Intriguing. I wonder if a pair of the 6k models can adequately cool the 7250-24p or if they'd be constantly cycling between high and low speeds.
An update on my situation: Today on a warm summer day the fans did throttle up with switch still all idle, only one gigabit ethernet cable connected. So at low mode these don't move enough air. When they spin up they are audible, not noisy though. Nothing even close to one original fan at low speed.
It would be interesting to hear what the 15K version sound at low speed, but atm i'm planning on ordering 3x noctua nf-a4x10 flx, which are 10mm deep, and attach them to each ASIC heatspreader, and feed the directly from the 12V line coming from the PSU, and maybe even use the included voltage reduction adapter. I already have the T-Splitters to get 12V.
I believe the combination of 3 fans directly over the ASIC's and the 3x Arctic should get me a silent switch. The best thing about the S4028-6K's is they work perfectly at both 4.5V and 12V, and when they do spin up to high speed they actually move air decently, and the switch cools down, throttling down after a while.
Today i've been experimenting with adding the 2 extra arctic's from the valuepack inside the case, in the free area between the PCB and the rear, and blowing towards the rear in the same direction as other fans. I've strapped them with zip-ties to the PCB screws beneath the PCB. I'm missing some wires like splitted 3-pin fan cable extenders so i'm having to improvise some just to test. So far i have the 4th Arctic working by feeding it only voltage and ground from one fan header. I'm not connecting the tach pin so not to mess up the signal. It works and the 4th fan starts at full and throtthles down to 4.5V just as the other fans.
I'm still working on getting the other, 5th fan connected, and while i think this will help abit, i'm still probably gonna go with 3x noctua's directly on the ASIC chips. That combination of 3x noctua's and 3x Arctics i believe will result in a very quiet switch that still has the ability to ramp up the rear fans incase of heavy load.
Update. Now i have the 2 extra Arctic's connected, and the cover back on, so in an hour i'll know how much they help. It's not a silent switch, there is some minor noise at low speed now. You can hear it in a quiet room standing 1 meter away, but it's acceptable to me. Somehow attaching plus and ground to extra fans seems to have changed the sound of the rear fans abit, i dunno if i'm imagining it..
Update2. Now it's been running idle for an hour, and the temps have stabilized:
Temps kept rising even after an hour, and seem to have settled now.. This depends on window open or close, and outside temp ofcourse, but this ain't gonna cut it:
Code:
Fan controlled temperature: 61.5 deg-C
Fan speed switching temperature thresholds:
Speed 1: NM<----->65 deg-C
Speed 2: 56<-----> 79 deg-C (shutdown)
Sensor B Temperature Readings:
Current temperature : 57.5 deg-C
Sensor A Temperature Readings:
Current temperature : 61.5 deg-C
Warning level.......: 69.0 deg-C
Shutdown level......: 79.0 deg-C
So the 2 extra fans inside the case seem to have dropped the temps compared to only 3 Arctic rear fans by
2.5 1 degree for sensor B, and
5 2
degrees for sensor A.
It's a reasonable result in my opinion. Not much help. At low speed these don't move enough air.
The fact is that without direct airflow over the hot ASIC heatspreaders, the only way to compensate that is with very high general airflow throughout the whole chassis, which is impossible to do quietly.
I'm convinced now that it's not possible to make the 6450 close to silent without having 40 x 40 x 10 fans attached on the ASIC's.
Next comes splurging the big bucks for the Noctua's.
Edit3: Instead of the expensive Noctua's i decided to try
these 3V - 12V chinese fans, and connect them to the fan headers instead. Presumably at 4.5V they should rotate at low RPM, plus i can connect them with a simple fan cable splitter without tapping into the 12V line. that's 10€ instead of 45€.