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Old 27th April 2008, 03:52 PM   #1
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Default heat sinks and LM1875

I don't really understand how to calculate the amount of heat sink I'd need. I picked up a heat sink from a computer processor for $1, split into two sections of fins, each half is about 1x 1.3 x 2.5". The data sheet specs 1.4dC/W. Looking in a Mouser catalog, I see heatsinks rated at 3.5dC/W, 6dC/W etc. How do I use the spec to figure out how much I need. Thought about just putting the kits together on a breadboard and see how hot they get.
Any tips on this?
Thanks
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Old 27th April 2008, 03:55 PM   #2
AndrewT is offline AndrewT  Scotland
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Default Re: heat sinks and LM1875

Quote:
Originally posted by Sparky OR
Thought about just putting the kits together on a breadboard and see how hot they get.
Hi,
that's as good a way as any, if you are prepared to buy different bigger sinks when you experiment with your temperatures.

Try for the chip being cool to the fingers when no signal is present
Try for warm heatsink when a normal signal is present. The chip will be hotter than warm.

Or just read the National datasheet and double the suggested size.
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Old 27th April 2008, 10:33 PM   #3
ttan98 is online now ttan98  Australia
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Default Re: heat sinks and LM1875

Quote:
Originally posted by Sparky OR
I don't really understand how to calculate the amount of heat sink I'd need. I picked up a heat sink from a computer processor for $1, split into two sections of fins, each half is about 1x 1.3 x 2.5". The data sheet specs 1.4dC/W. Looking in a Mouser catalog, I see heatsinks rated at 3.5dC/W, 6dC/W etc. How do I use the spec to figure out how much I need. Thought about just putting the kits together on a breadboard and see how hot they get.
Any tips on this?
Thanks
I use computer CPU heatsink(pentium 4) with LM3886 or LM3875 and even Lm1875. I bought(*) the heatsink at computer swap meet for US$2.00 each, cheap and effective and compact, if you like you can activate the fan. I use the non insulated version and bolt directly to the heatsink with artic silver 5 cooling paste with no insulation which I prefer.

With the 1st 2 chipamps and powered from +/- 34Vdc, the chipamps are all very cool, <36deg centigrade(I measured not from chart), after hours of listening. If you drive them hard you may see an increased in temp., if you like activate the fan powered lower than 12vdc, eg 9Vdc so the speed is reduced also the noise level, the chipamp runs happily. Don't worry too much.

for LM1875 max supply voltage is +/- 30 volt from spec sheet.

I have at least 8 units and can make many projects.
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Old 27th April 2008, 11:17 PM   #4
TerryO is online now TerryO  United States
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IIRC, someone (Kurt Chang?) on one of the other forums, built an 1875 chipamp using Radio Shack transformers, etc., and laid it out on a board with a thin guage aluminum "L" section from the hardware store for a heat sink. Evidently it stayed cool enough as to not present any promblems at all.

Best Regards,
TerryO
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Old 27th April 2008, 11:42 PM   #5
ttan98 is online now ttan98  Australia
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Quote:
Originally posted by TerryO
IIRC, someone (Kurt Chang?) on one of the other forums, built an 1875 chipamp using Radio Shack transformers, etc., and laid it out on a board with a thin guage aluminum "L" section from the hardware store for a heat sink. Evidently it stayed cool enough as to not present any promblems at all.

Best Regards,
TerryO

a simple heatsink is ineffective if
1. LM is powered from high DC supply like 25V or higher.

2. if the speaker impedance is low, you are pushing the chipamp hard.

both above will increase chipamp temp. A simple heatsink is therefore ineffective.

The cpu heatsink is effective(with fins)in dissipating heat, cheap and compact. I use the fan when I drive low impedance woofers(2 in parallel) eg 4 ohm.
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Old 28th April 2008, 01:36 AM   #6
TerryO is online now TerryO  United States
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Quote:
Originally posted by ttan98



a simple heatsink is ineffective if
1. LM is powered from high DC supply like 25V or higher.

