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In Reply to: RE: Sorry but posted by Tre' on February 07, 2025 at 19:33:53
The JC-1+ can do 25 watts while all of the output devices are conducting 100% of the time and 450 watts total.
Yes, that's the thing.
Follow Ups:
Yes, that is the thing a Class AB amplifier does.
All of the power produced by a Class A amplifier is produced while the output devices are conducting 100% of the time. Saturation and cutoff is reached at the same time and at that point the amp has no where else to go.
The amp has no way to produce any more power.
That is Class A. If the amp is not doing that, then it's not Class A.
How can an amp that is not Class A produce Class A power?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
!
The Mind has No Firewall~ U.S. Army War College.
who design, test and sell them since you seem to be confused.
You are confusing sales hype with knowledge.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/08/25
Yeah, these guys (among others) don't know what they're doing. ;)
What is the collective noun for audio gurus?
among engineers.
Constant correction unnecessary.
?
A-ha, I do now see that it is a young Bernstein. But, I wouldn't know Copland if he a blew a fanfare in my face.
Edits: 02/11/25
Bernstein and Copland.
Another pair of distinguished artists in their craft.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
their AB amps run class A for tens of watts at lowest power levels.End of ridiculous discussion.
Edits: 02/08/25
A output device (tube or transistor) can't be operated all the way down to cutoff (or saturation) without entering the non-linear cutoff region.
A proper Class A amplifier is biased to idle in the middle of the most linear part of the operating curve, half way between cutoff and saturation and is intended to never be driven into those non-linear regions. (non-linear cutoff and saturation regions shown above)
A Class AB amplifier is driven into the non-linear cutoff region even while producing its so called "Class A" power. Just because someone chooses to call that Class A power doesn't make it Class A power.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Howard Tremaine Audio Cyclopedia second edition
Page 543 "What is a Class A amplifier. An amplifier in which the grid bias voltage is set to approximately one half the cutoff voltage to obtain linear operation."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"When a bipolar transistor is biased so that the Q-point is near the middle of its operating range, that is approximately halfway between cut-off and saturation, it is said to be operating as a Class-A amplifier. This mode of operation allows the output voltage to increase and decrease around the amplifiers Q-point without distortion as the input signal swings through one complete cycle. In other words, the output is available for the full 360o of the input cycle."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"The Last Word on Class A
First of all, what is a class A amplifier?
A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier. Or, as stated in the RCA receiving tube manuals: "The classification depends primarily upon the fraction of input cycle during which plate current is expected to flow under rated full-load conditions". The key phrase being "under rated full-load condtions", which is a requirement for amplifier classifications to be meaningful.. This is typically done by biasing the output stage halfway between cutoff and saturation, with the plate load impedance to an appropriate value that gives maximum undistorted output power."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think it fine to classify an amplifier that is class A only for a fraction of its specified output power as Class AB even if it might end up spending most/all of its time operating in a class A regime. But, as pure solid state class A amplifiers are uncommon and class B is very rare (AFAIK) I also think classification with such fervor pretty pointless.
In your follow-up posts I see the inclusion of single-ended amplifier load lines. These are not relevant to solid state push-pull output stages, which can produce load current above their bias point as required (but only in one direction, hence the need for push-pull). And then I remebered having this discussion once before on AA and after a search it turned out to be with you!
As I told you more than 6 years ago, I'm not a SS guy."I see the inclusion of single-ended amplifier load lines. These are not relevant to solid state push-pull output stages, which can produce load current above their bias point as required (but only in one direction, hence the need for push-pull)"
Should I understand, from what you said, that a transistor can't idle at the mid point between cutoff and max current?
The single ended load line may not be relevant as the current moves up from the idle point but isn't it relevant as the current moves down from the idle point?
Or do transistors magically drop all the way to cutoff in a perfectly linear way?
If they don't then that's my point. In a true Class A tube amplifier (single ended or PP) none of the devices are driven into the non-linear regions, up or down. It transistors are linear all the way to cutoff then there is no need for Class AB, Class B with be notchless. (crossover distortion less)
As the dynamic operating curve chart (not a load line plotted on a set of plate curves but derived from that) shows, with a tube circuit Class A only operates the tube(s) in the "most linear part of the dynamic curve". I forget which book I'm quoting but that is a quote.
