Many amps can produce X watts, but only for a few ms or seconds. The continuous power output for most amps is a lot lower than the max RMS power output.
So what limits continuous power ratings? One obvious thing is heat. Let's ignore that for now. The other obvious thing is the power supply. If a much beefier than needed power supply is used, and temperature is not an issue, would the amplifier be able to put out their max RMS power output continuously, or would something else limit it?
So what limits continuous power ratings? One obvious thing is heat. Let's ignore that for now. The other obvious thing is the power supply. If a much beefier than needed power supply is used, and temperature is not an issue, would the amplifier be able to put out their max RMS power output continuously, or would something else limit it?
In most power electronics, the primary limitation is thermal. Otherwise the circuit would be
very over designed and over cost. The power device junction temperature IS the issue.
In industrial systems, device voltage limitations are another serious issue.
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*Real* RMS Power is also called *Continuous* power for a reason: that´s the clean power you can count on rated load, under any signal you send it, including the toughest one: a continuous sinewave.
Its short term limitation is reaching power supply rails, beyond which signal can´t go, and onger term thermal: after a short time everything will heat up, including heat sinks, and it will of course let junctions heat even more.
We are all aware that typically power transistor dissipation is specified at very unrealistic 25 deg C *case* temperature, which would imply it being mounted to a 1 Ton cube of aluminum, kept at a *lower* than 25C temperature, to compensate for thermal resistance between transistor case and said ideal heat sink ... clearly an impossibility.
As long as the real heatsink (which weighs way less than 1 Ton 😀 ) heats up, so will transistor junction ... which are rated 150C .
So in a few minutes we reach dangerous temperatures and amplifier must be stopped or derated to avoid destruction, so in a way temperature IS a limitation, even if signal remains clean and pure.
That said, and considering this thermal time constant, many amplifiers can supply way higherpower for a short time.
Is it realistic?
Maybe in a Home Hi Fi situation, where Music is often played at well below Maximum anyway, to keep a nice dynmic range.
Now in a Pro situation, to me such short term ratings are meaningless and even deceitful, you can count on System being driven to the max. , for long hours.
I know old Peavey CS800, BGW750, QSC, Crown, etc. heavy power transformer Class AB amplifiers can deliver full rated power for ages; I was very disappointed finding modern "8kW" Class D SMPS fed amplifiers can only put out some 2kW sinewave continuous for any sensible period of time.
Nothing against, just label them "2kW" and it will be more useful.
Temperature, even when ignored, remains THE issue. All power supplies contain a transformer and that's a thermally limited item when its designed well.
Precisely. Some advanced power converters can modulate the control to maintain the power device
junction temperature within a well defined range, resulting in more reliable and efficient operation.
The question is, what do you achieve by using a much stronger power supply? The RMS power goes up, ofcourse. But the thing is, the amplifier runs a lot closer to its limit then. So what do you think prefer the manufacturers to produce?
a. Make amps with oversized power supplies, potentially damaging the amplifier at much higher production costs.
or
b. Let the power supply limit the amplifier output and save money at the same time?
With b. the point of failure shifts generally more towards the power supply but it's usually cheaper to rebuild/repair than the amplifier part. I don't know any manufacturer who opts for a. at class D amplifiers. Ofcourse everyone hates it if the amp delivers a lot less power than promised, I am no exception there, but let's face it: In praxis there is very rarely ever the need for a very high power RMS signal over an extended period of time - that's not what we are listening to (well, at least now, who knows what will change over time?). So that's okay-ish then with the low RMS power.
But, IMHO, that is NO reason or any excuse to lie to the buyer!
Power output vs. time can be plotted for any amplifier. The resulting line will have one or more inflection points, with certain slopes between points.
The inflection points and slopes come from thermal inertia and power handling of weakest components, and voltage/current availability. Voltage availability can become severely reduced with time due to power chain over-relying in bulk capacitance. Current availability can be actively reduced with time due to thermally-controlled limiting mechanisms.
In general, a long term amplifier power handling similar to the power handling of the motors of the speakers being driven can be considered adequate, considering "long-term" an interval long enough for thermal equilibrium of components, in the range of tens of minutes.
"Short term power" can be considered as power availability at the beginning of the power vs. time line, before the 1st inflection point. How much "short term" power? This depends on crest factor requirements. A 3:1 figure for short/long term power can be considered adequate for most applications.
What makes some amplifiers different from others? The amount of inflection points in the power vs. time plot and the slope of fall between them.
My preferred approach for this, as an amplifier designer, is to aim for several seconds of full power availability (which speakers can handle), then slowly falling to about 1/3 of that within several minutes. In other words, a s single inflection point after some seconds. This is possible when all components are sized mutually accordingly, and power supplies are actively regulated (for no rail voltage vs time/temp. fall), so that no component or stage is more bottleneck than another in power handling (this also results in optimum usage of natural resources, zero overkill).
The amount of inflection points depends on the number of stages used in power supply and amplification, and the mismatch between power vs. time handling of these stages, and the reliance on bulk capacitors fed from medium impedances and not employing any active voltage regulation.
