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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
onhwy61's avatar
onhwy61
5,150 posts
 
Express the difference in temperature in percentage terms.
larryi
3,760 posts
 
I find it interesting that the amp gets hot quicker playing loud. That suggests that it is not a pure Class A amp. With a pure Class A amp, delivering a lot of power to the load (the speakers) would decrease the amount of power the amp dissipates as heat. The temperature results should be just the opposite -- at idle, all of the power (Class A device is in full "on" condition all of the time)is being dissipated by the transistor as heat.
atmasphere's avatar
atmasphere
11,637 posts
 
In class A amplifier there is no AB operation at full power. If so it would be a class AB amplifier.

A 1 or 2 degree C temperature difference on the heatsink is negligible. The bias is likely set with a control in the unit , to get a certain amount of current through the output devices *not a certain temperature*! Normal variance in the devices are likely responsible for the very slight temperature difference. Nothing to be concerned about as long as the amp sounds OK.
mrderrick
292 posts
 
I'm guessing that you have a solid state amp. I would contact the manufacturer and request the instructions on how to set the DC offset and bias. More than likely the only tools required will be a meter and small tweaker screwdriver. I have a Classe CA400 that at idle was warmer on one side than the other. I do not recall the temp range but it was noticeable to the touch. The DC offset and bias was way off for both channels. After adjustments the amp sounded substantially better. I can only guess why the settings were off. Thermal expansion and contraction, vibration or maybe some fine tuning by ear?
aball
2,016 posts
 
Larryi's comments are correct. If your amp is true Class A all the way, it will be hottest at idle. I think your amp is in fact AB.

I have specialized in thermal design for some time now. A two degree difference is insignificant and may not even be related to the bias. For instance, do both heat sinks have the same support under them? If the airflow through the right side heat sinks is even slightly more impeded than the left, you will get a difference. It takes very little for such a small change.

Also, thermal resistances between the devices and the heat sink is mitigated by a thermal pad. The actual thermal resistance is determined by how tightly the devices are screwed down. It could be that the right side ones are slightly tighter than the left side (common in manufacturing) - but the bias may be the same for both.

What type of temperature probe are you using? Is it a thermocouple? If so, you HAVE to use thermal paste on the end of it. Otherwise its error can be +-3 degrees.... But then thermal paste will make a mess on your heatsinks very easily. If you are using a infra-red gun, their error is even greater and it isn't simply a bias error you can back out with relative measurements - it will depend on th reflection and emissivity of the spot you are shooting.

Arthur
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