CLASS A amp runs cool
- Thread starter qguy
- Start date
Hmmm. By this logic, only a power amplifier which operates ONLY in Class A can be called a Class A amplifier. Not many of these around. What would you call an amplifier that operates in Class A 90% of the time? It is certainly not a Class A/B amp.
It's a matter of output stage quiescent current. Class B has the stage very near cutoff, and where the signal crosses ground, there may be a discontinuity. AB is biased on a little harder to minimize the discontinuity, and class A is just cranked on hard enough so that the output stage runs constant current, regardless of signal. It's just a matter of bias point setting what the lettered class is.
That, and having the corresponding components, heat sinking, etc to deal with it.
I don't think you can just turn up the bias on any old amp and "convert" it to class A!
Conventional "Class A" requires all of the output stage to contribute to the signal at all times, this is where the "New Class A" and "Super Class A" fail. Yes the output transistors never turn off but then they don't always contribute to the signal. My PASS A40 is a 40W/8 Ohms Class A amplifier and each channel idles at 100W.
Craig
Craig
whoaru99
Epic Member
I believe there is a difference between Class G and or Class H topology vs. sliding bias, although perhaps some amps incorporate elements of both.
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Not really sure what you are getting at here. I was simply making the distinction between things like "New Class A", which is actually just a very good Class AB amplifier and an actual Class A amplifier. Huge difference between them. Also I have zero problem with calling an amp "Class A" if it operates as such at lower levels. But now you are simply nitpicking about what to call something. That really isn't important. What is important is how those amps operate and the differences between them. Those companies like Technics, JVC and Fisher knew what they were doing when they used terms like that. They were trying to indirectly tie their products to something else. I love New Class A amplifiers but I am under no impression that they are "Class A" in any way, shape or form. That was simply a marketing tool from the manufacturer... And I am OK with that because the actual amp worked well and sounded good with very little distortion...
Champco
Super Member
To understand the subject takes more then just thought. It requires multiple concepts, some of which may be misunderstood. like breaking bad habits.
The concepts all come together but there are few sources which do put it all together.
To some of these strings of conversation seem simple but to many here including myself it's helpful. Lets keep it entertaining and educational as much as possible. I think you are doing good work here.
I will keep mine around for a long time and and they sure put out some heat.....all true Class A amps put out some heat, but the hottest of them all of my amps is actually not a Class A. The Yamaha B-1 runs hotter than any of my Class A amps:
Yamaha BX-1s
Stax DA-100s
Sony TA-N9s
First Watt SIT-3
I know I shouldn't go where angels fear to tread, but I feel compelled to assert that this is incorrect. Conduction for anywhere between 50% and 100% of the duty cycle is certainly class AB by definition. (Original engineering definition, that is, from somewhere around the middle of the last century. I don't accept that the definition has changed, despite all attempts by marketeers.)
This fool will rush back out now. No offense intended. Cheers.
Really?
So the Yamaha amps that operate in Class A for the first 5-15 watts of their output are not really operating in Class A when putting out less than that?
Doesn't Pass offer some amps that operate mostly in Class A, then in A/B as they reach their full output capability? There are similar designs from other manufacturers.
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With respect, yessir. But I should probably substantiate my claim, at the risk of being Don Quixote for a few minutes more. From no less authoritative a source than a 1968-copyrighted version of the Allied/Radio Shack "Modern Dictionary of Electronics", I quote:
"class-AB amplifier - An amplifier in which the grid bias and alternating grid voltage are such that plate current flows for more than half but less than the entire electrical cycle."
Okay, done tilting now, really, I hope. Help me to be strong, Dios y Sancho Panza! And again, no offense, really, I hope.
Disagreement is not offensive. The smarm is.
As i understand it, the amps I am referencing do indeed meet the definition for Class A...for some portion of their output.
Full current for 100% of the duty cycle for first 5, 10, 15, 20 etc watts, AB thereafter.
As i understand it, the amps I am referencing do indeed meet the definition for Class A...for some portion of their output.
Full current for 100% of the duty cycle for first 5, 10, 15, 20 etc watts, AB thereafter.
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How is an amp class A and then class A/B unless there is a switch between the two???
Marketing???
Class A/B amps are class A until they are operating in class B, hence class A/B.
https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html
".........the Class AB Amplifier is a combination of Classes A and B in that for small power outputs the amplifier operates as a class A amplifier but changes to a class B amplifier for larger current outputs. This action is achieved by pre-biasing the two transistors in the amplifiers output stage. Then each transistor will conduct between 180o and 360o of the time depending on the amount of current output and pre-biasing."
https://www.passlabs.com/technical_article/leaving-class-a/
"Class B, Class AB, or Class A.
Class B has no bias current, Class AB has a moderate bias current, and Class A has a high bias current. Class AB push-pull amplifiers are hybrids between Class B and Class A. Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias."
Marketing???
Class A/B amps are class A until they are operating in class B, hence class A/B.
https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html
".........the Class AB Amplifier is a combination of Classes A and B in that for small power outputs the amplifier operates as a class A amplifier but changes to a class B amplifier for larger current outputs. This action is achieved by pre-biasing the two transistors in the amplifiers output stage. Then each transistor will conduct between 180o and 360o of the time depending on the amount of current output and pre-biasing."
https://www.passlabs.com/technical_article/leaving-class-a/
"Class B, Class AB, or Class A.
Class B has no bias current, Class AB has a moderate bias current, and Class A has a high bias current. Class AB push-pull amplifiers are hybrids between Class B and Class A. Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias."
In the case of the Yamaha amps, there is indeed a switch.
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I know there were (are?) some amplifiers that had switches.Regarding the OPs Luxman L-510 amp the Vintage Knob info says: "The switch from Class A to Class A/B is automatic when higher output levels are required."
Honest to gosh, I was really trying not to come across smarmy or condescending. I apologize that I did.
In my defense, shields were at full because I know this subject generates controversy. I think it doesn't help that the word "class" is so overloaded.
I think the "class A up to N watts" characterization is a pretty reasonable way to look at it. I'll still call that class AB 'til the cows come home, and I would call the above-N-watts operation class C - but as long as it's clear what actually happens when the rubber hits the road, I guess I can let the shields down.