HP Proliant MicroServer

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It was inevitable that SPCR modify the MicroServer for lower noise. What are the usual methods to reduce noise in an existing computer?

  • Reduce the transfer of vibration from the hard drives to the chassis. This is not really possible here, given the plug-in design of the HDD bays.
  • Lower fan noise by reducing RPM or replacing fans with slower, smoother ones. This is not a practical option with the PSU fan, which made very little acoustic contribution anyway. The 1100 RPM minimum 120mm fan is a perfect target for this approach.

We mentioned that this primary cooling fan is a 4-pin PWM type with a pin-pattern that is non-standard. This means you cannot plug any old PWM fan into the CPU fan header on the MicroServer meotherboard and expect it to work. Our first step was to examine the connector. A close comparison revealed that it is not quite the same as the 4-pin connector on other PWM fans used in PCs. The latter is designed to fit either 3-pin or 4-pin fan headers; the HP connector can only fit on a 4-pin header.

We also examined the information that SPCR forum members unearthed. Much thanks to vlrnmz and camlin for the information they shared. Here is the 4-pin configuration information for the HP MicroServer from camlin:

HP MicroServer
4-pin CPU fan header
Pin Name Color
1 Control green
2 GND black
3 +12V red
4 Sense yellow

What you need to know now is the normal pinout for 4-pin PWM fans. From AllPinouts:

Pin Name Color
1 GND black
2 +12VDC yellow
3 Sense green
4 Control blue

Pin Name Color
1 GND black
2 +12VDC red
3 Sense yellow
4 Control blue
Header on mobo
For some AMD CPU fans

So armed with this information, a couple of possible Scythe PWM fan candidates were identified, the connectors compared, and modifed. This requires a very small screwdriver to unlock the conductor pins from the connector and swap them around to the desired slots. The center guide on the plastic connector also needs to be removed, as it will stop it from fitting on the HP fan header. This was accomplished with a sharp utility knife.

The labels on the photo are self explanatory. The Scythe fan color code is different from either of the ones described by AllPinouts.

Rather than installing the fan, it was tried first, by plugging it into the fan header with the chassis door open, then powering up the system with the fan on the outside. The first fan to be tried was a Scythe SY1225SL12LM-P. This is a PWM version of the popular SlipStream series, with a rated maximum speed of 1300 RPM. After the initial full speed burst at turnon, the fan slowed to an extremely slow speed, which caused a fan error message to display and the system to power down immediately. The system wants to see a faster fan. Since the >3000 RPM stock fan ends up spinning at 1100 RPM, the PWM fan controller applies a duty-cyle of about 35%. That meant the Scythe must have been spinning at under 500 RPM.

The next fan to be tried was a Scythe SY1225SL12HPVC. This is a PWM fan rated for 1900 RPM, equipped with a manual speed control. With the control set to full speed, it would surely run fast enugh (~700 RPM?) for the system to boot. And indeed, it worked.

The Scythe Slip Stream120mm PWM Adjustable fan on the left worked as a replacement fan in the HP MicroServer. The 1300 RPM Scythe PWM fan did not run fast enough.

Then came the task of removing the stock fan. After the four Torx screws are removed from the back panel, follow the course of the cable to the motherboard header and carefully snip several plastic wire ties. The Scythe fan has a much a shorter cable, so it had to be run a bit differently. This required the removal of the motherboard. Finally, the dongle with the PCI slot cover plate and rheostat had to be put somewhere. The control could be removed from the plate and mounted somewhere on the back panel, but for testing, it was left resting behind the optical drive. Be careful that the fan wires don't catch on the fan blades. The fan experiments could now begin.

The Scythe fan controller was temporarily tucked behind the optical drive. Do not leave it this way.

Many drive combinations and loads were tried with various fan speeds, with all temperature and noise data recorded. All that is not presented here, to reduce information overload.

First, the manual fan control was used to determine the minimum fan RPM allowed before auto-shutdown. The BIOS fan monitoring screen was left on while the fan knob was rotated. It turned out to be somewhere around 500 RPM; the exact value was difficult to pinpoint. It is probably safe to consider 550 RPM as the minimum speed.

With the Scythe fan at 550 RPM, the baselline noise was just 16 dBA. However, a tonal peak at ~2.7 kHz which had been masked by the stock fan before was now apparent. This comes, evidently, from the power supply or its fan. It is not clear which is the culprit without examining the PSU by itself. In any case, since hard drives would always add to the overall noise, there is no point in keeping the fan speed so low. It was raised to 700 RPM, bring the SPL up to 17 dBA@1m, and this became the default for SPCR's Scythe-fan-modded MicroServer.

The Scythe fan @ 700 RPM brought the baseline noise down 6 dB from the stock fan baseline.

Other smooth sounding fans could be used, of course, with or without manual control. To achieve 700 RPM speed on the motherboard's built-in fan controller, if we assume 35% estimated fan duty cycle, you'd need a PWM fan with rated speed of no more than 2000 RPM. Note, however, that our 1900 RPM Scythe spun at 1080 RPM with its manual control set to maximum, which implies about 50% duty cycle. This means that the PWM controller in the board interacts differently with different fans.

Here is the essential cooling and noise data for the 700 RPM Scythe mod.

HP MicroServer w/ Scythe PWM fan @ 700 RPM
Windows 7 w/ 4 WD Green AV drives
20 dBA
20 dBA
* The temperatures of the hottest and the coolest drives.

HDD temperatures are not affected by the drop in fan speed, although the CPU ran 6°C hotter, to a still modest 59°C. This kind of extended 100% CPU load will never happen in a NAS or home server. The overall noise signature is much nicer than the 4 dBA reduction would suggest. You still might not want this system on your desktop, as HDD seek noise is clearly audible, but you'll probably be fine with it in the same room.

The Scythe fan @ 700 RPM brought the noise down 4 dB compared to the stock fan.

Finally, some audio recordings of the MicroServer.


These recordings were made with a high resolution, lab quality, digital recording system inside SPCR's own 11 dBA ambient anechoic chamber, then converted to LAME 128kbps encoded MP3s. We've listened long and hard to ensure there is no audible degradation from the original WAV files to these MP3s. They represent a quick snapshot of what we heard during the review.

Each recording starts with ambient noise, then 10 second segments of product at various states. For the most realistic results, set the volume so that the starting ambient level is just barely audible, then don't change the volume setting again while comparing all the sound files.

* * *

Discuss this article in the SPCR forums.

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