Heatsink approved needed for lm3886
Hi All,
I found someone selling these heatsinks.
The Dimensions are 15 & 1/10 inches long and 7 inches wide and the fins are about 5/8 inch tall.
I was thinking of using BrianGT's pcb boards with it, and having it lay horizontal as a top cover to the amp case.
Any opinions on if it's decent enough, and how many chips could I possibly run on it (with and without fan cooling on top).
Also, does anyone know if Brian's boards can be used with the lm3875 (with minor modifications).
Many Thanks Again,
Adrian
Hi All,
I found someone selling these heatsinks.
The Dimensions are 15 & 1/10 inches long and 7 inches wide and the fins are about 5/8 inch tall.
I was thinking of using BrianGT's pcb boards with it, and having it lay horizontal as a top cover to the amp case.
Any opinions on if it's decent enough, and how many chips could I possibly run on it (with and without fan cooling on top).
Also, does anyone know if Brian's boards can be used with the lm3875 (with minor modifications).
Many Thanks Again,
Adrian
Attachments
You might want to read National's application note here: http://www.national.com/an/AN/AN-1192.pdf
But that's a lot to read to just say this. Based on AN-1192's suggested heatsink for the Bridged or Parallel LM3886's (see page 19 of AN-1192), I would say that the extrusion you are considering could take 2 chips for every 3 linear inches of heatsink (one on each thick part of the extrusion cross section I recommend). That's about 10 chips for the whole thing! If you are going to be using this in a hot environment or are going to be bridging/paralleling these you might want to err on the conservative side.
I personally have a similar heat sink and similar application and will be using 8 chips on mine. I wouldn't hesitate to add the other two if I needed them though.
Regards,
David
But that's a lot to read to just say this. Based on AN-1192's suggested heatsink for the Bridged or Parallel LM3886's (see page 19 of AN-1192), I would say that the extrusion you are considering could take 2 chips for every 3 linear inches of heatsink (one on each thick part of the extrusion cross section I recommend). That's about 10 chips for the whole thing! If you are going to be using this in a hot environment or are going to be bridging/paralleling these you might want to err on the conservative side.
I personally have a similar heat sink and similar application and will be using 8 chips on mine. I wouldn't hesitate to add the other two if I needed them though.
Regards,
David
Cool.
Thanks guys. I was worried about the shallow fins, but it was designed for an old school rockford fosgate car amp, so I figured it must be something pretty decent.
He is selling them for $10, I was thinking of getting a couple.
He also has shorter ones, same design, same price.
I was thinking also of hunting down some black plexiglass for the front and back panels.
Adrian
Thanks guys. I was worried about the shallow fins, but it was designed for an old school rockford fosgate car amp, so I figured it must be something pretty decent.
He is selling them for $10, I was thinking of getting a couple.
He also has shorter ones, same design, same price.
I was thinking also of hunting down some black plexiglass for the front and back panels.
Adrian
To answer your other question about the LM3875. I know that the 2876 is pin compatible, but don't have personal experience with the LM3875. A short trip to the National website reveals the following pinouts:
They don't look pin compatible to me. I would have to assume that it would be not be worth trying to use the LM3875 if you could help it.
Regards,
David
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They don't look pin compatible to me. I would have to assume that it would be not be worth trying to use the LM3875 if you could help it.
Regards,
David
Hi,
mute and ground are the incompatible pins.
Magura's hacksaw will do wonders for keeping the chips cooler.
Try cutting that tall heatsink into two shorter halves. It will dissipate about 40% more heat if the two halves are placed side by side.
Similarly if it were cut into 4 equal lengths it's dissipation capacity is just about doubled, compared to mouting all the devices on one giant heatsink.
One 3.7inch tall sink might just be able to cool a pair of chipamps, try fitting one first and see how cool/warm/hot it runs.
mute and ground are the incompatible pins.
Magura's hacksaw will do wonders for keeping the chips cooler.
Try cutting that tall heatsink into two shorter halves. It will dissipate about 40% more heat if the two halves are placed side by side.
Similarly if it were cut into 4 equal lengths it's dissipation capacity is just about doubled, compared to mouting all the devices on one giant heatsink.
One 3.7inch tall sink might just be able to cool a pair of chipamps, try fitting one first and see how cool/warm/hot it runs.
When you have it flat, the air has to fill in from the side almost like water, filling a valley, which is much less efficient then when it just speeds up travelling upwards...
Cutting it in two, will allow you to use it as the sides of a case + give you the benifit of vertical convection...
A tablesaw also warks nicely for cutting these square..., just make sure you have the sink solidly attached to a large plank.
I preffer LM3875 as it is very easy to knock an amp together just useing stripboard, snipping off unused legs, and bending other legs to fit.
Cutting it in two, will allow you to use it as the sides of a case + give you the benifit of vertical convection...
A tablesaw also warks nicely for cutting these square..., just make sure you have the sink solidly attached to a large plank.