2. if the speaker impedance is low, you are pushing the chipamp hard.

both above will increase chipamp temp. A simple heatsink is therefore ineffective.

The cpu heatsink is effective(with fins)in dissipating heat, cheap and compact. I use the fan when I drive low impedance woofers(2 in parallel) eg 4 ohm.

I agree, although I believe that Kurt was driving HE horns and that while he used the chipamp for all kinds of uses, his preference was bi or triamping. I suppose that if the conditions you discribe are the intended usage, then perhaps a 3875 or 3886 might be a better choice.

Best Regards,
TerryO
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Old 28th April 2008, 12:59 PM   #7
eketehe is offline eketehe  Indonesia
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Quote:
split into two sections of fins, each half is about 1x 1.3 x 2.5".
hi there,
thats will not suficient.
pls check my last thread, i was using 2 of pentium heatsink for my stereo lm1875 ( the fail portable project ).
its a hot kind of chips, you must get suspicious if the heatsink stay cool.
the heatsink is coated, scratch by smooth sandstone paper, have some good paste, and a real thin micas, then you will find more actual heat.
the portable amp still running well till today ( with fully casing ), but i limit the pot to use 25K only, thats good and loud enogh to drive ineficient 4' or lighter 6,5'. but i use 3 times biger heatsink + thermostated blower fan for my biger project ( stereo pair, 10A, 34VDC, for 12' threeway ).
i don't know hows the heat if you use the full timer blowing fan, the sound is annoying, but i will try the ttan98 idea ( 9V for fan ).

i think the lm1875 need an ideal heat to perform the best sound.......... or just my ears adaptation??
i dont know, hope someone capable to explain this.

Brgds.
Eka
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Old 28th April 2008, 02:08 PM   #8
ttan98 is online now ttan98  Australia
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Quote:
Originally posted by eketehe



i don't know hows the heat if you use the full timer blowing fan, the sound is annoying, but i will try the ttan98 idea ( 9V for fan ).

i think the lm1875 need an ideal heat to perform the best sound.......... or just my ears adaptation??
i dont know, hope someone capable to explain this.

Brgds.
Eka
"the sound is annoying, but i will try the ttan98 idea ( 9V for fan ). "

At 9volt may be still loud for you, try a few volts lower, say 6volts, fan with lower speed and less noise is much better than no fan at all. If you feel like it design a sensor, to detect when a temp exceeds a specific temp, then turn the fan on and stops when it drops below a specified temp. A few designs on the net you can view and easy to build.

"i think the lm1875 need an ideal heat to perform the best sound.......... or just my ears adaptation?? "

This maybe true to some extend, what is the ideal temp., most probably when the amplifier temp stabilises and stop rising, and warm to touch and not hot to touch. The ideal temp, I don't know maybe there is none.
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Old 7th May 2008, 12:57 AM   #9
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Ideal temp. . . no idea. But, I do have some input on voltages.

From collected empiric data:
The chip isn't appreciated as much on 20vdc rails and nearby values.

Its more appreciated from 11 to 16vdc or (skip 18, 19, 20, 21,* 22, 23, 24) and its also appreciated at from 25 to 32**vdc.

*I highlighted the worst in bold. As it is the mathmatical average of averages, then its probably quite measurable.

**Operation at or above 30vdc is restricted to 16 ohm speaker applications or else thermal runaway may occur.

An especially quiet lineup of fans is made by CoolerMaster. This can be run from a simple power resistor.

An inexpensive bimetallic spring thermostat can be found at the hardware store, in the form of an attic fan controller.

An outdated bimetallic spring type air-conditioning thermostat can be inserted between the power resistor and the fan.
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Old 9th May 2008, 12:40 PM   #10
BrianGT is offline BrianGT  United States
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I helped my friend build our first LM1875 amp last night, and we used small heatsinks for to-220 devices. A single one was getting a bit warm, but putting 3 together seemed to suffice until we can get a real chassis design. We were able to turn it up pretty loud with the small heatsinks.

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