continuous power handling

[woodyc]

Say you have an amp designed for music with some rated clean output power.

You supply a sine at some midband frequency.

What is the max power (ballpark) you should be able to use where the amp would reach some reasonable equilibrium temperature after a while and run without incident thereafter for as long as you want.

25% of the rated power? 10%? Less?

[Jazz P Bass]

Man, I first read this post at lunch and I thought, "whoa, where is all the data?"
So, I am back to that.

Which amp are you refering to or is this a general question.
It sounds to me like an engineering challenge.

Which amp output type are you looking at?

Tube amps rely on convection.
So that begs the question of 'what is the room temperature & output wattage of the amp.'

If solid state, that depends on the output type.
The new class D hybrid amps pretty much do not even need a chassis fan.

[eschertron]

Which amp are you refering to or is this a general question.
It sounds to me like an engineering challenge.

As far as engineering challenge goes, I'd think an amp rated at "X" music program power would run forever at that power level. Now if the question is what percent of peak, or of RMS power is the "rated clean output", that's more reliant on the amp in question, as noted.

The first thing I'm thinking - if actually your intent is to run this test - is that you'd be better off using white noise instead of a sine wave, as I can't imagine you'd get anywhere close to rated power without serious distortion at a single frequency, which would render the power calculation harder to figure.

[e_c_mccollum]

Very good engineering question. I do remember Crown has this data on their old micro/macro amps. They do list the btu's according to the wattage output. I don't see that anymore. The amp has to be biased right. Too hot, and won't live long. Too cold, sound ugly.

Crown MA3600 thermal and power data: http://www.crownaudio.com/media/pdf/amps/138350.pdf

Let's use a qsc2450 for example. Was testing one and saw runs slightly warm. Qsc spec at that time was 90mv, no load or signal applied. Okay, but amp still ran little warm. When tested at 8ohms on a dummy load using 1khz, got pretty hot. So, started experimenting. Eventually, settled on 74mv. Amp runs cooler, sustains power longer, and sounds cleaner. Come to find out qsc tech on their forum is advising 2450's to be biased to a lower voltage.

I personally like older amplifiers. I think engineers did a better job at designing them. U have to take in account how much heat a certain device will make and be able to remove that heat to keep the device in it's "safe" zone.

[woodyc]

Man, I first read this post at lunch and I thought, "whoa, where is all the data?"

I thought you guys had the data

But here's a concrete example -- an Eden WT800...

The output transistors are 2SC3856/2SA1492. Its a 550W per side amp and there are 6 transistors.

I tried running one side with 440Hz continuous at quarter power (137W.) It seemed reasonable because at that power, each transistor only has to handle 23W, and according to the derating curve the temperature is permitted to go as high as 125C. In 3 minutes it was over 90C and still climbing fast. I shut it off because it was clearly going to exceed 125C and I was afraid it might fail before the mute kicked in.

So next I decided to find a power level where the heatsink gets to a reasonable temperature and stays there. The first question is, what is a reasonable temperature? One way to do this might be to take the max permissible temperature at the rated output, which in this case is 92W/bjt, where the derating curve suggests 65C as a limit.

In a 20.2C room, the power level turned out to be 21W per side or 42W total. That put both heatsinks at a fairly constant 65C over about an hour.

So by this measure (and if you have a better one I'm all ears) the the cooling system is good at removing 21W per side or 42W continuous output. In other words if you want all the volume this amp can put out, you can't exceed a duty cycle of 3.8%. Which I think is like being permitted to play only one staccato 16th note per measure?

[Jazz P Bass]

How about this.
Run that 137 watts into a hefty dummy load.
How hot does the load get compared to the amp heatsink?

[woodyc]

How about this.
Run that 137 watts into a hefty dummy load.
How hot does the load get compared to the amp heatsink?

The temperature rise in anything soaking up power is just a function of its mass and heat capacity. Could be undetectable. Could burn your finger. Its not related to whats going on in the amp.

I don't see the point of the question. Maybe you could elaborate?

[nickb]

I thought you guys had the data

But here's a concrete example -- an Eden WT800...

The output transistors are 2SC3856/2SA1492. Its a 550W per side amp and there are 6 transistors.

I tried running one side with 440Hz continuous at quarter power (137W.) It seemed reasonable because at that power, each transistor only has to handle 23W, and according to the derating curve the temperature is permitted to go as high as 125C. In 3 minutes it was over 90C and still climbing fast. I shut it off because it was clearly going to exceed 125C and I was afraid it might fail before the mute kicked in.

