Cooling an amp with poor cooling

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When the load increases and the heatsinks heat up, what do you do with an amp with a tiny heatsink? I have a Parts Express 120-watt subwoofer amplifier (hardly audiophile quality, but I like it quite well) driving a Blueprint 1001 10" long-excursion subwoofer, and it causes my amp to heat up.

My cooling solution is pretty cool (no pun intended) expecially when you consider that it's only improvised. I have taken two 8cm biscuit fans (like the kind you find in your computer's power supply) and arranged them over the big heatsink. However, I still noticed that the power transformer was still heating up more than I would have liked. The transformer was mounted underneath the panel with some thermal transfer material, so I took an old heatsink and fan that used to be on my computer's processor and squirted some thermal goo on the bottom, and stuck it to the panel. I have noticed that this keeps my amp much cooler than it used to be. One might think that running two 80mm fans like that would be noisy, but my little AC adapter that I'm using for the fans lets me switch the voltange to anything from 1.5v to 12v. Right now the fans are running on 4.5v (the motors stall at anything less) and my amp is quite cool. When I'm watching movies or listening to loud music, I turn the fans up to 6v or 7.5v. But when I turn the entire setup all the way up to 12v it sounds like a jet ready for takeoff.

I hope you enjoyed this suggestion for low-noise fan cooling: oversized fans running slow.
About touching hot copper and aluminium...

My experience tells me that 40 degrees Celsius one is able to touch for as long as one wants, and it just feels warm.

50 degrees Celsius starts to feel unpleasant, but one can keep his fingertips on it quite long, if he can take the minor pain.

60 degrees will begin to feel unbearable after a little time, maybe 5-10 seconds, and keeping fingers on as hot metal as this doesn't feel sensible.

70 degrees will give a burning-like feeling after a second or two, and most I know will promptly pull their fingers back.

Transformer's shieldings and such can usually take temperatures from 130 to 180 degrees C, but by default most are 130 degrees maximum. If you have a chance to try a transformer on it's rated power to a resistive load, you will get a good picture on how hot they can be. Just remember that the rated power is usually to a 25 degrees ambient.

An ordinary transformer winding running on it's limits will feel hotter than my 60 deg. example, but not as hot as the 70 deg. one. And you will have to keep your hand on it a bit longer because you don't directly touch the copper.

A well rated transformer warm to the touch in an audio amplifier is most propably getting it's heat from somewhere else. At least I've been able to warm my transformers only by prolonged and serious use.

You can try to estimate how much power it's made for by checking measurements at any transformer manufacturer's site or catalog. Or give me the dimensions and/or weight of your panel's transformer, and I will look up the nearest match from Trafox's catalog.

-Kimmo S.
If a transformer gets hot it´s too small for the job. Heatsinking from a distance doesn´t help at all. If the transistors get hot it doesn´t help them to have fans from a distance. you need big heatsinks or making the airflow through the heatsinks. It´s the junction temperatures that matter.
I know I'm late to the party and everything, but I've got two questions:
1) How hard are you playing this thing? Restate this as: Is it possible that you should consider a larger amp? 120W on a sub sounds a little light to me (yes, yes, I know, I'm on the lunatic fringe and most folks don't overdo things to the extent that I do, but...).
2) Are we certain that this amp is in good condition? No internal shorts? No bias problems? No lightning strikes back during the summer?

I also find it a little odd that its the transformer itself heating up...

Do you notice any differences in heat depending on where you touch the heatsink?

Also, what sort of chassis is the amp in? Is it built into your subwoofer cabinet? Does the transformer have its own housing on the back of the amp?
I don't know of any problems with the amplifier. It has never triggered the thermal overload protection. The amp is rated at 120 watts by the manufacturer, but Parts Express has measured it at 156 watts into 4 ohms. I play the sub with the volume control at the 1 o'clock position and the subwoofer level on my reciever at 10 (the level control goes from 0 to 15)
What Impedance is your speaker? Building our own we are rather picky about the designs we choose. While an A75 is designed to drive a .1 ohm load most cheapo amps work overtime driving 4 ohms. Not too many can drive 2 ohms without failure. Maybe your sub is too much for it. Then again if your underpowering your sub you could be clipping the amp, though that would eventually damage the speaker as well. Double check your matchup. I always try to stay within 25% either way on wattage just to be safe. If not be careful how hard you push it. Your fan may just be masking a problem, and problems generally don't go away. Good Luck
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