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Old 23rd September 2002, 04:03 AM   #1
mbroker is offline mbroker  United States
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Question Thermal design for Class AB amplifiers

Hello, all. I've found a lot of information on heatsinking class A amplifiers but haven't found much on class AB. Being unable to search for "class ab" on this site doesn't help much....

A typical class AB amplifier rated 250W into 4 ohms at clipping may dissipate 150W as heat at 250W output. This level of output is very unlikely to occur for more than a few seconds at a time, so why design for this level? What's a (the?) generally accepted percentage to design a thermal solution or a class AB amp? (that sounds like a buzzterm from Wakefield or Avvid Thermalloy ) Trying to squeeze ample heat rejection into a 4U chassis for 6 channels of 250W output rather reminds me of Cinderellas' sisters' experiences with a glass slipper.... (Basically, take
this amp and add another channel.... ) I'm thinking 60% of full power (90W in my case) would be a good number?

My friend, who's working on this with me, thinks we should be able to run the amp at full power on all channels in the sun on a hot day with no fans or meltdown .... I must have laughed for 5 minutes straight then told him he can use HIS amp for that "test".

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Mark Broker
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Old 23rd September 2002, 05:19 AM   #2
jleaman is offline jleaman  Belgium
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i want to see the melt down yuo shuold getyur friend to let you drive your car at high speeds in the rain and seeif he can stop befor he hits the MOOSE. LOL humm i think id put small fans under the heat sinks to cool them just incase it is better to have the ability to build the case to house fans than build the case blow up yur amp that you worked verry hard on and have it melt down. i was to thinking about building a 5 channel mabey 6 channel illbe using e-holtons hitachi lateral amps there small and 200 watts is perfect for me and cheap to make probably 50bucks canadian per channel ..
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Old 23rd September 2002, 06:55 AM   #3
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http://www.onsemi.com/pub/Collateral/AN485-D.PDF
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Old 24th September 2002, 03:03 AM   #4
mbroker is offline mbroker  United States
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Quote:
Originally posted by JasonL
yuo shuold getyur friend to let you drive your car at high speeds in the rain and seeif he can stop befor he hits the MOOSE. LOL humm i think id put small fans under the heat sinks to cool them just incase it is better to have the ability to build the case to house fans than build the case blow up yur amp that you worked verry hard on and have it melt down.
ROFTL. Perhaps you mean "get your friend to let you drive HIS car...."? ROFTL. I'll have to use that analogy next time he suggests it... We need an ROTFL smiley!

Fans are a last resort, more due to cost than anything else. Though I would prefer to use a thin fan mounted on the heatsink CPU-style than one placed underneath. This will help maintain airflow, too, since the fan isn't [partially] blocking the vents.

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Mark Broker
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