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proSilence PCS-350 Fanless PSU

Fanless operation is the Holy Grail of silent computing: The proSilence PCS-350W is the first fanless drop-in substitute for a ATX12V Power Supply. It is not a product for everyone, but in the right PC configuration, this PSU gets you one step closer to the fanless ideal.

June 25, 2003 — by Mike Chin

Product proSilence Fanless PCS-350W power supply
Manufacturer Silentmaxx
Supplier Silicon Acoustics
MSP US$235

Fanless operation is the Holy Grail of silent computing, whether it’s a heatsink, case or power supply. There have been a few fanless PSUs for PCs introduced in the last year or two. Some of them are listed on our Recommended PSUs pages and one has been reviewed in the context of John Coyle’s Fanless (or not) system article. But none have been drop-in compatible with ATX form factor power supplies, and none have delivered enough power to be considered seriously by many computing enthusiasts. Silent Systems’ (GMBH) proSilence PCS-350W introduced earlier this year broke new ground by being physically compatible with the ATX case / PSU form factor and by having a seemingly normal 350W power rating.

Silicon Acoustics, a web retailer dedicated to silent computing components, sells the proSilence PCS-350W. They kindly provided a review sample for SPCR to examine. The sample is a version 1.1, which has recently been displaced by version 1.2, which provides higher power output and offers a better power switch; my apologies to all about the undue length of time this review has taken to complete.


The PSU comes in a long box considerably larger than usual.

As you can see below, the packing is good, and an instruction manual is provided. A copy of this can be downloaded from the manufacturer’s site; or just click here for the English language PDF.

The sample came with the usual power cord and 4 mounting screws, which were already screwed into the mounting holes on the PSU. The bottom two screws are nonstandard; they are a bit bigger and have a different thread than the norm.


The most prominent and unusual features of this PSU are the bright blue glossy paint job and the large heatsink that protrudes on the back panel in place of the usual exhaust fan. The heatsink measures 4″ by 1.5″ by 2.5″ and probably adds at least half a pound to the weight. The PSU is a fairly hefty unit, but it is not excessively heavy at over 5 lbs. For a PSU that is fanless, the vent slots seem a bit on the skimpy side, but there may be good reasons for this choice.

As shown in the photo directly above, the power switch and AC input socket are the only features on the back panel other than the heatsink One thing to note: Silicon Acoustics says:

The external heatsink requires a full size PSU case cutout. If your PSU cutout is restricted then you will need to trim the opening to allow for proper clearance. Restricted PSU cutouts are very common. Be psychologically prepared to make some modification to your case. It’s worth it!

The sample is a 110V unit; there is no voltage change switch for 110-220V. A 220V version is also available.


Connectors – There are 5 wire sets:

  • 2 cables, 31″ long, each with three 4-pin IDE drive connectors and 1 floppy drive connector
  • 1 cable, 12″ long, with main 20-pin ATX connector
  • 1 cable, 12″ long, for 12V (P4) connector
  • 1 cable, 12″ long, for 3.3V connector

It’s an odd mix: The 31″ cables seem too long and the 12″ cables are too short, especially as the cables exit the PSU from the top corner (as opposed to bottom), increasing the distance to the motherboard connections.

The usual label on the side of the PSU shows output ratings for the voltage lines.

The label on the sample unit says:

AC Input
110 VAC, 4A, 47-63 Hz
DC Voltage lines
Max Output
Peak Output
Total Output

One has to wonder where that 350W is? There is no information regarding load or line regulation tolerances.

The silentmaxx website says the proSilence PCS-350W v1.2 features:

  • Passive PFC (power factor correction)
  • ATX 2.03 compatible
  • P4 compatible
  • Operational in the temperature range of -22 to 167F (?!)
  • Improved power switch

Version 1.2 also has increased output on all its lines.

DC Voltage lines
Max Output
Peak Output
Total Output

It is clear that the 350W rating of this PSU was obtained by using the normal convention of simply adding up the maximum output capability of each of its voltage lines. This does not, as their total output specification indicates, tell you anything much about the total power than can be delivered simultaneously, which would be normal in a real-use situation. Given that the real power capability (of version 1.1) is just 170W, the “350W” embedded in the model number is an unfortunate stretch. It seems rather misleading.


