the Kill-A-Watt. In simple terms, it tell us how much AC power is lost to harmonics (unnecessary electromagnetic energy) while driving the PSU. In practical technical terms, it is the difference between the measured V(oltage) x A(mperes) and AC power in Watts. PF varies somewhat depending on load. The ideal PF is 1.0, which means no AC power is lost. A PF of 0.5 means that to deliver 100W in AC to a PSU, your electric company actually uses 200W and this can be shown in your electric bill as savings (depends on your electric utility company and your account with them). 100W is lost or wasted. Active PF Correction (PFC) power supplies usually have a PF of >0.95. Passive PFC units usually run 0.6 - 0.8. Non-PFC units usually measure 0.5-0.7.

PF is not significant in terms of noise, heat or performance for a PC, but it is relevant to electricity consumption and energy conservation. If you are running large numbers of PCs, there's absolutely no question of the benefits of high PFC and, to a lesser degree, high efficiency. In the EU, Japan, China and many other places, PF is mandatory for all electrical devices that draw more than a certain power (usually ~75W).

The Heath / Zenith SM-2320 digital display multimeter, a fairly standard unit, is used to measure the fan voltages and the line voltages of the PSU output. The latter is done via the terminal pin on the front panel, above the connections for the DC outputs from the PSU.

The Test Lab is a spare kitchen measuring 12 by 10 feet, with an 8 foot ceiling and vinyl tile floors. The acoustics are very lively and allows even very soft noises to be heard easily. The PSU under test is placed on a piece of soft foam to prevent transfer of vibrations to the table top. Temperature in the lab is usually ~20C. This is something of a problem as PSUs usually operate in environments that easily reach 45C. Sited next to or above the CPU, the PSU is always subject to external heat. This brings us to the next topic...

In-case Thermal Simulation

The solution is a AC bulb in an empty case with the PSU mounted normally. The distance between the bottom of the PSU and the top of the bulb is about 7 inches. All the case back panel holes are blocked with duct tape. The only significant exit for the hot air in the closed case is the PSU, which is then subject to a fair amount of heat. Still, the bottom front panel case intake hole is very large. In testing, the front of the case is moved so it hangs over the edge of desk, over free air, to ensure good fresh convection airflow.

A 60W bulb is used for the 65W and 90W load tests; it seems a more realistic heat source for those lower power loads. The higher power tests use a 100W bulb.

The PSU must cope with the heat generated by the light bulb plus whatever heat it generates within itself. In real systems, there would be other air exhausts paths, and mostly likely at least one case fan. So a Panaflo 80mm Low speed fan is mounted on the back panel of the test case and connected to a voltage controller. The PSU is run through its load range with and without the fan turned on, to 7V, which is about the level at which most PC silencers would run their case fan. It is a reasonable low noise PC simulation.

Noise Measurements

For this review, I used a highly accurate calibrated B&K model 1613 sound level meter on temporary loan from the University of BC's acoustics lab.

This professional caliber SLM dates back to 1978, weighs over 10 pounds, and is completely analog in design. It has a dynamic range that spans over 140 dB. The microphone used has a 1" diaphragm that's very responsive to low sound levels and low frequencies. The unit's absolute sensitivity reaches below 0 dBA -- at one point in the midband (1kHz) I was seeing -4 dBA for background noise in the UBC anechoic chamber.

Noise readings were taken with the microphone positioned at 1 meter distance, facing the control panel. The ambient noise level in the live test room was ~15 dBA.

Being done at 1 meter distance, the noise measurements are somewhat comparable to manufacturers' specs. Note, however, that differences in the temperature of the test conditions, and the fact that they are made in a live room rather than an anechoic chamber, makes such comparisons not quite valid, either. Because of the higher background noise level in the live test room, and the high degree of reflections from boundaries (walls, ceiling, floor), these measurements generally run higher than they would be in an anechoic chamber.

As usual, noise measurements are accompanied by descriptions of subjective perceptions. The measurements provide only part of the picture.

TEST RESULTS

Measurements were made at 5 output power levels: 65W, 90W, 150W, and 350W. The PSU was allowed to run for 10~15 minutes at each power level before measurements were recorded. The room temperature was 21C.

A. Load on the PSU

+12V
24
36
60
156
+5V
20
20
40
110
+3.3V
16.5
26.4
42.3
78
-12V
2.4
3.6
3.6
3.6
-5V
1
2
2
1
+5VSR
1
2
2
1
Total DC Output
65W
90W
150W
350W

B. On test bench, in 21C ambient temperature

AC Power Draw
103W
135
215
516
Efficiency
63%
66.7%
69.8%
68%
Power Factor
0.62
0.67
0.68
0.71
Fan Voltage
4.1 V
4.2 V
4.3 V
10 V
Noise (1 meter)
<20 dBA
<20 dBA
22 dBA
38 dBA

C. In thermal simulation case, over light bulb

Total DC Output
65W
90W
150W
350W*
AC Power Draw
103W
135
215
525
Efficiency
63%
66.7%
69.8%
66.7%
Light bulb
60W
60W
100W
100W
Fan Voltage
4.1 V
4.4 V
7.2 V
10.8 V
Noise
<20 dBA
22 dBA
30 dBA
43 dBA
Case Temp
29
30
33
38
Exhaust °C
33
33
36
40

* Only after the tests were conducted did I realize that 350W total DC output represents a 20W overload for this model. It's one explanation of why in this thermal simuation high power test, the AC power draw went up: We may be seeing a tiny bit of thermal overload in the circuits.

