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Old 16th September 2011, 01:37 AM   #1
diyAudio Member
Join Date: Nov 2004
Location: Florida
Exclamation Concerned about amp temperature at half load.

I just repaired the power in on an old Alpine 3525 and now it works great.

After testing bridged with 8ohm load the amp gets really hot at high volume, almost too hot for hand. I can't keep my finger on any of the outputs for more than a few seconds before I nearly burn my finger. If it gets that hot at 8 ohm, I'm scared to try 4 ohm for any length of time.

Is it ok to let them run so hot?

I'm considering switching the A1265/C3182 outputs with mjw21195/96 to beef it up, but only if necessary.
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Old 16th September 2011, 01:46 AM   #2
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Join Date: Nov 2003
Location: Louisiana
Is the idle/bias current the same when hot and when cold?
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Old 16th September 2011, 03:57 AM   #3
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Join Date: May 2010
I see that your title says "half power" and the body says "high volume".

What are you using for a source signal, and by 8 ohms do you mean a dummy load (power resistor)? And by half power, do you mean measured output (ie 50 watts output on a 100W amp, or 1/2 input voltage with a known sensitivity for full power)?

If it's a sine wave input (or some other continuous source) and half power continuous output, that's a very demanding load you're placing on it; you won't normally be driving the amp at that load with music. It's a more difficult load than music that runs to full power, without clipping on transients, into a 4 ohm loudspeaker.
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Old 16th September 2011, 11:35 AM   #4
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Join Date: Nov 2004
Location: Florida
Half load, meaning 8 ohm speaker instead of rated 4 ohm. I use an old 10 inch 8 ohm home woofer as an expendable test speaker, with normal music to test. Amp driven at high volumes gets hot, but stays cool at idle or low volume.

Maybe this amp was made to run hot. I'll try it with 4 ohm speaker and see how much hotter it gets.
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Old 16th September 2011, 06:57 PM   #5
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Join Date: Dec 2005
Location: Northern California
It will likely get hotter with a 4 ohm load due to higher current demand across the outputs. I remember these amps well and yes you can upgrade the outputs. I did long ago but I used 2SA1302 and 2SC3281 devices at that time and it was for a very special client with $20K sound system invested in his $18K car. It won lots of trophy's but I do not recommend changing things inside unless your willing to debug all the little issues with doing such a thing

These old Alpine amp ran HOT, They were high bias'ed to begin with from the factory, and yes I have seen them too hot to touch in some cases. The above car had 1000 watts total Alpine power back in 1992. They have at least three temp sensors inside them to protect the amp so they will roll back as they heat up, and they can cut off if they are driven past certain temperatures.

If the amp is being driven too hard it will over heat and shut down. This tells me that either its bias'ed too high or you just need a bigger amp. The Bias setup can be tested with a voltmeter and checked to match factory recommendations but other then that if the amps is too hot you likely need a bigger amp or a class D amp which do not get hot...
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