Lacking the tool to measure it, I cannot tell you how thick the metal of the case is except to say that it seems about double the thickness of ordinary PSU cases. It certainly feels hefty. The heatsink that covers the bulk of the visible area under the cover also looks very hefty. As the photos show, it appears that all the main heat producing components are mechanically connected to the big heatsink for cooling. The external heatsink is bolted directly to the internal one via 4 substantial bolts, and some thermal interface material can be seen around the edges where the two heatsinks meet.

The concept is simple enough: The heat generated by the PSU is meant to be transferred to the internal heatsink, which is physically and thermally coupled to the external heatsink. The external heatsink dissipates the heat to the outside via convection cooling.

The photo directly above shows a couple of unused white pin connectors directly in front of the coil. It’s hard to see in this photo, but they look for all the world like fan connectors. It seems safe to presume that the main PSU board has been adapted from a conventional ATX PSU.

One other significant deviation from the norm is that the printed circuit board sits on the bottom of the PSU when it is installed in a case. In a conventional PSU, the PCB goes on the top, and the components hang down from it. It appears that the reversal was made to in order to take advantage of naturally rising heat convection. Note that the cover has ventilation holes to allow this heat to escape the PSU. In midsize ATX cases, however, this will provide only minimal cooling, as the top of the PSU is very close to the top panel of the case, and there is nowhere for the hot air to go. In a taller case, it would provide some cooling advantage if the case has ventilation vents on the back panel above the PSU. One has to think that the main cooling is normally via the heatsink on the back.


Parameters Tools
DC load on PSU DBS-2100 PSU load tester
Ambient temperature
Any number of thermometers
DC voltage regulation
Heath / Zenith SM-2320 multimeter
AC power
Kill-A-Watt Power Meter

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.

The DC output connector closest to the PSU on each set of leads is hooked up to the load tester. This ensures that the current delivered is distributed to as many short leads as possible. The heat generated in the wires can be an issue when pushing a PSU to its rated output,

The proSilence was tested at 4 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 midrange desktop PC.
  3. 150W: For higher power machines.
  4. 170W: The maximum

Care was 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 is often 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.

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. With the proSilence, a 60W bulb was used for all the load tests. It seems unlikely that this PSU would be used in a system that generates gobs of power and heat. The bottom front vent of the case and its bezel have been modified for unrestricted airflow; the approximate size of the intake hole is the equivalent area of a 92mm fan,

The PSU must cope with the heat generated by the light bulb plus whatever heat it generates within itself. In a real system, there would be other air exhausts paths and mostly likely at least one case fan. So a Panaflo 80mm L fan was mounted on the back panel of the test case and connected to a voltage controller. The PSU was run through its load range with the fan turned on to 7V, about the level at which most PC silencers would run their case fan.

Noise Measurements
For once, none were needed!


Measurements were made at 5 output power levels: 65W, 90W, 150W, and 170W. The PSU was allowed to run for ~10 minutes at each power level before measurements were recorded. The room temperature was 23C.

Table A. Load on the PSU


Table B. On test bench, in 23C ambient temperature

AC Power
Internal Heat
Power Factor
Max Internal Temp*
Internal HS Temp*
External HS Temp*

*Please see text under COOLING for full details

1. VOLTAGE REGULATION was very good, within -/+3% on all lines in any combination of loads tried (somewhat at random). It was often within -/+2%. The low and high voltage seen on each of the main lines is shown:

  • +12V: 11.66 to 12.43
  • +5V: 4.75 to 5.28
  • +3.3V: 3.28 to 3.4

It should be noted that I have no way of testing line regulation, so AC conditions are steady-state, not dynamic as it would be (potentially) in a real PC; I have no way to vary input AC voltage at this time. The AC line voltage in the lab as measured by the Kill-a-Watt power meter is usually within a couple of volts of 120V.

2. EFFICIENCY was very good, measuring a relatively high 68% even at 65W, where most PSUs fall to 65% or below. By 90W output, it hits 71%, and improves to 73-74% at the highest measured levels. This is very good performance, especially at the sub-100W levels. Most desktop PC systems actually operate between 65-150W so the proSilence is well optimized for real world application.

