"Class A" nonsense

[Chuck H]

What defines a "race car?" The design or the use? My mom doesn;t think of her car as a race car, but I could certainly take it and race it.

My mother drove my brother and I around in a '68 Plymouth Fury with a 383. At the time she wouldn't have called it a race car. But by the time I was 14 I knew what it was. I wish I had that car now.


Chuck

[Steve Conner]

Yep, and with the right choice of race track a Geo Metro can beat a Lamborghini. (Or the wrong choice of race track, if you're the Lamborghini owner.)

The amplifier classes are all defined under the assumption that the amplifier is meant to be used without clipping, so the clipping behaviour is ignored as irrelevant. If you want to understand the old-school standards then you must remember this.

I prefer a new-school approach, where the clipping region is regarded as a normal part of the amplifier's use. I like to think of the transition into clipping as a transition from one class (the amplifier's original design class, as per the old-school theory) to another, which will normally be Class-C.

The old standards and design methods did not cover this at all, because at the time they were written, no designer could get their head round the idea that you might actually want to overdrive an amp, let alone manipulate its behaviour under overdrive with a view to making it musically useful. So by those standards, an amplifier was always simply the class that it was designed to be, de jure if not de facto.

[mooreamps]

To decide if an amp is class A, you need only ask a simple question:
If a device goes into cut-off, does the output power continue to increase?

If the answer is no then we have class A. This is why single-ended amps can only be class A (or C, which doesn't really apply to us). As soon as it goes into cut-off (or grid-current limiting) the output signal stops increasing, we cannot squeeze any more out of it.

If the answer is yes then we have class AB or B. One device may be in cut-off, but another device continues to provide output power. This is why Class AB or B amp always need at least two devices.

All this maybe just fine and dandy, for guitar amps. But, probably going to be limited to "only guitar amps". I would not make the mistake of thinking these same trains of thought would go over with discussions of the "rest of the amplifier world."

Also, BTW, Class B and Class C more commonly uses only one tube. So, when you say things like this, make sure you make the point clear it's only applying to audio power output stages.


-g

[Merlinb]

All this maybe just fine and dandy, for guitar amps. But, probably going to be limited to "only guitar amps". I would not make the mistake of thinking these same trains of thought would go over with discussions of the "rest of the amplifier world."

The point I made applies to all audio amps (not RF which is completely different and way off topic).

Also, BTW, Class B and Class C more commonly uses only one tube.

You mean for RF? Certainly not for audio.

[Old Tele man]

...by definition, Class-AB *is* push-pull only.

...and, Merlinb is correct about Class-A power not increasing when pushed beyond cutoff...it's a "hallmark" characteristic of Class-A operation.

...some relevant reading (including RDH references):
A) http://www.transcendentsound.com/ClassAorAB.htm
B) http://www.aikenamps.com/ClassA.htm

...some more "fodder" for the nonsense fire (as correctly stated earlier by others):

Class-A PP optimum RL = 1/2 of reflected load
Class-B PP optimum RL = 1/4 of reflected load

PS--at the plates (PP), its a variable frequency (20~10KHz)/variable amplitude, semi-rectified AC-voltage riding on a DC-voltage (actually a subtractive addition, where no signal = full plate voltage and maximum signal = minimum plate voltage as defined by the reflected plate-to-plate load).

...and, a 'working' example of a class-B tube is the rectifier, or a triode/tetrode/pentode being operated as a rectifier.

[balijukka]

Can we really say that one can call an amplifier a "class A amplifier?" We can say it is designed to operate in class A, but that doesn't guarantee that it will be run that way.

That's the way see it.

The classes are suppoosed to be operational classes, created by the engineers, so that they can easily communicate what's happening within an amplifier stage, be it preamp or power amp.

So for example you can say: "when overdriven the power amp goes from class A1 operation to class A2 operation" or the voxish "from class A operation at low volumes to class AB operation at higher volumes".
I think everybody here understands what's going on.

Of course one can argue about these things, and I believe every now and then we should do so in a civilized way, just to see if anyone has anything new to offer. Then we can go back tweaking the amps

jukka

[Merlinb]

That's the way see it.

Hmm, I'm not sure I agree. Anything which involves the statement "supposed to" is not a reasonable basis for classification. Either it's class X or it isn't.

For example, we can talk about a class AB amp being "class A at low levels", but really that's a description to aid understanding of how it works. It's still a class AB amp, we don't class it Class A and B.
We could drive it really heavily, until the bias shifts and the whole thing ends up working in class B. But it's still a class AB amp. It's just overdriven, and that's what happens when you overdrive a class AB amp. It doesn't 'become a class B amp at high levels', it becomes an overdriven class AB amp.

