CLASS A amp runs cool
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Champco
Super Member
Not unusual to run cool without signal and getting hot processing audio signal into noise.
Try dialing in the class A idle current on a Yamaha CA-1000 at 940mv and tell me it don't get hot at idle.
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Champco
Super Member
Never said it cant be made to run hot with no signal. Isn't that what biasing is for?
Try dialing in the class A idle current on a CA-1000 at 940mv and tell me it don't get hot at idle.
High bias is KEY to class A..
Champco
Super Member
High Bias is?????
Relatively speaking.. For instance.
Take a Yamaha CA-1000 Integrated Amplifier.
Class B bias is 47mv = 50 ma idle current --- 2.35mw dissipated as heat at idle
Class A is 940mv= 1000 ma idle current --- 940mw dissipated as heat at idle
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hornbostel
Well-Known Member
My Schiit Aegir’s run HOT HOT, no doubt.
My old School Yamaha M40 which I’ve continued to run for over 20 years in class A mode also runs hot as Schiit. Both amps sound utterly fantastic with all many speakers I’ve owned. True Class A has to run hot. There is no physics around this fact.
My old School Yamaha M40 which I’ve continued to run for over 20 years in class A mode also runs hot as Schiit. Both amps sound utterly fantastic with all many speakers I’ve owned. True Class A has to run hot. There is no physics around this fact.
Champco
Super Member
Bias on my HK12 set's between .8 &.13 MV. When that number was high, say 30+mv, the transformer, on that side gets hot but the outputs get very hot. As I tune it down into the recommended range you can feel the outputs cool.
Class A bias is set to switch the outputs on but just barely conducting. This is for linearity in reproducing the full signal as efficiently as possible for linearity.
My AMP has been playing for 4 hours at about 25% signal volume and the outputs are barely warm to the touch. The Transformers are both very warm but not close to hot. They all heat up as I turn up the volume.
Class A bias is set to switch the outputs on but just barely conducting. This is for linearity in reproducing the full signal as efficiently as possible for linearity.
My AMP has been playing for 4 hours at about 25% signal volume and the outputs are barely warm to the touch. The Transformers are both very warm but not close to hot. They all heat up as I turn up the volume.
W9TR
Super Member
What you are describing is class AB or class B biasing. The defining principle of Class A operation is that all of an amplifier’s output devices must be conducting through the full 360 degree cycle of a waveform. An amp running true class A will actually cool down as you increase the volume and power output.
jhjove
Well-Known Member
I always thought class A setups were to bias the devices into conduction (idle, never shut off, therefore no crossover distortion) but not saturation.
This sounds like the reverse of amplifying, if a device is in max current (saturation) then any increase on the input does not provide any increase (amplification) on the output.
whoaru99
Epic Member
I don't believe enough bias to conduct full 360 of a particular voltage/current swing of the output is necessarily synonymous with saturation, but maybe I'll learn something new.
jhjove
Well-Known Member
I'm not sure either anymore.
I guess the idea is if there is a signal and a load to accept the extra power the device runs cooler?
EDIT: So I was reading, the bias is the Q point, never cutoff and never saturation, and so not much amplification; the load takes some of the heat.
EDIT2: not to beat a dead horse, but this is making sense to me:
To achieve high linearity and gain, the output stage of a class A amplifier is biased “ON” (conducting) all the time. Then for an amplifier to be classified as “Class A” the zero signal idle current in the output stage must be equal to or greater than the maximum load current (usually a loudspeaker) required to produce the largest output signal.
As a class A amplifier operates in the linear portion of its characteristic curves, the single output device conducts through a full 360 degrees of the output waveform. Then the class A amplifier is equivalent to a current source.
So the stage is always max power, with a signal the load (loudspeaker) takes some of the power thus the output device runs cooler.
<Ok got this out of my mind>
--Joe
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Champco
Super Member
I have been looking to shore up your description of Class "A" to Class 'AB" with biasing adjustments.
I found this page
Class A Amplifier
The most commonly used type of power amplifier configuration is the Class A Amplifier. The Class A amplifier is the simplest form of power amplifier that uses a single switching transistor in the standard common emitter circuit configuration as seen previously to produce an inverted output. The transistor is always biased “ON” so that it conducts during one complete cycle of the input signal waveform producing minimum distortion and maximum amplitude of the output signal.
This means then that the Class A Amplifier configuration is the ideal operating mode, because there can be no crossover or switch-off distortion to the output waveform even during the negative half of the cycle. Class A power amplifier output stages may use a single power transistor or pairs of transistors connected together to share the high load current. Consider the Class A amplifier circuit below.
https://www.electronics-tutorials.w...onfiguration is,to produce an inverted output.
A lot here to absorb so I have yet to find it.
qguy
Super Member
Is that on one side or both?
Not sure how many transistors the Luxman L-550 has, but the owner said that he ran the amp overnight and the amp was just warm, the L550 is a 50 watts class A and is known to get hot, so I got hesitant and decided not to push through with the purchase. My L510 is 8 watts Class A and 100 AB and that amp gets hot.
Not sure how many transistors the Luxman L-550 has, but the owner said that he ran the amp overnight and the amp was just warm, the L550 is a 50 watts class A and is known to get hot, so I got hesitant and decided not to push through with the purchase. My L510 is 8 watts Class A and 100 AB and that amp gets hot.
whoaru99
Epic Member
A "full on" Class A amp will run hot. Typically they draw multiple times rated output from the wall all the while turned on, regardless if putting out 0 power or full power. For example, the manual for an old school Krell KSA-80 (rated 80 wpc pure class a) says the amplifier draws 8 amps at idle. That's nearly 1000W of dissipation. That is a lot of heat and it has to go somewhere and that somewhere is through heatsinks that become hot doing it.
There are other types of "Class A" amps that use a sliding bias. That type may still run hot, but not the kind of hot as the "full on" type of Class A amps like the Krell KSA-80 example.
There are other types of "Class A" amps that use a sliding bias. That type may still run hot, but not the kind of hot as the "full on" type of Class A amps like the Krell KSA-80 example.
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Some people here need to not get caught up in how things are named and what they actually are. Technics used a method that they called "New Class A", even though it was really just another Class AB amplifier with very little distortion due to the output transistors always being on and idling even when not in use. But the amplifer was not true Class A. Still very good and I like those amps but there is a difference. JVC had "Super A" and I beleive Fisher had something called "Class A II". But none of them where actual Class A amplifiers. And I do understand that some amps are designed to run in true Class A at lower volume levels and then they switch over to AB or B at higher ouput levels. But one should not confuse the two...