3. POWER FACTOR was only modest.

4. NOISE was nonexistent. Even at the highest output power levels, I had to press my ear right up against the top of the unit to barely hear just a trace of electronic noise, but the level was so low that it is easily dismissed. The proSilence 350 really is silent.

5. COOLING: TEMPERATURES were measured because it is a question mark hanging over any fanless PSU.

For the open bench tests 3 temperature probes were used for the following measurements:

  • Max Internal Temp – Underneath the main internal heatsink, at a spot that seemed to provide the highest temp readings. Where the thermal sensor actually ended up is not known because it was not visible; the wire with the probe at its end was simply snaked through an intake vent slot.
  • Internal HS Temp – On the top center of the internal heatsink by inserting the thermal probe through a top vent slot.
  • External HS Temp – A thin-film sensor in between the internal heatsink and the and external heatsink.

The results of the open bench tests are shown in Table B above. The column Internal Heat is the difference between AC power input and DC power output; it is how much heat the PSU generates as a byproduct of converting the AC to usable DC. It’s the amount of heat that the heatsinks are trying to dissipate.

Internal and external HS temperatures seem safe at any power level up to the maximum 170W. There is a temperature difference of 2-3 degrees indicates a bit of heat transfer loss between the two heatsinks.

Maximum internal temp was just an attempt to get an idea of how hot components might run. It’s an arbitrary measurement that may have little meaning. In a fan cooled PSU, some components also run very hot, and they are rated to run safely at such temperatures.

For thermal simulation case tests, the internal and external case temperature probes were kept the same; the third thermal sensor was moved inside the case to a position about 1 inch from the PSU intake vents. Tests were run only at the maximum 170W output. As mentioned earlier, the back panel case fan used was a Panaflo 80mm low speed model (SPCR’s reference standard).

Note that the case used for the thermal simulation has a 4″ vent hole cut on the top panel directly over the PSU location. This case is the same one used for the quiet P4-1.6A system article I wrote about over a year ago and shown in the photos directly below. (The system has long been retired and its parts scattered throughout the testing room.)

The top vent came in very handy to consider the cooling effect of such a vent hole for the proSilence 350 PSU.

The wires going into the PSU are attached to temp probes. To simulate a case without a top vent, a manual was placed over the hole, as shown below.

The results of the thermal simulation case tests are shown below in Table C.

Table C. In thermal simulation case, at full 170W output power over 60W light bulb. Each reading after 10 mins.

Top vent
Case fan
Case Temp
Internal HS
External HS
7 volts
7 volts
open; top CD bay cover removed; bottom PCI slot cover removed

As long as as the back panel Panaflo fan was running, the PSU temperatures remained modest. The top vent dropped the temps by 4C, which is as expected.

For a completely fanless PC simulation, the case fan was removed so that the case was cooled only by convection through the back panel fan hole,and the top vent was left open for PSU convection cooling. Case temps went up by just 5C, yet HS temps jumped very substantially.

Finally, in an effort to improve convection airflow in the case, the plastic cover on the front bezel for the top CD drive bay top and the cover for the bottom PCI slot were removed. It did not do much.

For an extreme stress test, the PSU was left running for 30 minutes without a case fan as per the last setting. The internal HS temp was measured at 64C and the external HS temperature reached 56C. The external heatsink felt too hot to keep running.

A final comment on load testing:

Full power testing of PSUs for any length of time is a very demanding test, generally tougher than what real use conditions can demand. SPCR’s bench testing is steady-state and can be extended indefinitely until the PSU burns; in real world applications, PSUs in PCs rarely get anywhere close to this kind of abuse (except maybe in a serious server room, which is a different application altogether), and the power demand on them varies up and down in a much more dynamic way, with average power loads rarely exceeding 150W for desktop PCs.


