Cooling fan in my LM3886 stereo amp

Painkiller82 · 9th October 2007, 04:48 PM

Hi again,

I have built a Gainclone based on this schematic: http://diyaudioprojects.com/Chip/LM3.../LM3886_CA.htm

The heatsinks get quite hot, and I was thinking about putting a fan on the inside that sucks air out, and drill holes around the chips, so that most of the air is sucked in from that area.

But how can I design the fan circuit? I thought about having 9 to 12 V on the fan, and I tried to put a 220k in series with a 470k, parallell to the +32V. But I guess the fan acts like an inductor, because when I put the fan across the 220k, the voltage across the resistor fell to 1V and the fan did not work.

Does anyone have any ideas to how I can get it to work?

AndrewT · 9th October 2007, 05:06 PM

Hi,
do not run the fan off an audio PSU.
Supply it from a separate PSU, either from a separate winding off the transformer or from a separate transformer.
Select the Vac to suit the fan motor.
Arrange for some kind of temperature control. Off when cold. Medium speed when warm and fast +warning light when hot.

Be careful to ensure it starts when on medium speed.
It might need a kick to overcome friction and then slow to medium speed.

I would also fit a high temp mains switch to the sink in case of failure of the fan/control circuit.

Painkiller82 · 9th October 2007, 05:37 PM

Thanks! I think I'll skip the fan then. I have added a couple of heatsinks on the chassis instead. That helped a bit.

But is there no easy way to run it from the audio PSU?

BTW, the amp sounds great! I'm very happy with the result.

Nordic · 9th October 2007, 05:55 PM

I once had a transformer made, with an extra secondary for a fan... sadly I could never get rid of the noise the fan made elctricaly, tried filters etc... then gave up and got proper heatsinks....

A good heatsink for a chipamp should cost no more than $5 to $10 (if you are feeling fancy) per channel.

SpittinLLama · 9th October 2007, 11:39 PM

I have run a fan off the main amp PSU with just a resistor to slow it down and there was no audio noise. I have also done it with lots of audio noise. For no noise it requires maybe some shielding and good PSU PCB design. I had to shield the input wires and the fan wires. You can also get a 12V fixed regulator to run off the main PSU lines. This helps with isolation of the electrical noise on the PSU lines. The best option is to not use a fan at all.

-SL

Painkiller82 · 10th October 2007, 07:52 PM

Thanks for the answers! I dropped the fan, and hope that it won't be too hot. It hasn't failed yet at least.

Here's the final result:
http://folk.ntnu.no/espenta/bilder/hifi/DSC_0022.jpg

9th October 2007, 04:48 PM	#1
Painkiller82 diyAudio Member Join Date: Sep 2007	Cooling fan in my LM3886 stereo amp Hi again, I have built a Gainclone based on this schematic: http://diyaudioprojects.com/Chip/LM3.../LM3886_CA.htm The heatsinks get quite hot, and I was thinking about putting a fan on the inside that sucks air out, and drill holes around the chips, so that most of the air is sucked in from that area. But how can I design the fan circuit? I thought about having 9 to 12 V on the fan, and I tried to put a 220k in series with a 470k, parallell to the +32V. But I guess the fan acts like an inductor, because when I put the fan across the 220k, the voltage across the resistor fell to 1V and the fan did not work. Does anyone have any ideas to how I can get it to work?

9th October 2007, 05:06 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, do not run the fan off an audio PSU. Supply it from a separate PSU, either from a separate winding off the transformer or from a separate transformer. Select the Vac to suit the fan motor. Arrange for some kind of temperature control. Off when cold. Medium speed when warm and fast +warning light when hot. Be careful to ensure it starts when on medium speed. It might need a kick to overcome friction and then slow to medium speed. I would also fit a high temp mains switch to the sink in case of failure of the fan/control circuit. __________________ regards Andrew T.

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9th October 2007, 05:37 PM	#3
Painkiller82 diyAudio Member Join Date: Sep 2007	Thanks! I think I'll skip the fan then. I have added a couple of heatsinks on the chassis instead. That helped a bit. But is there no easy way to run it from the audio PSU? BTW, the amp sounds great! I'm very happy with the result.

9th October 2007, 05:55 PM	#4
Nordic diyAudio Member Join Date: Sep 2005	I once had a transformer made, with an extra secondary for a fan... sadly I could never get rid of the noise the fan made elctricaly, tried filters etc... then gave up and got proper heatsinks.... A good heatsink for a chipamp should cost no more than $5 to $10 (if you are feeling fancy) per channel.

9th October 2007, 11:39 PM	#5
SpittinLLama diyAudio Member Join Date: Apr 2005 Location: In the Wild, Wild West	I have run a fan off the main amp PSU with just a resistor to slow it down and there was no audio noise. I have also done it with lots of audio noise. For no noise it requires maybe some shielding and good PSU PCB design. I had to shield the input wires and the fan wires. You can also get a 12V fixed regulator to run off the main PSU lines. This helps with isolation of the electrical noise on the PSU lines. The best option is to not use a fan at all. -SL

10th October 2007, 07:52 PM	#6
Painkiller82 diyAudio Member Join Date: Sep 2007	Thanks for the answers! I dropped the fan, and hope that it won't be too hot. It hasn't failed yet at least. Here's the final result: http://folk.ntnu.no/espenta/bilder/hifi/DSC_0022.jpg

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