fan cooling?

[Raj1]

Hi,

I was kindly posted this by Hugh Dean of AKSA, when I enquired about a couple of heatsinks

Have you considered fan cooling? On a Class A amp (Aleph by
any chance??) it is really is well nigh impossible to
achieve good cooling with still air, particularly in a high
ambient situation - yea verily, even the Brit summer.

Fan cooling with a large (120 x 120mm) fan rated for 2700rpm
or so and reduced to half voltage with a fluoro ballast is
VERY effective. If you bring a 32dB fan to half voltage,
the flow drops to about a third, yes, but the speed halves
and the noise drops to around 22dB; scarcely audible, even
in a quiet room, and MUCH quieter than a PC fan, which we
all have to live with constantly........ Most 120x120 fans
can easily generate 90cfm flow.

Fan cooling at 22cfm (12.7l/s) into a well designed heatsink
such as the flat backed MF18-151.5 will give an effective
thermal performance of 0.13C/watt. That should do it, as it
means for 100W input, only 13C above ambient, which should
guarantee MTBF on an IRF mosfet of around 50,000 hours!!

Two such heatsinks facing each other can have the semis on
opposite sides, will be 120mm x 120mm end on, and 151.5mm
long. For minimum noise, it is suggested you use a 4" long
120mm x 120mm plenum chamber between fan and heatsink; this
stops turbulent noise of fan blades against the fins.
Unfortunately I do not have photos, but from the description
you should get the picture. There are 18 fins on this sink.

Hope this is helpful, Happy New Year, and sorry about the
delayed response.

Cheers,

Hugh

Hugh R. Dean
Sales & Research
Aspen Amplifiers P/L
www.aksaonline.com


Problem is I'm having difficulty in visualisng how it's done like this, anyone know where there's any pictures?


Thanks
Raj

[Apogee]

Raj,

This will give you an idea of what he is talking about. Picture two heatsinks that when faced fin to fin they create a cube similar to the picture...

The only other tidbit is that he recommends using a 4" plenum. The plenum in this case is nothing more than a 4" square spacer that's hollow between the fan and the heatsinks...

You then mount your semiconductors on the "ledges" that you see in the picture... In the case of the heatsinks he is referring to, I believe that there would be one mounting "ledge" on each side instead of two like in the picture...

If there is no ledge on the heatsinks you choose then you would simply distribute the devices down the flatback of the heatsink on each side...

Hope this helps!

[joensd]

Hi,
whilst talking about fans:
Does anybody need a heatsink like the one shown from Apogee?
It´s the LA1 02 with two segments from Fischer Elektronik.
I want to sell it for 20€.

Jens

[pr]

Fan cooling at 22cfm (12.7l/s) into a well designed heatsink
such as the flat backed MF18-151.5 will give an effective
thermal performance of 0.13C/watt.

Raj,
can you tell us please:
What is the thermal performance without the fan?
What are the dimensions of the sink ?
That woul answer the question, how much does a nearly inaudible fan reduce the thermal resistance
of a heat sink.
Thank you.

[BDP]

Raj1

I constructed this case with fan cooling for my A75. The fan is a 24 volt run at 12volt, very quit and cooool. It is pretty much out of the A75 article, the heatsinks were surplus and this seemed the best way to provide for the heat dissipation. If I want to increase the bias I just have to turn up the fan. Plus I think it is a great looking toaster, or that is what most people call this amp.

Bryan

[Circlotron]

Originally posted by Raj1
Fan cooling with a large (120 x 120mm) fan rated for 2700rpm
or so and reduced to half voltage with a fluoro ballast is
VERY effective. If you bring a 32dB fan to half voltage,
the flow drops to about a third, yes, but the speed halves
and the noise drops to around 22dB; scarcely audible, even
in a quiet room, and MUCH quieter than a PC fan.

I never thought of using an AC fan but it should be quieter than a brushless DC one. The DC ones make a brrrr sound at low speeds because of torque pulsations whereas an AC one running off a sinewave should be much quieter. I have heard of using a series capacitor to reduce the voltage to an AC fan but the fluoro ballast method sounds good.

[fezz]

hmm im not sure if i agree with this - yes it is a great way for cutting costs BUT it adds noise and interferance - the former is definatly bad for any sort of audio produce and the latter caused by the magnet) isn't goin to help much

if u are planning to use a fan, look for a better way to control the noise - such as this Pulse Width Modulation controler : http://www.bit-tech.net/article/51/

also bear in mind that it is possable to get very low c/w heatsinks for not much - i infact have two 0.2c/w heatsinks comming in the post for me that i got for free as samples from aavid

[Raj1]

Hi,

I have no idea what a fluoro ballast is! Can anyone tell me, where can I get it/one from?


Her's the web address for the fan I have in mind.

http://www.tekheads.co.uk/tekheads/product?id=600801

SPeed selectable 12v fan which runs at 21db whilst churning out 25cfm.

Bryan what type of fan did you use? Was it dc 24v or ac?

Also what type of transformer did you use?

I was thinking of using a walwart dc regulated psu for the supply to the fan
The heatsink is 151.5mm long

120mm wide

and the fins are 60mm long
I think the base is 7mm thick, the standard dissipation of this heatsink without air cooling is 0.46 c/w

If you apply 13.5 cfm this drops to 0.13 c/w

At 27 cfm it drops to 0.11 c/w

however I am keen to do this as quietly as possible so if you think the fan I've chosen is no good please let me know!


Thanks
Raj

[wuffwaff]

Fezz,

I tried out the pwm stuff (with an MIC502 IC) but wasn´t pleased cause the pwm signal turned up everywere despite using a seperate supply.

Raj1,

22dB(A) isn´t that silent inmy opinion. I use 12dB(A) fans at a lower voltage (i guess around 10dbA for the two) and only now I can say that I can´t hear them. At 15 dB(A) for the two of them at 12V you could hear them at over 3m distance.

william

[Tazzy]

The Papst 4412F/2GL is a nice 21Db 120mm Fan, I've got 2 of those in my PC for extra cooling

As long as the fan is used for intake the fan makes virtualy no noise on 12v. The same counts for outtake on 7v