ANALYSIS

1. VOLTAGE REGULATION was good, within -/+2% on all lines in any combination of loads. The low and high voltage seen on each of the main lines is shown:

  • +12V: 11.87 to 12.34
  • +5V: 4.90 to 5.31
  • +3.3V: 3.29 to 3.42

We have no way of testing line regulation, so AC conditions are steady-state, not dynamic as it would be (potentially) in a real PC. The AC line in the test lab as measured by Kill-a-Watt is usually very stable, within a couple of volts of 120V.

2. EFFICIENCY was modest throughout the power load range and never quite reached 70% even at high loads. This is a bit below average performance compared to other PSUs tested by SPCR.

3. POWER FACTOR was as expected, ranging from a low of 0.64 at low loads to a high of 0.72 at maximum power load. This was expected, given the use of passive PFC.

4. FAN VOLTAGE: The fan receives full voltage (10-11V) for a couple of seconds upon startup to ensure that it always starts. The default voltage just after a cool start is ~4V. In the thermal simulation case, the first significant change comes at the 150W output level with the 100W bulb. The fan voltage jumps from 4.2V (without external heat) to ~7V. At full power without additional outside heat, the fan speed stabilizes at 10V.

5. NOISE was measured at 1 meter from the exhaust grill. The test environment is live, so readings are higher than would be obtained in an anechoic chamber readings. (See explanation in Test Methodology section above.)

Subjectively, the Nexus NX3500 is very quiet. After the startup burst, the 120mm fan has virtually no bearing noise, and sounds much smoother than the fan in the Fortron-Source Aurora. As the speed increases, the fan retains its smooth sound. Even up past 6V, the primary noise is the relatively benign whooshing sound of air turbulence. No bearing chatter, whistling or whining. It is substantially smoother sounding than the SuperRed 120mm fan used in the Seasonic Super Tornado as well.

When the fan was deliberately disconnected or halted, some coil hum could be heard with the output above ~150W. Normally this noise would be masked by the fan noise. At the highest fan speed, wind turbulence noise dominates completely, accompanied by a bit of humming.

The measured noise at minimum is a couple dBA better than the Seasonic Super Silencer 400 or the Nexus NX3000. Only the Seasonic Super Tornado and the ExoticPC-SilentX 14 dBA PSUs manage to best it for residual noise. However, we all know this is only where the noise begins. The real question is where it ends up under load in a real system.

6. THERMAL IN-CASE SIMULATION

The temperature of the case was monitored with a thermal probe positioned about 1" below the PSU intake vent and about 1" away from the center. The temperature of the exhaust air from the PSU was measured with a thermal probe positioned about 1/2" away from the center of the PSU exhaust grill panel.

Judging from the upturn in fan speed at the 150W output level with the 100W bulb, in order to minimize noise from a system using this PSU, it is probably best to keep total system power draw to under 150W. This is not difficult to do even with a very capable system -- one with a CPU rated to >2.5 GHz, a couple of hard drives and a mid-line gaming video card. It is probably possible to run such a system without a case fan, with just the fan in the PSU and with a quiet fan on the CPU heatsink. In this room ambient temperature, once the load reaches 150W, however, the fan speed jumps to ~7V and the fan noise reaches 30 dBA at 1 meter, which is not so quiet any more.

7. WHAT ABOUT WITH A CASE FAN?

At the 150W and 350W loads, measurements were repeated with a Panaflo 80mm low speed fan (FBA08A12L1A -- our reference) mounted on the back panel of the case, fed 7V via a Zalman Fanmate1 controller. There is some question about how this fan interacts with the 120mm fan in the PSU.

Power
150W
350W
Case Fan
Off
Exhaust
Intake
Off
Exhaust
Intake
Fan V*
7.2 V
8.1 V
7.1 V
10.8 V
11.1 V
10.8 V
Noise
30 dBA
33 dBA
30 dBA
42 dBA
43 dBA
42 dBA
Case Temp
32C
33C
29C
33C
33C
32C
Exhaust °C
37C
38C
33C
39C
40C
34C

At the 150W power load, turning the exhaust case fan on had the effect of increasing the fan voltage from 7V to over 8V, with a concomitant 2-3 dBA rise in noise. This implies that the temperature seen by the internal PSU thermistor increased. It is the same result found with another 120mm PSU.

