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Heatsinks for your poweramp

Started by joecool85, April 05, 2006, 07:37:26 PM

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Quote from: spud on April 07, 2011, 12:06:38 PM
How about an old CPU heatsink from a dead computer?  Would that be usable given that you have some thermal goop to put on there.


A computer CPU heatsink is good for a small 5-10w amp without a fan.  With a fan it should be good for 50-100w depending on the heatsink.  That said, I never recommend using a fan on a heatsink.  It's one more part that could fail.  Lots of companies do it though, mostly with good success.
Life is what you make it.
Still rockin' the Dean Markley K-20X


I've used heatsink USA (barrredboss on Ebay) a couple times and his stuff is excellent and good value.
A great (cheap) source for heatsinks are old amps/receivers at Goodwill, thrift shops etc. They don't need to be working. These are also good for a wealth of screws, standoffs and other hard to get items - transformers too. Worthwhile grabbing a couple or more to scrap for parts.

The side heatsinks of this monster are from HeatsinkUSA:



Why not just use bad ass cpu heatsinks? :-p or water cooling:-p

Most computer fans will operate on 5 volts - you wont hear that at all, fan wise.  I'd run 2 of them for a push pull set up, if 1 failed... still got 1 to go on.  Set up some LED indicators for each fan, if fan fails to start led lights up or something.

You could also go as far as using thermo resistors, higher temp would allow for more current/voltage flow.  Means fan will go faster. If its hot enough to make it go fast - you wont hear it mechanicaly.  Computer fans are also brushless, so not difficult to silence the eletricty.

If i build any high powered amps soon, I'll be using heat pipes.


My main worry with brushless DC motors would be EMI... haven't tried anything myself though.

Has anyone had any experience with that?

J M Fahey

I always use 12V CPU power supply type fans (not the small microprocessor ones but the 3 1/4" ones) on my 300W amps.
Yes, they *do* cause some interference, not RFI type (they are brushless) but magnetic: the rotor itself is a rotating multi-pole magnet, plus the coils are being switched on and off by squarewaves very quickly.
So: no reverb pickup heads close to them, and no sensitive preamp inputs either.
I put them on an end of the cabinet, and "shielded" from the electronics by the power transformer, which itself is a noise source, so both "live" on the opposite end of the inputs.
No problem at all with my Bass amps; but on my Powered Mixers, with inputs all over the place, sometimes I have problems with some buzzy hum injected in the Master section ... the culprit is easily found just by blocking rotation with a finger.


Silly question:

Why cant i use some piece of steel framing (steel galvanized) as heatsink? I have a lot of pieces of them, and it comes in several sizes...

J M Fahey

No, not steel.
Buy *aluminum* rails used to make doors and windows, they may be used, if at least 1.5 or 2mm thick , plus having at least 10x10cm (4"x4"), preferrably 10x20cm (4"x8") or larger.
If *somewhat* narrower (say, 3", they should be proportionately longer.
Somebody who makes such doors and windows may have lots of leftover pieces, in small sizes useless for him, but good for you.
At least part of the surface where you bolt the chip must be flat and make good thermal contact.
Always use thermal grease.
Some examples of extruded aluminum which can often be had for free , or peanuts, if leftover useless for the metal shops:


^ Cool idea, cheap, I like the recycling aspect of it, and with some thought process it likely could also make a great aestetic design detail in the vein of another aspect I really like: function defines form. Have to store that to my brain and use it when I run out of cannibalised heatsinks.

Oh, generic trash metal yards will be packing literally tons of this stuff.

But I think I wouldn't use that stuff for any heavy-duty heatsking, though; my impression with those leftover frame pieces is that they are kinda flimsy (effective heat spread throughout the heatsking requires a thicker baseplate) and the convection would be somewhat diffucult to arrange since that stuff doesn't have too many "fins" and overall the shapes are pretty weird. I had some of that stuff around and actually thought about heatsink purpose for e.g. regulator chip but back then (and still) I had a good supply of small heatsinks scavenged from equipment found from trashpiles.


