Heatsinks for your poweramp

[joecool85]

First - Have a good power supply circuit and a big enough/proper voltage transformer
Second - Big heatsink, there is no such thing as too large of a heatsink

Proper heatsink size for two LM1875 chips, or one LM3886:


Overkill(which isn't a bad thing though):

[skey]

Neither of the above heatsinks above seem to take advantage of thermal airflow. 
Sometimes smaller is better if it's less bulky.  Is there any convenient way to figure out a useful minimal size you'd need?

[RDV]

Neither of the above heatsinks above seem to take advantage of thermal airflow.
Sometimes smaller is better if it's less bulky. Is there any convenient way to figure out a useful minimal size you'd need?

The one in the 1st picture does not take advantage of thermal airflow(though it's big enough to not need it), but the 2nd one is outside the cabinet. How much airflow do you need? It's in free air.

Please don't say that a smaller heatsink is better because it's not. The smallest heatsink you can get away with is best for space saving purposes, but there is no heatsink that is "too big".

There is no convienient way to calculate heatsink size other than doing the math from the datasheet.

Regards

RDV

[joecool85]

The first one does, maybe poorly, but it does.  I drilled 16 holes in the bottom, then 9 holes in the top to allow cold air to come in the bottom and hot air to rise out the top.  It works, trust me I've never had it over heat.  That amp is my computer speaker amp and I've had it on 3/4 volume (really really loud btw, I had no neighbors at the time  ) for 2 hours or so and no problems.

[teemuk]

The one in the 1st picture does not take advantage of thermal airflow(though it's big enough to not need it), but the 2nd one is outside the cabinet. How much airflow do you need? It's in free air.

Free air means no "concentrated" airflow. Imagine this: Do you feel cooler on a sunny day when it breezes or when it doesn't? It's the same thing with heatsinking, thus fans are the ultimate solution. If you channel the airflow through the heatsink fins (hot air automatically tends to rise up when cool air replaces it) it will be a lot more efficient than when the heatsink is just in free air. For same reason vertical fins are better than horizontal. (Unless the air flows horizontally for some reson i.e. fan). In the first pic I see some holes at the bottom of the chassis, which means there will be a cooling airflow through the fins. IMO this is better than picture 2 solution. However, the heatsink in picture 2 is massive and therefore dissipates more heat. Same size sink in picture 1 configuration would dissipate heat even more efficiently. If you want a very efficient cooling, channel the airflow up through the fins and use a fan to push a huge amount of air for the heatsink. The cool air would replace the warm by itself but the fan just speeds up this process. If the cool air has paths to "leak" away from the fins you loose efficiency. Thus free air heatsink is always less efficient. However, temperature in the chassis may rise higher than temperature outside it. Best solution would be to isolate fins to a separate and well ventilated (airflow from down to up only) compartment in the chassis.

[joecool85]

We know what you are talking about, but there are several problems with fans in guitar amps. 

1.)  You need to get power to it (not a huge deal)
2.)  They make noise, and who wants fan noise in their amp
3.)  If not done properly you will lose your chip from overheating
4.)  Even if done properly, if the fan fails you will overheat your chip
5.)  Big heatsinks look cooler (this is the most important one)

And yes I know about using fans on CPUs and controlling air flow.  I have a dual 2ghz G5 powermac that (if it would let you) run with the case open it would overheat and toast it's CPUs due to improper (or non existant in certain areas of the case) air flow.

* Ok, so I was kidding about #5, but it is true.  Who wants a girly CPU fan in there when they can have a monster like I have on the back?

[joecool85]

Oh, and both of those amps up top are mine.  The reason for the inefficient huge monster is because it was all I could find on eBay for a sufficient heatsink at the time.  I haven't changed it because I like the comments I get when people see it 

[teemuk]

I agree that fans have their problems. I wouldn't stick a fan anywhere near the preamp sections and a separate supply should be used to isolate fan noises. My intention was not to imply that one should save in heatsink size by using a fan. Of course it can be done but I'd rather use both huge heatsink and a fan. IMO, this combination looks even cooler than a massive heatsink alone. But regarding of saving costs, I'm quite certain that a well-built forced cooling solution would cost as much as someone would save in selecting a smaller heatsink - so why bother. However, a configuration that switches the fan on (with hysteresis) when the heatsink temperature rises to a certain level plus a thermal relay to switch off the power when the heatsink temp is too high is the most failsafe solution, don't you agree.

[joecool85]

I think the most failsafe fan solution would be to have a heatsink that is just barely large enough to use passively, then put a fan on it.  This way, if the fan dies, you can still limp along till you fix the fan.  And yeah, a big sink with a fan on it would look cool.  I didn't bother on mine since it is so huge, plus I wanted a simple solution.  And there is nothing more simple for a heatsink than just grabbing the biggest one you can afford and bolting a chip to it lol.

[RDV]

With proper design, filtering, and load, these amps run fairly cool though really. A huge heatsink is really not neccessary but doesn't really hurt either. If you run two chips bridged or parallel, there almost isn't a big enough heatsink period, and fan cooling is almost a neccessity. This is more true as a guitar amp than in music reproduction however due to the dynamic range of the guitar.

RDV

[teemuk]

I wouldn't trust only on heatsinking a high power amplifier in gig situations. The ambient temperature on stage is huge. It's safe to assume the power dissipation might start from heatsink temp of 40 degrees of celcius not nominal 25.

[RDV]

I didn't bother on mine since it is so huge, plus I wanted a simple solution. And there is nothing more simple for a heatsink than just grabbing the biggest one you can afford and bolting a chip to it lol.

The only way you'll overheat that one is to actually outdo the heat transfer rate. It's possible with a 4 ohm load but unlikely with an 8 ohm load. The Spike protection would finally kick it at that kind of level.

RDV

[RDV]

I wouldn't trust only on heatsinking a high power amplifier in gig situations. The ambient temperature on stage is huge. It's safe to assume the power dissipation might start from heatsink temp of 40 degrees of celcius not nominal 25.

If I ever use a DIY chipamp for gig purposes it will be fan-cooled. I doubt it ever happens though. There's too many inexpensive commercial amps out there. I just use them as stereo equipment and practice amps.

RDV

[Stompin_Tom]

uh... I was looking at this guy's site (he sells kits and completed amps) and his heatsink is pretty small (about half way down the page):

http://www.audiosector.com/chassis_integrated.shtml

Is there something special about copper that you could use a chunk like that instead of the standard heatsinks like Joe used? AND what *possible* effect could the heatsink have on the audio performance of the amp!?!?!?

[teemuk]

Copper has a better heat transfer than aluminium. A cool amplifier would perform better than a hot one (at least the semiconductors would live longer if nothing else).

But anyway, in my opinion that heatsink doesn't look very efficient: It is indeed pretty small and even more, it has no fins! I bet that piece of copper has also been a lot more expensive than a "less sexy", but adequate aluminium heatsink. The most stupid part is, who's going to see it when it's inside the chassis anyway! And speaking of the chassis, where are all the ventilation holes! Got to admit that the chassis looks pretty sturdy though, I bet it acts as a heatsink itself. Doesn't look that impressive IMO. Also, I wonder what "the possible sonic improvements" and "better damping" are... So far no-one has been able to prove that heatsink material, aside effects of heat transfer of course, would do anything to operation of the amplifier.