Flipping the case fan around to blow outside air into the case had an immediate effect. Both case and exhaust temperatures dropped slightly with increase in the PSU's minimum fan speed. Note that the presence of a CPU heatsink and cooling fan directly below the PSU may have an impact on airflow; experimentation is recommended.

CONCLUSION

The Nexus NX3500 is a good contestant in the 120mm fan PSU sweepstakes. The smoothness of its fan is a very endearing quality and provides unobtrusive low noise PC operation with a modicum of care in system assembly. This brightly colored Yate Loon is probably the smoothest, least obtrusive 120mm fan I've heard yet. (And no I have not hear them all!) The Nexus NX3500's fan noise characteristic puts it in at the top of the 120mm PSU class. As with other 120mm fan PSUs, it may be possible to run a mid-range power system without any other case fans due to the large air-moving capability of that big fan.

The Nexus NX3500's strengths include:

  • Very smooth, quiet fan
  • Good stability and voltage regulation
  • Good self-cooling
  • Good directed airflow design

There are a couple of weaknesses:

  • Linear-line fan control may produce more noise than absolutely necessary when pushed hard.
  • AC/DC conversion efficiency could be higher.

Our thanks to our sponsor EndPCNoise for the Nexus NX3500 review sample and for their continued support.

* * *

POSTSCRIPT: Unraveling the Mystery of PSU Noise Vs. Watts Graphs

Consider the noise / power output chart provided by Nexus below. It compares their earlier NX3000 PSU, a highly successful and popular model, with the NX3500. Just looking at the graph at face value, it's clear the noise advantage of the NX3500 comes only below 150W or above ~260W. In that middle range, the NX3000 stays 2-3 dBA quieter than its bigger sibling. That's at face value.

A Key Fact: We do not know at what temperature the information on the graph above (or a similar one supplied by Seasonic and other PSU makers) was collated.

This is a most important point: There are NO ATX power supplies that adjust fan cooling speed in accordance with power load. Rather, they adjust their fan speed in accordance with the temperature of a thermistor within the PSU housing, usually clamped to a heatsink. When graphs like the one above are shown, the power points for the changes in fan speed are NOT absolute, they are quite dependent on environmental or ambient temperature. The fan speed-up point is plainly and simply preselected by the designers to a specific thermistor temperature, and any combination of factors which causes the thermistor to reach this temperature will have the effect of turning the fan speed up.

Consider what this means: After subjecting the new model to a battery of torture tests, the PSU design engineers decide that the thermistor must stay below 50°C in order to keep key components in the PSU within their safe operating range. Just because the thermistor is at 50°C doesn't mean those other components are at the same temp; they could be running usually 30°C hotter. We don't know. In any case, they have chosen 50°C to be the maximum safe allowable temperature of the thermistor.

So. Let's say tests have shown that at the rated output of 300W, this new PSU dissipates 100W of heat (which means an AC/DC efficiency of 75%). The engineers test further and determine that given the characteristics of the PSU heatsinks and casing, and the anticipated temperature in which the PSU will be tested for safety certifications, a directed airflow of 30 CFM is adequate to keep the thermistor at just below 50°C. They prefer a healthy margin of error, so they choose a fan of 40 CFM airflow capability.

They want to minimize noise, and so design a fan control circuit that begins speeding the fan up from its minimum consistent start voltage of 5V at around 35C, going up linearly to 50°C where the fan will get the full 12V. Above that thermistor temperature, the engineers know that the efficiency of key components will become less linear so that power output actually begins to decline. But that's OK, they know that with 30 CFM or more at the test temperature, the PSU can keep working indefinitely at full power output and not exceed 50°C.

Remember, all this assumes a certain predefined environmental temperature! If the ambient temperature is at a very cool 20°C, then the PSU may be perfectly capable of 350 W output before overheating begins to take place. On the other hand if the ambient temperature is a scorching 40°C, that 50°C temperature may be reached at a low 250W, beyond which thermal overload behavior will ensue.

The final point is simple: Fan speed (and thus noise) is NOT a simple function of power output, and noise vs. power graphs as presented by many PSU makers is simply an idealized behavior curve they would like to see and to present. Real operating conditions are subject to far greater variations -- such as actual environment temperature dictated by climate and season, air conditioning or the lack thereof, amount and effectiveness of system case ventilation, the amount of heat dissipated by other components in the PC, the efficiency of the PSU, the noise/airflow curve of the fan used, and the workload on the system as a whole.

The system builder's challenge is to keep any of these noise-reduced PSUs operating at or below the speed-up turning point of the fan curve. The rules for any prudent silent PC system builder to follow are:



1. Choose the highest efficiency PSU with the quietest fan.


2. Provide this PSU with the lowest temperature adequate intake airflow.

This second rule brings up the idea of a PSU fresh air duct, which I have mentioned many times in the forums, and which forum member Lilla took and ran with in the thread: Building a PSU Channel.

It's obvious that one day I have to witness a PSU to getting tested for certification by a safety agency like CSA or UL. One day.

* * *

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