Confused about temperatures and 45w vs 65w CPU AMD

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pete6032
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Confused about temperatures and 45w vs 65w CPU AMD

Post by pete6032 » Wed Jun 02, 2010 8:58 am

I am trying to put together a Micro ATX system to be placed in a Silverstone TJ08 case. I will be using it for video encoding and Photoshop CS4. I would like the system to be quiet so I started looking at 45w processors, however, I read somewhere that 45w processors aren't necessarily the coolest processors, rather they are just low-power processors that could run very warm (and thus need a faster spinning CPU fan). I would like to stick with either a 45w or 65w CPU (power limitations). Should I be looking for a 65w CPU or a 45w CPU if I want the quieter of the two? (I realize that the HSF will be the real decision, but assuming that more heat = more fan noise, then are 65w CPU's cooler and less prone to causing higher spinning fans than 45w processors)?

MikeC
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Post by MikeC » Wed Jun 02, 2010 9:26 am

Facts about TDP & CPUs:

1. All CPUs of a specific type (say Athlon64 dualcores w/512k cache) come off the same assembly line, like peas in a pod. They are checked and binned according to power consumption and maximum stable speed -- because there are always variations in the products -- then multiplier locked and labeled accordingly. (see step 10 here)

2. TDP indicates the highest power draw of any sample of that model under the most extreme conditions. It is therefore the maximum possible under load. At idle, 65W and 45W TDP Athlon X2s, for example, probably have the same power draw. The difference will show up only under load.

Whether a 65W or 45W TDP cpu is quieter really is not that answerable a question. The way I'd answer it is that they can be made to run equally quiet, especially if used in a PC that won't be under high cpu load all the time -- ie just about all desktop PCs. It's typical for most computers to run at idle >90% of the time that they're powered up.

A big enough heatsink will allow either cpu to run cool enough with equally quiet fans. To maintain the same temperature at high load as the 45W model, the 65W cpu will need a bit larger heatsink but that's the gist of the difference. This assumes good adherence to well accepted quiet computing guidelines for the whole pc.

m0002a
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Post by m0002a » Wed Jun 02, 2010 10:14 am

If you want a cool-running CPU, make sure you get one with the smallest form factor, which is usually 45 nm. A 45 nm CPU will run cooler than a 65 nm or 90 nm processor when both are running at the same speed. Generally speaking a CPU with a 45 watt output will never run hotter than one rated at 65 watts, and will often be cooler, although as noted above they might have similar temps in many situations (assuming the same number of cores). But I would go with the 45 watt version unless you really need the extra speed that you might be able to achieve with a CPU at higher frequency, especially if it is a processor specifically designed by the manufacturer to be an energy conserving CPU.

Note: Form factor (nm) and TDP (watts) are not the same, so it is a coincidence that common form factors of 45 nm and 65 nm exist, and also that a CPU TDP is often rated at 45 or 65 watts.

BlackWhizz
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Post by BlackWhizz » Wed Jun 02, 2010 12:40 pm

65w can be passive cooled by a Ninja so i wouldnt be worried. Just get a 65w cpu which has the most performance :)

Klusu
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Re: Confused about temperatures and 45w vs 65w CPU AMD

Post by Klusu » Wed Jun 02, 2010 12:42 pm

pete6032 wrote:just low-power processors that could run very warm (and thus need a faster spinning CPU fan
No. Lower power CPU creates less heat.

Wayne Redpath
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Post by Wayne Redpath » Wed Jun 02, 2010 8:53 pm

m0002a wrote:A 45 nm CPU will run cooler than a 65 nm or 90 nm processor when both are running at the same speed.
Not necessarily true.

A larger die size, whatever the process resolution, can allow the more accurate build of the transistors which often results in more power efficient transistors and therefore lower power consumption.

A smaller process can give more units on a wafer and probably there will be more units that are not trashed by a wafer flaw.

quest_for_silence
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Re: Confused about temperatures and 45w vs 65w CPU AMD

Post by quest_for_silence » Mon Jun 07, 2010 6:28 am

pete6032 wrote:Should I be looking for a 65w CPU or a 45w CPU if I want the quieter of the two?

I guess there will be almost no difference, if you design the system with just a bit of care: and You also have the option of 30w CPU (such as the Clarkdale-based dual processor Xeon).

However I currently use a 45nm-45W quad core AMD, cooled by a modest top down Scythe in a very small case: with around 30°C ambient it ran >1 hour stable at ~51°C with 4 session of Prime95 and its fan whispering at <700rpm (or ~48°C if fan ~900rpm), while the PSU noise (very quiet but still audible at 50cm) cannot let you perceive that whisper.

Regards,
Luca

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