Quote from: spud on December 15, 2010, 12:45:13 PM
Also, what size would be appropriate for a LM1875 chip amp implementation running about 15 - 20w? Probably 24v or so.  I figure the small 1 inch one would suffice since it's rated at 2.15o C/W/3 or am I not understanding the rating they give? 

Just to take these numbers as an example.

Amps power output = 20 watts
Heatsink rating = 2.15 degrees C per watt

Assuming the amp has a pretty typical efficiency of 50%, then the heatsink has to dispose of 20 watts.

20 * 2.15 = 43 degrees C

This is the temperature rise at full power over the ambient temperature.  Normally we would assume 25 degrees C ambient, but on-stage under lights a much safer assumption is 40 degrees C.  To get the actual operating temperature at full power we add the rise to the ambient;

40 + 43 = 83 degrees C.

We haven't accounted for the thermal resistance of the chip to case, or case to heatsink (via any insulating washer), so we can guess that the actual chip temperature will be closer to 100 degrees C.

As a rule of thumb the absolute maximum chip temperature shouldn't be hotter than 100 degrees C, so this setup would be shaving it a bit too fine for comfort (or overall chip life), so really a heatsink with a much lower thermal resistance is required.

The quoted thermal resistance of 2.15 degrees per watt also assumes optimum mounting of the heatsink, fins vertical, in genuine free air, and not exposed to any external heat source (such as stage lights).

Heatsinks are made the shapes they are for good reasons, thick where the devices(s) mount so they conduct heat well out to the thinner fins.

A couple of "rules" of heatsinking are that you always need more heatsink than you think; and quoted thermal resistances tend to be optimal, or even optimistic.  On the plus side, even a small amount air movement from an under-run fan makes a large difference to heatsink performance; however even a strong blast of air won't turn a seriously undersized heatsink into a wonder heat dissipator.

First design for sufficient passive cooling, then add a thermally controlled fan to cover those nasty extreme situations that crop up.

Here's my indicating proportional "thermofan" design that has been used in several amps including a re-creation of an Acoustic 360 by J.C.Maillet;

If you say theory and practice don't agree you haven't applied enough theory.


Quote from: Roly on May 02, 2012, 11:27:30 AM
Here's my indicating proportional "thermofan" design that has been used in several amps including a re-creation of an Acoustic 360 by J.C.Maillet;


VERY cool!  If you don't mind, I'd love to have you start a thread here about this and include schematics, diagrams, pictures, whatever you can. 
Life is what you make it.
Still rockin' the Dean Markley K-20X

bry melvin

On using computer heat sinks: I've been adding these (P4 or socket7 heatsinks  to Fender15G/R amps that I fix (TDA2050) on a regular basis.  I've been using a pair of these on 4ohm cabs in my studio. I've been buying returned  Fender squier/amp/guitar combos( repair and resell) for a while most of these have a burned out TDA 2050. For larger amps I've found the massive P3 passive (no fan) heatsinks Compaq used to use in P3 Servers and business desktops work quite well.

Also STAY AWAY from some of those Silver Computer heat sink goops some of them are CONDUCTIVE electrically.



What do you think of a design like this one? An aluminium chassis (2mm thick Al sheet) with sides bent to 6 cm
The power board with it's transistors attached to the chassis, and the back side of the chassis with riveted fins
made of an U shapes structural Al bar.

This design may be useful? Or are the fins useless because of the poor thermal contact with the rest of the chassis?
I've seen this kind of heatsinks used in 7.1 surround sound equipment, (aluminium plate with riveted fins, but the PA
for this equipment is a class H chipamp that should run very cool all the time, or most of the time.

J M Fahey

IF the chassis is 2mm thick, power transistors are near the back panel and U shaped channel aluminum is not only riveted, but you add thermal grease between them and the back panel .... yes, they will work very well.


Does an opamp  like the lm386 need a heatsink? While testing a breadboard build i pressed down on the chip and suddenly went ouch! as it had got pretty hot. (It may have been connected wrong ny the wau)t least I willl remember the name of the chip as its branded on my finger.

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