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Old 20th March 2003, 08:28 AM   #1
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Default Pls Help Choose a Fan

I need a fan to cool my amps heatsink, which do you think is better, AC or DC powered fan? Fan noise is not an issue but if I use an AC(Mains) powered fan, will it cause noise in my amp?

How many blades does a fan need to give more air? Someone once told me that a fan with 3 blades pushes more air than a fan with 5 blades. Is this true?

Jojo
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Old 20th March 2003, 11:49 AM   #2
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us in old computer fun.... is a 5 blades... i us from 4 computer funs to big car amplifier....

the computer fun is a 2800RPM +- 12VDC 100mA.
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Old 20th March 2003, 12:22 PM   #3
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I am planning to use 2 12v fans in my amp,but they run from a seperate supply , isolated from the amp ( they run off the 12v rail that the soft-start / amp monitor cpu uses ) . In my case it's essential to use 12v computer fans which have a tacho feedback - in case either fan fails the cpu shuts down the amp.

AC fans typically use a brushless 'squirrel cage' type of induction motor, but even these can induce noise onto the supply.

as for the airflow, look at the manufactures figures, they should tell you the airflow in cfm or l/min . the number of fan blades is not normally an issue , but try to get a fan which has an odd number of blades with respect to the supporting struts for the motor - i.e. 5 blades/4 struts , or 7blades/3 struts etc.
This minimises vibration, noise and resonances.

Try looking at some of the computer overclocking websites for lots of info on fans, there are some real keen folks out there.

you might be able to add an additional secondary winding to your psu transformer ( e.g. toroidal one especially ) and power your 12v fan from that via a diode bridge. This keeps the fan supply nicely isolated.
Ive done this trick once or twice myself to cool amps.

ray
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Old 20th March 2003, 12:26 PM   #4
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Default concerns

I'm really interested in the AC powered fans since they are cheap in our country (surplus?).

However, I'm worried that it may induce noise since it will be installed inside the amp and use the same AC(mains) supply.

Anyone?
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Old 20th March 2003, 12:55 PM   #5
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Default fans are specified for SPL's

my one budding architect son has a very "overclocked" computer for doing his CAD work, and purchased a pair of low SPL fans -- these are easy to find on the web.

Unless you are a complete head-banger, you might wish to consider using a controller chip for the fans which would respond to the temperature on the heat sink. If you are driving the amp to high levels of output the ambient noise from the fan will rise, but the s/n ratio is going to remain pretty low. If OTOH you are listening to something quiet the fan noise will be at a low level also.

there are quite a number of fan controller chips. Analog Devices has one with built in thermistor and it only costs a buck or so.

Jack
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Old 20th March 2003, 01:14 PM   #6
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there are quite a number of fan controller chips. Analog Devices has one with built in thermistor and it only costs a buck or so.

you might as well build your own controller, it only takes a thermistor , a darlington transistor (e.g. TIP120) and a couple of resistors...
as for the fan, brand new they cost about 6UKP ( maybe $8 ), much less from surplus outlets ( last lot I got were about $2 each)

ray
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Old 20th March 2003, 01:30 PM   #7
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Quote:
Originally posted by TheFettler
[B]
you might as well build your own controller, it only takes a thermistor , a darlington transistor (e.g. TIP120) and a couple of resistors...
as for the fan, brand new they cost about 6UKP ( maybe $8 ), much less from surplus outlets ( last lot I got were about $2 each)

ray
The analog devices part is cheaper than a thermistor combo and only takes one resistor for T(set) -- no cutting and pasting (time=money).
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Old 20th March 2003, 01:48 PM   #8
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The analog devices part is cheaper than a thermistor combo

coo... I didn't realise they were that cheap

fair 'nuff.

ray
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Old 20th March 2003, 07:16 PM   #9
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Default I think I was wrong on the price

when they first came out with the chip it was in the $2.20 area -- but I see it's now around $5.70. Mea culpa.
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Old 21st March 2003, 02:11 AM   #10
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Default So it's DC

Ray,

Thanks for the info, that is exactly what I did to my other amp. I used a separate winding to power it.

What do you mean when you say soft start?

jackinnj,

Can you be more specific with the Analog Devices chip. Part # pls.

Thanks,
Jojo
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