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Old 1st July 2006, 05:20 AM   #1
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Default noise from a cooler (two different ones)

hi,
i've built a LM1876 based gainclone, and the main chip is mounted on a pentium II heatsink with it's own fan.
fans are noisy, i know. but the noisy produced by the air turbulence doesn't annoy me since the gainclone is installed far away from my listening position. yet there is another noise, the one coming out of my speakers.
i can hear a "trrrrrrrr" sound, i guess in the range of 800-4000Hz (since it can be heard out of my midranges) and it's also produced by the fan (i can tell because when the fan is starting it's rotation the "trrr" accelerates from zero as well).
the fan is a dc brushless 12v, running on a 7812, hooked up to the 25vdc not regulated power supply that suplies the lm1876 V+ lead.

where is this sound produced? how does it interfere with the circuit? any suggestions on how to eliminate it?
thanks a lot people!
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Old 1st July 2006, 10:06 AM   #2
ir is offline ir  New Zealand
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all the pentium II heatsinks i've encountered would be sufficient to cool an LM1876 (2x20W) without a fan
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Old 1st July 2006, 10:19 AM   #3
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I've had simillar issues with fans,when they are installed physically close to circuits. It seems the amplifier picks up the magnetic pulses from the little coils inside the fan (brushless motor after all..)

I had a problem with a small fan giving the pulsing/oscillating fits to an RF amplifier it was on. I had to move it farther away to eliminate the problem.
Of course,a bit of filtering on the fan's power leads can't hurt either.
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Old 1st July 2006, 10:20 AM   #4
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The first, and easiest thing to do is check your lead dress. Make sure the supply leads to the fan are twisted, and are run well clear of all other cables. If that doesn't work, we might have to get complicated.
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Old 1st July 2006, 01:35 PM   #5
poobah is offline poobah  United States
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Put a 0.1 uf ceramic as close the motor terminals as you can. Use a 1/4" or 6mm hex nut (a small ferrite toroid works much better), pull both motor wires, at the same time, through the hole 5-6 times, then twist the wires tightly. Magnetism from the motor & wires can couple into your circuit... voltage spikes can capacitively couple too.

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Old 1st July 2006, 02:48 PM   #6
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Quote:
Originally posted by ir
all the pentium II heatsinks i've encountered would be sufficient to cool an LM1876 (2x20W) without a fan

mine is something like this one

Click the image to open in full size.

and my lm1876 is doing 2x25w.

if that is enough without the fan, that's best news ever!!


if not, i'll try the other posts pieces of advice.

thanks everyone!
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Old 1st July 2006, 04:24 PM   #7
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I don't like fans. They make noise that isn't very needed and there is a big workaround of just heatsinks...




Drill the proper holes on the outside of the case and heatsink if needed...

Use some thermal grease by putting some on the back of the heasink or on the case of the amp and align the heatsink's hole up to the hole on the case...

If you have an *isolated chip, just attach the chip and board to the inside of the case with thermal grease and align it up to the holes on the case and heatsink.

If you have an isolated chip, use a screw to attach the heatsink, case, and chip together. Make sure all of the pieces (heatsink, case, and chip) are snug together.

That should be it for a better heat transfer


*(IF YOU DO NOT HAVE AN ISOLATED CHIP, YOU MUST USE AN ISOLATING MATERIAL! IF YOU DO NOT, THE ENTIRE CIRCUIT COULD BE EXTREMELY DAMAGED AND COULD RESULT IN USER INJURY! IF YOU DO NOT HAVE AN ISOLATED CHIP, DO NOT USE A SCREW THAT WILL CONDUCT THE METAL TAB AND THE CASE! MAKE SURE TO USE AN ISOLATOR OF SOME SORT INSIDE OF THE HOLE ON THE METAL TAB OR USE A SMALL PIECE OF ISOLATING MATERIAL TO ISOLATE THE INSIDE OF THE HOLE. DO NOT FORGET TO USE THERMAL GREASE ON THE HOLE. REMEMBER - THERMAL GREASE IS ELECTRICALLY CONDUCTIVE SO BE CAREFUL!)
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Old 1st July 2006, 05:34 PM   #8
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my amp case is a plastic one, so that is a problem.
my chip is isolated, and screwed to the heatsink, using thermal grease.
the case has holes where the air comes in to the fan, and out from it.
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Old 1st July 2006, 05:37 PM   #9
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Oh...

Maybe you could use some metal plates and cut out a side of the case or something? Just an idea...
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Old 2nd July 2006, 04:20 AM   #10
ir is offline ir  New Zealand
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Quote:
Originally posted by facundonu
and my lm1876 is doing 2x25w.

if that is enough without the fan, that's best news ever!!
You'd be surprised.

my best advice would be to disconnect and remove the fan, run the amp for a while, check the heatsink and chip temp every so often. it will get warm but shouldn't be hot as such. if you're comfortable with its running temp, simply leave the fan off - no point in running a fan you don't absolutely need to.
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