Hi, No it doesn't, significantly, for decent quality music programme, rgds, sreten.
Are you sure you really want to argue this point sreten? The energy dissipated as sound is no longer dissipated in the devices.
With zero signal applied at the input of the class A power amplifier, ac power developed across the load reduces to zero and therefore all the power fed to the transistor is wasted in the form of heat. Thus, a transistor dissipates maximum power under zero-signal condition. Thus the device is cooler when delivering power to a load than with zero-signal condition.
Now, you can argue it's significance if you like, but it certainly doesn't run hotter.
This is absolutely basic.
The key work is 'significantly', consider program audio with say a mere 10dB peak/average ratio (very low for anything exept highly limited pop), turned up so that the peaks are just below clipping, then the average power delivered to the load is 10dB down on the input power to the class A stage....
10dB down is 10% power, so the power stage is still dissipating 90% of the input power under these conditions.
Even if you go for a mere 6dB peak/average ratio, you still get 75% of the no signal dissipation, and that is a completely unrealistic condition.
By the time you have average level 20dB below clipping, you have 99% of the no signal power dissipated in the power device, and I suspect that is close to the usual case.
It does not appear that there is a significant reduction in heat from a class A stage when audio is applied under realistic conditions for music reproduction.
Regards, Dan.
10dB down is 10% power, so the power stage is still dissipating 90% of the input power under these conditions.
Even if you go for a mere 6dB peak/average ratio, you still get 75% of the no signal dissipation, and that is a completely unrealistic condition.
By the time you have average level 20dB below clipping, you have 99% of the no signal power dissipated in the power device, and I suspect that is close to the usual case.
It does not appear that there is a significant reduction in heat from a class A stage when audio is applied under realistic conditions for music reproduction.
Regards, Dan.
There's an absolutely serious point to this.
The way to practically evaluate the heatsinking on a class A amplifier is to leave it switched on with no signal source, as this is the condition in which it will get hottest, regardless of whether the difference is large or small, and this is also a likely condition.
Unlike a class A/B (depending on the exact bias conditions) or class B amplifier, a class A amplifier does not cool down when the operator forgets about it and walks away, leaving it switched on. It gets hotter, until the unit achieves equilibrium with the ambient. It's not a question of 'withstand operation at full volume for 3 hours'. It's 'withstand operation at zero volume in perpetuity.'
Amplifiers are frequently switched on and left idle. Bias may not be thermally stable. Obviously we try to ensure that thermal runaway is obviated, but a good heat-soak in worst-case conditions is a sensible precaution, because it's in the extreme conditions that a problem is likely to reveal itself.
Plenty of pieces of kit I've tested went in an oven.
The way to practically evaluate the heatsinking on a class A amplifier is to leave it switched on with no signal source, as this is the condition in which it will get hottest, regardless of whether the difference is large or small, and this is also a likely condition.
Unlike a class A/B (depending on the exact bias conditions) or class B amplifier, a class A amplifier does not cool down when the operator forgets about it and walks away, leaving it switched on. It gets hotter, until the unit achieves equilibrium with the ambient. It's not a question of 'withstand operation at full volume for 3 hours'. It's 'withstand operation at zero volume in perpetuity.'
Amplifiers are frequently switched on and left idle. Bias may not be thermally stable. Obviously we try to ensure that thermal runaway is obviated, but a good heat-soak in worst-case conditions is a sensible precaution, because it's in the extreme conditions that a problem is likely to reveal itself.
Plenty of pieces of kit I've tested went in an oven.
I figure everyone knows worst case class A is zero output, but that putting audio through it does not reduce the dissipation in a meaningful way is possibly somewhat less obvious.
I have had a class AB biased RF deck get into trouble when switched on at very low ambient temperature as the FET die heated faster then the heatsink and the bias compensation thermistor failed to track fast enough, it tripped the overcurrent hard fault shutdown when Id reached 5A @ 300V up from a designed 0.1A standing bias....
The fix was to reduce the drain voltage to reduce the slope enough that the short term thermal behaviour became reasonably stable at a sane dissipation.
You always test at BOTH extremes of the designed temperature range if you have any sense, environmental chambers exist for good reason.
Regards, Dan.
I have had a class AB biased RF deck get into trouble when switched on at very low ambient temperature as the FET die heated faster then the heatsink and the bias compensation thermistor failed to track fast enough, it tripped the overcurrent hard fault shutdown when Id reached 5A @ 300V up from a designed 0.1A standing bias....
The fix was to reduce the drain voltage to reduce the slope enough that the short term thermal behaviour became reasonably stable at a sane dissipation.
You always test at BOTH extremes of the designed temperature range if you have any sense, environmental chambers exist for good reason.
Regards, Dan.
Hi,
No it doesn't in reality in any meaningful way.
The dynamics of music and the fact most people don't listen at clipping
levels, usually -6dB to -10dB below that should be your normal level if
your amplifier is wisely chosen to match your system, means the drop
of dissipation is very low, and is hardly what one would call "cooler".
There is more misinformation in your statement than the real fact.
Which is course is true. If you have a class A amplifier you will never
notice. It will never appear to run cooler unless you put full power
continuous sine waves through it, which are no fun to listen to.
(Or ply it with one of those hideous no dynamic range CD's at
full power, but then what would be the point of a class A amp ?)
rgds, sreten.
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