For example: Classic big-toroid/big-caps approach would exhibit an inflection point just at the origin, with power availability continuously falling from that point to another inflection point, located at a few tenths of a second, where the caps reach steady state and the thermal handling of components starts to be the limiting factor, sometimes producing further inflection points at time in the order of minutes.
More complex power supplies may exhibit more inflection points.
The inflection point at the origin is only removed with active regulation like SMPS, but not all SMPS. It could be said that a SMPS can't be properly designed for audio if the designer does not have experience about thermal handling vs. time of speaker drivers and requirements of various trends of world party music.
What about ratings commonly given in amplifier ads and spec sheets? It's almost always the power available for less than a tenth of a second. Remember that the presence of an inflection point at the origin makes the big difference here, some designs can still sustain the power rated after 1 or 2 seconds, others only 50%~70% of that.
From the stuff I have designed I can cite as examples about how to properly design power handling:
- A subwoofer amplifier module capable of 6kw for 3 seconds, then slowly falling to 2kw for indefinite time (obviously intended to drive 2kw worth of voice coils).
- A multi-channel amplifier capable of 1800W for 3 seconds, then slowly falling to 300W without fan in transformer (as for hi-fi) or to 600W with variable fan (as for PA). Again, obviously intended to drive 2x 300W motors in PA and smaller ones in hi-fi.
The inflection points and slopes come from thermal inertia and power handling of weakest components, and voltage/current availability. Voltage availability can become severely reduced with time due to power chain over-relying in bulk capacitance. Current availability can be actively reduced with time due to thermally-controlled limiting mechanisms.
In general, a long term amplifier power handling similar to the power handling of the motors of the speakers being driven can be considered adequate, considering "long-term" an interval long enough for thermal equilibrium of components, in the range of tens of minutes.
"Short term power" can be considered as power availability at the beginning of the power vs. time line, before the 1st inflection point. How much "short term" power? This depends on crest factor requirements. A 3:1 figure for short/long term power can be considered adequate for most applications.
What makes some amplifiers different from others? The amount of inflection points in the power vs. time plot and the slope of fall between them.
My preferred approach for this, as an amplifier designer, is to aim for several seconds of full power availability (which speakers can handle), then slowly falling to about 1/3 of that within several minutes. In other words, a s single inflection point after some seconds. This is possible when all components are sized mutually accordingly, and power supplies are actively regulated (for no rail voltage vs time/temp. fall), so that no component or stage is more bottleneck than another in power handling (this also results in optimum usage of natural resources, zero overkill).
The amount of inflection points depends on the number of stages used in power supply and amplification, and the mismatch between power vs. time handling of these stages, and the reliance on bulk capacitors fed from medium impedances and not employing any active voltage regulation.
For example: Classic big-toroid/big-caps approach would exhibit an inflection point just at the origin, with power availability continuously falling from that point to another inflection point, located at a few tenths of a second, where the caps reach steady state and the thermal handling of components starts to be the limiting factor, sometimes producing further inflection points at time in the order of minutes.
More complex power supplies may exhibit more inflection points.
The inflection point at the origin is only removed with active regulation like SMPS, but not all SMPS. It could be said that a SMPS can't be properly designed for audio if the designer does not have experience about thermal handling vs. time of speaker drivers and requirements of various trends of world party music.
What about ratings commonly given in amplifier ads and spec sheets? It's almost always the power available for less than a tenth of a second. Remember that the presence of an inflection point at the origin makes the big difference here, some designs can still sustain the power rated after 1 or 2 seconds, others only 50%~70% of that.
From the stuff I have designed I can cite as examples about how to properly design power handling:
- A subwoofer amplifier module capable of 6kw for 3 seconds, then slowly falling to 2kw for indefinite time (obviously intended to drive 2kw worth of voice coils).
- A multi-channel amplifier capable of 1800W for 3 seconds, then slowly falling to 300W without fan in transformer (as for hi-fi) or to 600W with variable fan (as for PA). Again, obviously intended to drive 2x 300W motors in PA and smaller ones in hi-fi.
And this is specially dedicated to all those folks dreaming about bulky power supplies. Know where the stuff comes from and how it is extracted from Earth.
https://www.freedomunited.org/advocate/nevsun-in-eritrea/
Bisha High Grade Open Pit Copper-Zinc Mine | Nevsun Resources
Those "workers" don't even have a stereo, they dance to the sound of the assault riffles. Of course this promotes war, in the sense commonly used of the word (with weapons), because there is another sense of "war" without weapons, humans have a complex chain of biological mechanisms to make some of the others sick of the organs they misuse if the global brain reaches with more certainty than them the conclusion that they are proud of a bad habit.
I'm proud of getting solid 600W long term out of about 150 grams of copper.
https://www.freedomunited.org/advocate/nevsun-in-eritrea/
Bisha High Grade Open Pit Copper-Zinc Mine | Nevsun Resources
Those "workers" don't even have a stereo, they dance to the sound of the assault riffles. Of course this promotes war, in the sense commonly used of the word (with weapons), because there is another sense of "war" without weapons, humans have a complex chain of biological mechanisms to make some of the others sick of the organs they misuse if the global brain reaches with more certainty than them the conclusion that they are proud of a bad habit.
I'm proud of getting solid 600W long term out of about 150 grams of copper.
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