An AB tube amplifier doesn't do that and neither do SS circuits (unless transistors are linear all the way down to cutoff).
Side note, with a PP Class A tube amp, it you drop the idle current a little to get more power then it's not a Class A amplifier anymore and none of the power is Class A power. Even the part that is still operating in the "most linear part of the dynamic curve". The total rated power of the amplifier has to be power produced while all the tubes are operating most linear part of the dynamic curve. That is what all the old books discribe and show as examples of Class A.
Maybe with SS it's different. If a transistor stays linear all the way to cutoff. Tubes don't do that.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/10/25 02/11/25
And likewise I'm still not a tube guy :)
I've kind of lost the thread a bit but to answer your question nothing is linear! The further up and down the output current vs. Input voltage curve you go then the more distortion there is. The smaller the current excursion around a bias point then the less the distortion (with the limiting case being zero distortion for zero current change = small signal conditions). I don't know what the load line for an emitter/source follower would be but it would probably show the analogous effect that you are describing by tending to cut off the follower if it has to sink current from a load. Followers are great at supplying current to loads but that only works in one direction hence the need for push-pull unless you went with a single-ended follower biased so that it can sink the maximum load current before turning off (or getting too close to turning off) and that would be class A. But, IMHO, even if you did go class A, a push-pull would have the advantage of going into class AB for lower load impedances e.g. it could be 'pure' 50W class A into 8 ohms but give 100W into 4 ohms class AB, whereas a single ended 50W class A could only provide 25W into 4 ohms (current limited).
Transistors do not stay linear all the way to cutoff...otherwise, there would be no need for Class A to begin with as you would not have serious distortion at the handoff between transistors in a push/pull amp.
With push pull, there would be no need for Class AB because there would be no notch distortion in Class B.
I just thought of an analogy. It maybe not a good one but here goes.Class AB is a "fix" for Class B much like a suppressor grid in a pentode is a fix for the tetrode.
When they added the screen grid to a triode to block internal feedback path between the plate and the screen that lowered the gain and reduced the distortion, they created the problem of secondary emissions and had to add the suppressor grid to fix it.
When they went from Class A (push pull) to Class B (push pull), the non-linear behavior of amplifying devices in the cutoff region created cross over distortion and they had to put some of the idle current back to "fix" it.
A pentode is a fix for a tetrode but that doesn't make it a triode.
A triode is it's own thing.Class AB is a "fix" for Class B but that doesn't make it Class A.
Class A is it's own thing.I warned you that the analogy might not be very good. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/11/25
!
The Mind has No Firewall~ U.S. Army War College.
Are really AB in that they operate with significant idle bias current thru the finals. Theory indicates that a unity coupled OPT could operate successfully with B biased finals but that's not what Mac did in practice, at least for their consumer stuff.
Marketing is a very important part of any business. It is just easier to explain it that way rather than trying to explain that it is not really Class A, it just sort of acts like Class A.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
....it acts as A sometimes but it is not A in the true sense of the definition.
Yes, I think it is just easier to call the power an AB amplifier makes, while not reaching cutoff, Class A vs. trying to explain that it only acts like Class A is one narrow way but it's not real Class A. I believe that is the most likely reason designers (who for sure know better) say what they say.
I wish the wouldn't do that. They are confusing people. :-(Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/08/25
One thing is for sure, the distortion behavior is not the same as a Class A amp. Also, if you look at measurements, you can see that not all Class AB amps are sufficiently high bias to remove the zero crossing cutoff distortion.
As an aside, newish brand Infigo claim to operate always in class A using a novel sliding bias scheme but JA's measurements showed it rife with cross-over distortion though THD was still low and the reviewer loved the sound. I've no idea how much bias is required for a 'smooth' handover between push & pull and vice versa but many years ago Douglas Self wrote a series of articles about audio power amp design and claimed, IIRC, that optimally biased class B actually produced the lowest distortion. I didn't pay much attention at the time so don't know his reasoning but think it was that the variation of composite transconductance of the push-pull pair could be made much less through the cross-over region than for an AB pair. This was set in the context of applying as much loop gain as possible so, perhaps, with the right class B bias and a lot of feedback Topping has gone in this direction and try maximize efficiency to go against class D designs.
