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.
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???
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.

>> 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.

thanks for your response.
you bring up some good points. Maybe it's the different thermal resistances?
Express the difference in temperature in percentage terms.
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.
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?
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.

Larryi, Aball,

you guys have caught me on my symantics!! :-)

1stly, this IS a class-A amp. The manuf reassured me several times. I had similar doubts hence I did communicate w/ the dealer on this.

what I meant to convey when I wrote "At lower levels it takes a lot more time for the heat sinks to heat up" is that altho' I can feel the heat rising up thru the vents in the top-plate, the heat sinks are relatively cold. As the amp is played longer & longer, the heat sinks begin to warm up.
I agree with both of you that class-A amps are hotter at idle than when they are playing.
This bit of info was incorrectly communicated by me. Sorry!

I have a Fluke thermometer that comes with a 2-pin fairly long probe. The probe end is a bit of exposed metal. I will have to look @ it to give you the model #.
So, I just stick the probe between the heat sinks or thru the top-plate vents to take the temperature.

Both heatsinks have identical support & air-flow around them.

You should try to relax a little - I recommend Rockband - it is great fun through the main system (loud) - not audiophile sound quality of course but hit those drums and you will forget sound quality and just enjoy the music! They even have a song, which I suspect, may be just right for audiophiles such as you - Shirley Manson singing "I Think I'm Paranoid" ;-)
"If your amp is true Class A all the way, it will be
hottest at idle."
FYI - I have a Krell FPB300cx and it gets hotter the louder & longer I play it. It is aledged to be "Pure Class A" (key word here; aledged). I can play it at low volume for 10-hours and it stays cooler than being pushed. My point being, I question the above statement.
Good luck in your quest.

very funny!! ha, ha, ha!! :-)

OK, I get the point.
My inexperience w/ class-A amps "forced" me to ask the audio community at large.
Newbie to class-A cutting his teeth/getting his feet wet. Cut me a little bit of slack, chief!
That's the opposite of Class A. Class A amps are so good because they are always running at full power. The power has to go somewhere when not being used for sound, hence the heat. As you play louder, the heat dissipation reduces.
The Krell may sound better than many class A amps but it not. I'd say that Krells are very nice but not as "warm" sounding as Class A amps (pass, threshold, aragon...).
Were your measurements taken during playback or idle state?
I think I would agree with ralph's response from atmasphere
that there is nothing to worry about; if you notice some audible changes then I would investigate more.

they were taken soon after playback state (during the time I changed CDs or LPs).
Thanks very much everyone for responding to my question.
Krell, Coda and some Thresholds use a "sliding" bias, so they can run cooler at idle. It's still class A, but bias is automatically adjusted on demand.

How much class A is another question that has a lot to do with the load. For example, my amps run hot and are rated for 100W @ 8ohms, however, bridged into 4 ohms, 650W of class A is just unrealistic.

How much class A do I have? I don't care. It sounds great.
Elevick and John, many Krell amplifiers have a patented (although not patented by them) sliding class A system that cuts back on the idle current at low volume levels. As the signal increases, the bias current is increased with it.