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For people with experience with Class-A amps

Hi All,

I had a question for all you audio folks who have experience with Class-A power amps as I am new to this myself.

This is bugging me so I finally decided to ask - I have a class-A power amp (name will be with-held to protect the innocent) where I notice with no shade of doubt that the right heat-sink is warmer to the touch than the left heat-sink after about 2 hours of play/program material.

I got hold of a thermometer & measured the heat sink temperature (stuck the thermometer probe deep into the heat-sink fins & waited for the temperature read-out to increase/decrease by 0.1 degree Celcius).
What I found was that the right heat-sink was 1.5 - 2 degrees Celcius HIGHER than the left heat-sink. I have about 4 readings so far. Altho' not a lot of data points, each measurement has the identical trend - right heat-sink was 1.5 - 2 degrees Celcius HIGHER than the left heat-sink.

This indicates to me that the right channel current bias is more than the left channel current bias. Am I correct in my thinking?
2ndly, should I be worried about this?
(in that, over time, this temperature differential could diverge & the amp could run away thermally?).
Should I get the dealer to adjust the bias so that both channels are more or less equal?

I realize that "more or less equal" is a fuzzy term. To that effect, in your respective experiences, what should the temperature differential be between the 2 channels? 0.5 degree Celcius? Less? More OK? if more, upto what point OK?

Thanks very much in advance for your help.
128x128bombaywalla

Showing 1 response by gs5556

gs5556
1,218 posts
 
It could simply be due to a different thermal resistance between the sinks and the transistor casings. If the thickness of the mica insulators vary slightly, it could throw off the final temperatures due to a different rate of heat transfer.

Could it be bias? Sure. I'd think it's possible that those resistors have either drifted slightly or it's normal within their rated tolerances.

BTW - I measured the temperatures of my Class A amps when I suspected there was a problem and the tech told me that since I measured a greater than 20 farenheit difference between sinks then it was to him a symptom. Based on that I would assume that a 2 celsius difference would not be outside normal.
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