The proSilence PCS-350W fanless PSU actually works! That is, it provides enough power to run most PC systems and does so without any noise. No fan noise, no buzzing, squealing or whining. The only caveat is that you need to provide adequate case cooling, because unlike fan-cooled PSUs, this one cannot provide any. A fanless PSU by definition is not really ATX12V compatible, because the standard actually calls for the PSU to provide at least some case heat evacuation. (See the case airflow drawing in the Recommended PSUs.)

This is a primary point to consider: If you buy this PSU thinking that it will make your system silent, think again. It will do that ONLY if your system has no need for case cooling fans or any other fans, and if your hard drives are silenced. If you have a higher power system, chances are, your case cooling fans will still be running the same as before. The overall noise improvement may not be large.

In typical systems, the proSilence should be able to cool itself adequately. This unit may have been adapted from a conventional PSU rated at 350W (a natural conjecture considering the model number). But attempting to obtain that kind of power from the proSilence PCS-350W without forced air cooling would clearly result in thermal runaway failure. The use of a one-piece HS to avoid the thermal loss at the junction between the separate inner and outer heatsinks would probably provide improved performance. One might also wonder whether a larger external HS might improve performance; the increased distance from the heat sources may reduce any cooling benefits.

The biggest downside of this PSU centers mostly on the effect it will have on your other components and your silent PC fantasies. After completely eliminating a major source of PC noise, many of you will undoubtedly be even more annoyed by any noise your other components make. For others, a fanless PSU is one step farther along the road to the completely silent PC.

Some of you may think that the proSilence PCS-350W lacks power to be really useful. Consider that I ran it for a while in the following system (which draws peaks of ~120W DC) without any stability problems and no change in fan speeds (all at 12V with hot weather now). The PSU it replaced was a Seasonic 300W:

  • P4-2.8G
  • 512mb PC3200 RAM
  • 2 x Seagate Barracuda IV 40G HDD
  • GF4-4800TI 128 mb VGA
  • 2 Optical drives
  • 1 Floppy drive
  • SCSI scanner card
  • 3 Panaflo 80L fans

* * *

The proSilence PCS-350W fanless PSU’s strengths:

  • Heavy build quality, with massive heatsinks
  • Excellent stability
  • Very good voltage regulation
  • Good self-cooling within rated power limits
  • Truly silent: 0 dBA!

The proSilence PCS-350W fanless PSU’s weaknesses:

  • Output power could be higher
  • Misleading 350W in model designation
  • Short ATX and 12V cables to motherboard
  • Price

The proSilence PCS-350W is a solid, viable fanless PSU well-suited for use in the vast majority of quiet desktop PCs. Its use does not obviate the need for fans elsewhere in the PC. A fan for case cooling is necessary to evacuate the heat generated by the CPU, hard drive, RAM and other components. Most systems will also require a fan for CPU cooling. Regardless, the proSilence PCS-350W deserves recognition for being the first fanless drop-in substitute for a ATX12V PSU.

Our thanks to Silicon Acoustics for this review sample and to SilentMaxx for their kind support.

* * *

POSTSCRIPT: May 24, 2004

Almost a year after this review, I am compelled to add this cautionary note. Over the past 11 months, I have heard about too many proSilence PCS-350 problems, both directly and in the PSU forums of SPCR. Despite the positive experiences many users have had, there appears to be a significant number problems. These problems include:

  • Buzzing or humming noise, either constant or whenever under load. Some users have gone through two replacements before getting a non-buzzy unit.
  • Early failures in just a few weeks or months.

It’s good to note that the manufacturer and suppliers appear to have provided good service with warranty obligations.

I believe a large part of the problem is overheating, especially in systems that don’t have good case airflow. In a typical tower case, the hot air naturally rises to the top and tends to pool there unless an evacuation path is available. In tower cases that employ the proSilence PCS-350, such an evacuation path does not exist, because the back panel of the proSilence is sealed. This is a dramatic contrast to the typical fan cooled PSU, which has a vent with a fan at the back; this fan does much in evacuating the heat that rises to the top of the case.

My suggestion here is that users of the proSilence and other fanless ATX PSUs take special care in providing hot air evacuation paths out of the case; a top panel “blow-hole” may be very effective for such systems. The stress over time in the temperature difference between a no-airflow PSU and a low-airflow PSU must be great, especially in hot weather.

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