I found that amp review on Stereophile (see above). I don't take too much stock in what the reviewer liked and didn't like. You can always find someone to like your product. Having had amps early in my audiophile career that were both Class A SS and what I would call close to Class B (meaning later measurements revealed zero crossing distortion), I can tell you the one with zero crossing distortion sounded nasty and congested and was not very long in my system.
The Naim measurement I found also has zero crossing distortion and I don't know about you, but I find all Naim amps to sound dry (as in no harmonics and poor low level resolution needed for ambience retrieval) and flat. Horrible amps that traded on the selling point of having PRAT...because they had nothing else that sounded like music.
To the best of my understanding, you cannot remove distortion caused by zero crossing with negative feedback. I would need to read the reference I learned that from again to know the exact reasons why. Might have something to do with the fact that it is not an inherent transistor property but related to the stitching together two transistors to make a whole waveform and the glitch created by the imperfect handoff.
Distortion is created if the transfer function is anything other than a straight line (with slope = gain). For a push-pull output stage, if there is a little wiggle in the line around the zero crossing that creates cross-over distortion. The larger the deviation, the higher the distortion and sharper wiggles create higher frequency harmonics. Feedback can only suppress any open loop distortion by the amount of loop gain at the frequency of interest. So, higher frequency harmonics are suppressed less because amplifier loop gain is always rolling off as frequency increases, and this applies to all distortions not just cross-over.
One danger, I think, is that biasing too hard into class B could create a 'dead-zone' at the cross over point where there is a small region with no transconductance and that will give low, or no, loop gain in that region and, hence, no suppression. It would be interesting to study but I don't get much spare time and I'd rather listen!
Conversely, class AB output stage should have twice the slope in the region where both devices are conducting (class A region) and a signal transitioning between A & B regions will be distorted by that slope change and there will also be a halving of loop gain in the class B region. So open loop distortion is created by that transition and the amount of suppression is modated. This is the so-called 'gm doubling' effect that is in the literature. Most recently it was tackled by Jason Stoddard in his blog, he came up with something he calls a 'continuity bias'to get around that problem. But, IIRC, that biasing burns significant power that is still lower than staying class A all the time but much more than a traditional AB.
Where both finals are in cutoff at idl, it's class "C" not "B" . . . Not good for audio. Almost all audio amps claiming "B" operation are really "AB" with idle bias current set low but high enough to prevent both finals from being cutoff simultaneously under real-world conditions of manufacturing and operation. It could be argued that true class B exists only at a single operating point where "AB" and "C" have an infinite number of points.
:)
Interesting how this amplifier class discussion arises every few years and involves some of the same persons each time. The basic amp classes were defined nearly a century ago . . .
However, that means the performance around zero crossing is very non-linear and will create a lot of distortion that is of a very obnoxious nature.
Look at the stereophile measurements I shared above. You can see the sharp discontinuity they have around the zero crossing. As one can imagine, this is essentially only high order dissonant distortion components. Even very low levels of these harmonics will audibly damage the sound.
As Tre was pointing out, even AB is not putting the bias in a very linear part of the transistors transfer function, which will also result in increased higher order harmonics.
Yes, sliding bias is a whole different thing. I mentioned that early on in this thread.
"I think there are some designs that operate in both Class A and (when needed) Class AB but that requires "sliding bias"." See link.
"with the right class B bias and a lot of feedback"
I wasn't aware that there was different types of class B bias. I thought class B was biased to where the devices are just off and ready to come on at the first sign of a positive signal voltage? I thought all Class B amps were biased at the same spot?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think bias is a spectrum so you could under bias heavily into class B such that both devices are off for a region and make a dead zone around the zero crossing (which would be bad) or over bias until you end up in class A. I suppose an optimum class B is something that gives the best trade off between distortion and power dissipation. I don't have direct experience of this but, I think, Self delves into it and his work is online and he justifies his choice of output stage biasing.
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