So next I decided to find a power level where the heatsink gets to a reasonable temperature and stays there. The first question is, what is a reasonable temperature? One way to do this might be to take the max permissible temperature at the rated output, which in this case is 92W/bjt, where the derating curve suggests 65C as a limit.

In a 20.2C room, the power level turned out to be 21W per side or 42W total. That put both heatsinks at a fairly constant 65C over about an hour.

So by this measure (and if you have a better one I'm all ears) the the cooling system is good at removing 21W per side or 42W continuous output. In other words if you want all the volume this amp can put out, you can't exceed a duty cycle of 3.8%. Which I think is like being permitted to play only one staccato 16th note per measure?

It's quite common for the fan to kick in when the heatsink gets hot in these bigger amps. Is it running on this amp?

What power you can run at depends on the design. A conservative design ( usually specified as continuous power out) should run, well, continually. Sometimes they specify "music power", in that case all bets are off as it depends on what they define as 'music', this would be a less conservative design.

How hot it get depends in equilibrium depends not on thermal mass but on thermal resistance. Just like ohms law Temperature = Power/Thermal Resistance. A big heat sink with a fan blowing over it will have low thermal resistance. A small heatsink will have a high resistance.

[e_c_mccollum]

What power you can run at depends on the design. A conservative design ( usually specified as continuous power out) should run, well, continually. Sometimes they specify "music power", in that case all bets are off as it depends on what they define as 'music', this would be a less conservative design.

Exactly

Crown tried to solve this question by incorporating an ODEP circuit. An analog computer that would sense the operating conditions and adjust the signal to keep the amp stable. Of course wasn't perfect and they could fail to.

[woodyc]

It's quite common for the fan to kick in when the heatsink gets hot in these bigger amps. Is it running on this amp?

What power you can run at depends on the design. A conservative design ( usually specified as continuous power out) should run, well, continually. ...

Yes the fan was on. There is a 50C switch on each heatsink, and either one closing turns the fan on.

About running continually... I googled up an FTC rule for consumer gear which is kind of interesting. They require an amp to handle full power for at least 5 min after a 1/8 power warmup of at least an hour, in a room of at least 25C still air.

Its not a particularly rigorous test, but I bet a lot of pro amps would fail it.

FTC "Amplifier Rule" in its entirety

[fredcapo]

This is an interesting topic... Is there some basic spec or requirement that some government agency like the FTC (govt is all over our lives so why not here) that requires some level of performance when an amp is rated or advertised as being X watts? Or is a wattage rating just considered a marketing tool?

[Mick Bailey]

I thought you guys had the data

But here's a concrete example -- an Eden WT800...

The output transistors are 2SC3856/2SA1492. Its a 550W per side amp and there are 6 transistors.

Did you run the amp with the cover off? I've found they will fry in minutes at full power with the cover off.

[woodyc]

Did you run the amp with the cover off? I've found they will fry in minutes at full power with the cover off.

No, all I did was take out one screw and inserted a thermocouple in the screw hole. Also I would not run that amp at full power for more than a few ms.

Maybe somebody else could check this, and maybe I didn't state this clearly enough before, but the facts seem to be:

1)In order to get full power from the transistors the manufacturer says you need to keep them under 65C. The derating curve is here: http://www.sanken-ele.co.jp/en/prod/...56e.pdf

2)The heatsinks got to 65C at 42W average output power in a 20C room.

By the way, there is nothing special about the Eden. It just happened to be on my bench when Mr. Jazz Bass was complaining about the lack of specificity in my original question.

EDIT: I tried it again this morning at 10% of the rated power and took a pic this time, if it helps...

[Jazz P Bass]

Uh, I was not complaining.
It was such a generalized question that it took me back a bit.

[Mark Black]

Say you have an amp designed for music with some rated clean output power.

You supply a sine at some midband frequency.

What is the max power (ballpark) you should be able to use where the amp would reach some reasonable equilibrium temperature after a while and run without incident thereafter for as long as you want.

25% of the rated power? 10%? Less?

After reading through the posts I think you have hit on the answer: Each amp design is different and would require the kind of experimentation you have documented to arrive at the equilibrium point for sine wave signal testing. Add in component tolerances, ambient temp, fan condition (if present), dust/dirt, and there you go - even between duplicate designs there could be differences.

Luckily most amp designs don't have to spend much time amplifying pure sine waves in actual use.