I preffer LM3875 as it is very easy to knock an amp together just useing stripboard, snipping off unused legs, and bending other legs to fit.
Amazing as you. You guyz know your stuff!
If I have to cut the heatsink, I might just instead get the 10inch long ones that the guy offers and make them upright thin amps.
I figure I can put 4 amps on one 10inch long heatsink upright with no problems.
I think I will get the pcbs from brian and do some pin leg manipulation to aline the correct pins the the holes on the pcb.
At $3 per pcb, i think it's a pretty good deal, nice quality.
My origianl plan was to use heatsinks from some PII chips that I was tossing away, but I am scared that they are too small.
Is it better to use 220uf or 100uf on the rails on the chips pcb board close to the amp chip?
Thanks,
adrian
If I have to cut the heatsink, I might just instead get the 10inch long ones that the guy offers and make them upright thin amps.
I figure I can put 4 amps on one 10inch long heatsink upright with no problems.
I think I will get the pcbs from brian and do some pin leg manipulation to aline the correct pins the the holes on the pcb.
At $3 per pcb, i think it's a pretty good deal, nice quality.
My origianl plan was to use heatsinks from some PII chips that I was tossing away, but I am scared that they are too small.
Is it better to use 220uf or 100uf on the rails on the chips pcb board close to the amp chip?
Thanks,
adrian
two chips on a 5inch high sink will run cooler.
one chip on a 2.5inch high sink will run cooler than either of the suggestions above.
I assumed that all combinations had the sink standing up and external to the case.
The cooling capacity of a particular vertical sink is roughly proportinal to the square root of the height. Double the height and one gets about 1.4Times the dissipation capacity. Quadruple the height and one gets about double the dissipation capacity.
If each chip is putting out 3W quiescent power and fitted to a single heatsink with Rth s-a of about 2C/W then the sink operates at about 6C degrees above ambient temperature.
Put 4 chips on on a sink 4times taller with Rth s-a of about 1C/W and the sink will operate at about 12C degrees above ambient.
You have just increased the Tc of the chips by 6degC.
Now put some power through the chips, let's assume each chip is putting out just 2W of signal output. Do the numbers.
BTW,
if you lie the sink flat with the fins pointing up, it's dissipation capacity drops significantly. i.e. your chip heating gets even worse. A very wide and very long sink relative to the fin heght suffers more than a sink with tall fins in this position.
Yup, that's the guy selling them on ebay. He also has the newer rockford heatsinks which are powder coated texture for a bit more.
Andrew, thanks again. I had a feeling the using a wide foot print would not make up for short fins. That heatsink is actually fan cooled with a shroud from rockford.
One of the helpful members here forwarded me a good spotting of surplus heatsinks at A1 electronics. It might be easier for me to go local.
But If I have to cut the heatsink down, any recommened methods without buying a new chopsaw. I do have table saw, which blade do I need?
Thanks,
Adrian
Andrew, thanks again. I had a feeling the using a wide foot print would not make up for short fins. That heatsink is actually fan cooled with a shroud from rockford.
One of the helpful members here forwarded me a good spotting of surplus heatsinks at A1 electronics. It might be easier for me to go local.
But If I have to cut the heatsink down, any recommened methods without buying a new chopsaw. I do have table saw, which blade do I need?
Thanks,
Adrian
A fine Tungsten carbide tipped blade will cut aluminium well. It will leave a few little burrs but these can be filed off easily.
Take it slowly, use a blade with lots of teeth, NOT a ripping blade.
Use a bit of wax to lube it,
Do NOT stand in line with the blade - if the blade grabs any fine offcuts it will throw them at you at high velocity, wear a face shield and safety glasses because metal will be going everywhere.
Take it slowly, use a blade with lots of teeth, NOT a ripping blade.
Use a bit of wax to lube it,
Do NOT stand in line with the blade - if the blade grabs any fine offcuts it will throw them at you at high velocity, wear a face shield and safety glasses because metal will be going everywhere.
svbear,
I use PII (2.5w*5l*2h) heatsinks for my lm3886 powered computer speakers. I've not had any problem with this configuration, but I have a low voltage supply (18-0-18 at rails). Not a demanding application my any means, but just an example for you!
added: I love the blade advice given above. That is experience speaking at it's finest! Please do be careful if you try to cut the heatsinks. I have actually seen the carbide tips fly off a blade while cutting aluminum and chip a cinder block wall. Please heed the advice given above.
I use PII (2.5w*5l*2h) heatsinks for my lm3886 powered computer speakers. I've not had any problem with this configuration, but I have a low voltage supply (18-0-18 at rails). Not a demanding application my any means, but just an example for you!
added: I love the blade advice given above. That is experience speaking at it's finest! Please do be careful if you try to cut the heatsinks. I have actually seen the carbide tips fly off a blade while cutting aluminum and chip a cinder block wall. Please heed the advice given above.
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