cooling fan attenuation

Disco-Pete

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Paid Member
2009-11-30 10:04 pm
TIA. I have a great sounding Altec 9440A. The problem is the cooling fans are noisy. I can unplug the fans at the back of the amp but then it gets hot. I'd like to attenuate the speed of the fans to the minimum required for the given load. Is there a control I can install that won't introduce it's own noise into the amp?
 
Something linear. Not Pulse Width Controlled, but plain stupid analogue.
Assuming 12V fan, I envision it more or less like this:
4V at 25 deg C (measured adjacent to output transistors).
5V at 30 deg C.
6V at 35 deg C.
7V at 40 deg C.
8V at 45 deg C.
9V at 50 deg C.
10V at 55 deg C.
11V at 60 deg C.
12V (full voltage) at 65 deg C.
Something with an op-amp, controlling a series pass transistor.
Something with a termistor bolted direct on top or next to one of the output transistors.
The trick would be to shape the Output_Voltage = f( temperature ) in an optimal way.
Whatever optimal may be.
Just a wild guess.

P.S. The fan should get a "kick_start" on power-on, just to get it start spinning. Once it starts spinning, it should actually self-sustain spinning, even at 4V.

P.S.S. Also include a last resort, additional, crude, thermal cut-off, in case the circuit with the op-amp fails.
 
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Disco-Pete

Member
Paid Member
2009-11-30 10:04 pm
Something linear. Not Pulse Width Controlled, but plain stupid analogue.
Assuming 12V fan, I envision it more or less like this:
4V at 25 deg C (measured adjacent to output transistors).
5V at 30 deg C.
6V at 35 deg C.
7V at 40 deg C.
8V at 45 deg C.
9V at 50 deg C.
10V at 55 deg C.
11V at 60 deg C.
12V (full voltage) at 65 deg C.
Something with an op-amp, controlling a series pass transistor.
Something with a termistor bolted direct on top or next to one of the output transistors.
The trick would be to shape the Output_Voltage = f( temperature ) in an optimal way.
Whatever optimal may be.
Just a wild guess.

P.S. The fan should get a "kick_start" on power-on, just to get it start spinning. Once it starts spinning, it should actually self-sustain spinning, even at 4V.

P.S.S. Also include a last resort, additional, crude, thermal cut-off, in case the circuit with the op-amp fails.
The oem fans plug in to the switched output on the back of the amp. They are 120v. I just want to slow them down manually with a suitable attenuator. Nothing fancy. Would a 120v dimmer introduce noise?
 
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According to the fan kit manual, these fan seem to be AC jobs, simply plugged into the amplifier's switched outlets. Not likely to be controllable very well.

I suggest going with an external 12 V supply and a set of computer DC fans of the right dimensions (looks like 120 mm). While the usual suspects do make temperature controlled fans with thermistors attached, it may be easier to use an external fan controller (for DC-controlled fans, I don't think they make them for PWM fans but might be wrong).
Would suggest mounting with rubber fan nipples. Any resulting gaps could be covered up with tape or something if needed.
Fan wise, you could go cheap but good with some Arctic Cooling jobs (perhaps the plain P12 which is tuned for static pressure as would be required here, or the lower-RPM P12 Silent; they also make some ball bearing ones if you don't trust their regular FDBs, though there seems little reason to) or fancy with Noctua (some of the best fans you can get in some of the most ugly colors).
Get some extra fan grilles if you conside this necessary.

You will probably be glad you went this route - old fans can be rather noisy indeed. After 40 years, the bearings in yours may well be dried up and appreciating some TLC anyway. They can often be restored (bearings cleaned & relubed, or replaced if standard industrial ball bearings). Always running at full throttle is very industrial indeed though.
 
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All that … just barely overlooking the obvious … new fans of the ultra-quiet type! Almost surprisingly, most fans come in very standard sizes. So, hunt out the ultra-quiet type.

From a 2 second Google search: https://www.amazon.com/AC-Infinity-Cooling-Ventilation-Projects/dp/B009OWVUJ0

Probably not a good fit. BUT it was a 2 second search. The words “muffin fan” are key, since they've long been generically known as that. Kind of like how nose-napkins are known as kleenex, photocopies as xeroxes, and real estate agents as realtors.

Just saying,
GoatGuy ✓
 

Disco-Pete

Member
Paid Member
2009-11-30 10:04 pm
All that … just barely overlooking the obvious … new fans of the ultra-quiet type! Almost surprisingly, most fans come in very standard sizes. So, hunt out the ultra-quiet type.

