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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
elevick
2,636 posts
 
Could be many reasons. Have you opened up the amp? Maybe things are not symetrical inside?
Are your speakers drawing the same power? Check the impedance on them?
Is this difference when listening quietly or loud? Most class A amps will switch to B when pushed hard. They tend to only be class A for the first 10 watts or 10% (not all of them though).
Is your pre-amp the culprit? Is the balance off or is the sound even out of both sides? and on and on???
gs5556
1,235 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.
bombaywalla's avatar
bombaywalla OP
1,958 posts
 
Elevick,

>> Have you opened up the amp? Maybe things are not
>> symetrical inside?
I've opened the amp & done a very good inspection - things are very symmetrical inside.

>> Are your speakers drawing the same power? Check the
>> impedance on them.
Not sure 'cuz I did not check but I'll do so.

>> Is the balance off or is the sound even out of both
>> sides?
The sound is even out of both sides.

>> Is this difference when listening quietly or loud?
I've played it at lower & modest levels.
At lower levels it takes a lot more time for the heat sinks to heat up.
I've measured after 2+ hrs of program material at whatever level I was listening depending on time of day (lower at night time, louder during the day).

>> Is your pre-amp the culprit?
can you elaborate on this one? thanks!

Just FYI: with the prev class-AB power I did not have this issue. Both heat sinks were as hot after 2+ hrs of program material.
bombaywalla's avatar
bombaywalla OP
1,958 posts
 
Gs5556,

thanks for your response.
you bring up some good points. Maybe it's the different thermal resistances?
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