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,

>> 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.
Gs5556,

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