From a 2 second Google search: https://www.amazon.com/AC-Infinity-Cooling-Ventilation-Projects/dp/B009OWVUJ0

Probably not a good fit. BUT it was a 2 second search. The words “muffin fan” are key, since they've long been generically known as that. Kind of like how nose-napkins are known as kleenex, photocopies as xeroxes, and real estate agents as realtors.

Just saying,
GoatGuy ✓
I just placed an order for the fan speed controller also listed. The one available on the .ca site comes with dual connections. Perfect. Low, Med, High. We'll see. It's worth a try at 23.00


Thanks:)
 
Noctua, Nordic Cooling, BeSilent - all very interesting options in terms of low-noise fans.
You may also look for the Low-RPM versions of some of these products, because lower RPM at 12VDC usually means less noise during nominal operation.
And this can obviously be regulated "down" with the controller thing, which you could in plain vanilla version purchase also from the distributor of such PC fans.
I would seek the versions with a "potentiometer", and then try to replace the potentiometer with a termistor of sorts.
 
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Disco-Pete

Member
Paid Member
2009-11-30 10:04 pm
All that … just barely overlooking the obvious … new fans of the ultra-quiet type! Almost surprisingly, most fans come in very standard sizes. So, hunt out the ultra-quiet type.

From a 2 second Google search: https://www.amazon.com/AC-Infinity-Cooling-Ventilation-Projects/dp/B009OWVUJ0

Probably not a good fit. BUT it was a 2 second search. The words “muffin fan” are key, since they've long been generically known as that. Kind of like how nose-napkins are known as kleenex, photocopies as xeroxes, and real estate agents as realtors.

Just saying,
GoatGuy ✓
Okay, I received the controller, hooked it up. It has a variable speed dial with infinite control from min to max. Set to min, I can still hear the fans but very acceptably. I sit 10ft. away from the amp set between the speakers. I cannot hear them with music, even quiet passages. The heat sinks stay under 100F.
Thanks again;)
 

Disco-Pete

Member
Paid Member
2009-11-30 10:04 pm
Just an update. I purchased a pair of muffin fans at my local surplus store for $10 each, same size, to see if I can quiet it down even more. Well these are so quiet with the control turned to min that I cannot hear them hovering right over the amp. But the heat sinks still stay at around 100F even when the amp is pushed hard. The fan shroud is an enclosure with slots top and bottom. The fins on the sinks are rather minimal and a bit short imo. This could explain why it gets hot without the fans but it seems if they would have just incorporated adequate sinks, the fans would not be needed at all. Strange.
 

PRR

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Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> Never heard the term "muffin fan" used to refer to these before..

Trademark of one brand, Rotron.
 

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Disco-Pete

Member
Paid Member
2009-11-30 10:04 pm
> even when the amp is pushed hard.

They will be hotter if you push steady tone to dummy-load at a large fraction of maximum power. Speech/music, even "hard", is "easy".

Altec had to consider odd uses like motor-drive and way-low load impedances, so they went to fan.
Turning up the volume as loud as my ears can stand, I have to speed up the fans a bit but I still cannot hear them at my listening position. So this system is more than adequate. I'm using it with a pair of Acoustat Model 3 which I think have a working impedance of 4 ohms; not sure though
 
Turning up the volume as loud as my ears can stand, I have to speed up the fans a bit but I still cannot hear them at my listening position. So this system is more than adequate. I'm using it with a pair of Acoustat Model 3 which I think have a working impedance of 4 ohms; not sure though
If you really want to stress the amp to the limit, try this at 68% of the total possible signal level. Possibly into a dummy load, because your ears would probably not stand it, if the load would be the speakers.
Funny as it may seem, but the maximum stress for many an amplifier is not at "100%", but at somewhere around 68%. My gut feeling tells me that at the 68% of signal level is the point at which you will be running hottest.
 
fan noise is a common complaint, i wonder if anyone has tried a baffled duct to attenuate fan noise?
does blade geometry/shape play a role? perhaps a different blade shape would minimize blade noise while maintaining airflow?
Interplay of design + material
I guess that a baffled duct to attenuate fan noise would be counter productive. It would introduce far more flow resistance to the air stream, than actual noise attenuation. Fans are all about laminar flow.
The shape of the fins is critically important for the overall noise performance.
See the fin details in some of the top fan models of Noctua. Very interesting detail.
 
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