[raiken]

Hmm, I'm not sure I agree. Anything which involves the statement "supposed to" is not a reasonable basis for classification. Either it's class X or it isn't.

For example, we can talk about a class AB amp being "class A at low levels", but really that's a description to aid understanding of how it works. It's still a class AB amp, we don't class it Class A and B.
We could drive it really heavily, until the bias shifts and the whole thing ends up working in class B. But it's still a class AB amp. It's just overdriven, and that's what happens when you overdrive a class AB amp. It doesn't 'become a class B amp at high levels', it becomes an overdriven class AB amp.

I agree 110% with this statement.

Randall Aiken

[balijukka]

The point I made applies to all audio amps (not RF which is completely different and way off topic).

Me being the asshole here. just for arguments sake.
What if you make a a very high gain geetar amp with a single tube S output stage. The preamp gain so high that you'll only get a square wave in the power tube input with a MV pot there.
Now you bias that outpot tube cold. Start turning uo the volume. Very early it hits cutoff. After that the output still keeps increasing. Output signal still being the same square wave.
By your definitions it can not be class A since the output increases after cutoff and it must be class A being single tube.

jukka

[Old Tele man]

...then it's simply an "over-driven" single-ended Class-A tube.

...or, you might simply call it a "clipper" circuit.

[Robert M. Martinelli]

I agree 110% with this statement.

Randall Aiken

( a heart-felt ) +1!

That was the point I tried to explain in my previous posts ( seemingly, failing miserably ) - the class of operation is to be stated at one ( and only one ) point - the full un-clipped output. An engine rated power and torque are given at a certain number of RPM. You can drive the car at any speed, the engine can behave differently at different RPM, but that doesn't change its rated power and torque and the RPM at which they're obtained.

All the best

Bob

[Koreth]

( a heart-felt ) +1!

That was the point I tried to explain in my previous posts ( seemingly, failing miserably ) - the class of operation is to be stated at one ( and only one ) point - the full un-clipped output. An engine rated power and torque are given at a certain number of RPM. You can drive the car at any speed, the engine can behave differently at different RPM, but that doesn't change its rated power and torque and the RPM at which they're obtained.

All the best

Bob

And like a tube amp, those engine ratings are taken at peak output. Unfortunately, unlike a tube amp, you can't push past that peak. When you do, your morale gets crushed by the fact you just used a $2800 set of rods to ventilate a $1500 block casting, and a $1000 crankshaft to launch a $600 set of pistons into a low earth orbit.

[Robert M. Martinelli]

And like a tube amp, those engine ratings are taken at peak output. Unfortunately, unlike a tube amp, you can't push past that peak. When you do, your morale gets crushed by the fact you just used a $2800 set of rods to ventilate a $1500 block casting, and a $1000 crankshaft to launch a $600 set of pistons into a low earth orbit.

Hi Koreth,
We all know amps can be driven in and beyond the red zone, something engines can't stand, neither I believed that analogy of mine to be a perfect fit, I just wanted to point out that those ratings are specified at a precise and certain point, similarly to amps' classes. Both amps and engines can work all the way from their quiescent status ( silent amp on=still car @ min.RPM ) to their maximum power. An amp doesn't go beyond its rated power even when fully cranked, but at least it doesn't launch pistons into a low earth orbit, as you correctly noted .

Cheers

Bob

[balijukka]

( a heart-felt ) +1!

That was the point I tried to explain in my previous posts ( seemingly, failing miserably ) - the class of operation is to be stated at one ( and only one ) point - the full un-clipped output.

I agree that it makes sense, if you want to give an amp a class.
It is NOT what the standards say.

An engine rated power and torque are given at a certain number of RPM. You can drive the car at any speed, the engine can behave differently at different RPM, but that doesn't change its rated power and torque and the RPM at which they're obtained.

That is a very good analogy, just wrong insight. An engine can change it's type of operation, for example you have turbo that switches on and off.
Then it changes it's class from being not charged (class A) to charged (class AB).
The rated power, torque and RPM are different parameters. In an amp they could be for exaple rated power , frequency response, and input voltage.

jukka

[balijukka]

Hmm, I'm not sure I agree. Anything which involves the statement "supposed to" is not a reasonable basis for classification.

That supposed was 100% jukka. Trying to soften the claim.
It just is so that there is no standard definition for amplifier class.
There is the standard for classes of operation and that class of operation can change if the input signal changes enough.
Perhaps those ancient engineers were smarter than we think.
And not affected by the MBAs.

jukka

PS. since when you and Randall have been on the same side. Who's the turncoat ?