Class A amp running cool, not hot?

[pyonc]

Hi friends,


Recently I purchased a famed Class A amp, namely Krell KSA 50 to run bass of 4343 in bi-amp mode. The other day I brought Krell KSA 50 back home after having had it checked and fixed by an experienced tech nearby. He said he diagnosed it and repaired a couple of parts including a small 'lytic cap'. He said he "adjusted bias, along with load and audio test". Currently it's running bass in a bi-amp mode for my JBL 4343 system, producing a much more dynamic and deeper rich bass and larger sound stage than before. Now this is my question: as I understand, this Class A amp is supposed to run hot and that VERY HOT. But after several hours of running, it still is running cool, not even lukewarm around the unit cover when I touch it. I checked the fan-cooler inside, and it's running all right while it's on. Is this running cool normal? If it's so, is it because of bias adjusting rather low or high done by the tech? I appreciate any comment or feedback from you who had a similar experience in advance.

[grumpy]

http://www.diyaudio.com/forums/solid...s-setting.html

[10 Watt Street]

The fan is exhausting the heat, therefore it does not build up in the chassis. Class A operation consumes a minimum of 4 times the output. Your KSA-50 should consume 400+ Watts all the time.

The Krell KSA-200, a full time Class A amp of 200 Watts per channel, consumes 1750 Watts at idle or full power. It, however, is cooled by heatsinks not fans. It runs incredibly hot and heats the room significantly.

[pyonc]

http://www.diyaudio.com/forums/solid...s-setting.html

Thanks much for your useful link on this.
Though I'm not good at technical details, it still helps me get to understand this 'bias' thing a bit.
When I called the technician who diagnosed this amp, he mentioned something like 'between 2mA and 5mA', adding he set the bias up to the factory specs. I still don't get it, though. Anyway let me focus on listening to music from now on.

[pyonc]

The fan is exhausting the heat, therefore it does not build up in the chassis. Class A operation consumes a minimum of 4 times the output. Your KSA-50 should consume 400+ Watts all the time.

The Krell KSA-200, a full time Class A amp of 200 Watts per channel, consumes 1750 Watts at idle or full power. It, however, is cooled by heatsinks not fans. It runs incredibly hot and heats the room significantly.

Thanks a lot for your kind reply. Yea, I now know it's the 'fan' that cools down the heat.
After several hours of running this amp, though, I feel rather warm the underside of the unit where the fan is running, but the outer chassis doesn't feel warm to the touch at all.

[10 Watt Street]

Then it is working correctly. Mine ran cool on the outside.

You can easily check overall power consumption with a Kill-A-Watt, available for under $20.
http://www.p3international.com/products/p4400.html

[DavidF]

Thanks a lot for your kind reply. Yea, I now know it's the 'fan' that cools down the heat.
After several hours of running this amp, though, I feel rather warm the underside of the unit where the fan is running, but the outer chassis doesn't feel warm to the touch at all.

I had an old B&K amp that I ran with high bias. The heat sinks became just about too hot to the touch and the chassis around the sinks would get fairly warm. In warmer room ambient temps I would direct a small outboard fan at the heat sinks. It was remarkable how much the sinks cooled with just a little air flow to carry away the heat. Didn't make the room any cooler, for sure.

[Ducatista47]

My experience with Class A power amps is a little different, but could be taken as a cautionary tale. I have three First Watt amps, designed and built by Nelson Pass. I am fairly certain most Pass Labs amps, indeed most heat sink cooled amps, are the same.

The amps are optimized to run at the temperature reached with no active cooling. Installing a fan or mechanically circulating air through the heat sinks will degrade the operating specifications and the performance. Unmolested, the end result temperature is fairly stable. While that seems counter intuitive, the Fluctuation-Dissipation theorem demonstrates why. No surprise, Harry Nyquist once again. If concern manifests that the resulting temperatures will shorten component life, that may be true for poorly designed examples. In proper designs, the components that need to be hot are, and the rest should be much cooler. How many of us have seen failures from capacitors PCB mounted right next to hot resistors and transistors? One could certainly force cool and have one less anxiety to get between the recording and the listening experience. Once again, trade-offs.

For the engineers and math geeks reading this:

https://en.wikipedia.org/wiki/Fluctu...pation_theorem

[pyonc]

Thank you very much, guys.
All your feedback and comments help me enhance my knowledge of Class A amp.
As this is the first time I try it, I've become curious about it and now I think I get the answer.

[honkytonkwillie]

The fan is exhausting the heat, therefore it does not build up in the chassis. Class A operation consumes a minimum of 4 times the output. Your KSA-50 should consume 400+ Watts all the time.

You can easily check overall power consumption with a Kill-A-Watt, available for under $20.

I did this very measurement some time ago on several amps. My Class A Decware rated at 6W/channel sucks an even 90W from the wall, for an input/output ratio of 7.5x in my case.

[honkytonkwillie]

My experience with Class A power amps is a little different, but could be taken as a cautionary tale. I have three First Watt amps, designed and built by Nelson Pass. I am fairly certain most Pass Labs amps, indeed most heat sink cooled amps, are the same.

The amps are optimized to run at the temperature reached with no active cooling. Installing a fan or mechanically circulating air through the heat sinks will degrade the operating specifications and the performance.

My take on heat sink temperature is a little different. I don't think heat sinks are sized to ensure components operate at a minimum temperature; rather, they're sized to keep components from exceeding maximum safe operating temperatures.

Nelson Pass has a lot of write-ups on his First Watt site, and he tends to size heat sinks for around a 25-degree Celsius rise-above-ambient, giving a heat sink temp. of 50 degrees in a 25-degree room. This is handy for troubleshooting and guestimating operating conditions, because 50 degrees is about where things get painful for humans to touch for more than a few seconds.

He could just as easily design for a 10 degree or 45 degree rise of heat sink temperature - one has a whole lot more heat sink and the other gets so hot you fear the house will catch fire. Either way, the components are dissipating the same amount of heat.

[pyonc]

My take on heat sink temperature is a little different. I don't think heat sinks are sized to ensure components operate at a minimum temperature; rather, they're sized to keep components from exceeding maximum safe operating temperatures.

Nelson Pass has a lot of write-ups on his First Watt site, and he tends to size heat sinks for around a 25-degree Celsius rise-above-ambient, giving a heat sink temp. of 50 degrees in a 25-degree room. This is handy for troubleshooting and guestimating operating conditions, because 50 degrees is about where things get painful for humans to touch for more than a few seconds.

He could just as easily design for a 10 degree or 45 degree rise of heat sink temperature - one has a whole lot more heat sink and the other gets so hot you fear the house will catch fire. Either way, the components are dissipating the same amount of heat.

Thanks for your kind feedback and explanation!