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In Reply to: RE: mkuller is correct * posted by E-Stat on July 27, 2018 at 20:56:19
Do either of them make Class A only amplifiers?Ask yourself why?
Let me put it this way,
If you have an A/B amp and you think that the sound (while the amp is in "A" mode) is the best those transistors can do then go ahead and be happy in your ignorance and I'll just leave you alone.
I came to this thread trying to help people gain a deeper understanding but like they say "no good deed goes unpunished".
I'm out.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/28/18Follow Ups:
Do either of them make Class A only amplifiers?
One does.
Ask yourself why?
I know why.
If you have an A/B amp and you think that the sound (while the amp is in "A" mode) is the best those transistors can do then go ahead and be happy in your ignorance and I'll just leave you alone.
All of Nelson Pass' class A amps (XA line) have enough capability to leave class A and produce more power . I'm quite happy in his ignorance. LOL!
If it can "leave Class A" to produce more power then it's not a Class A amplifier. It's just a Class A/B amplifier with higher idle currents (which is a good thing but still not true Class A)."I'm quite happy in his ignorance."
Nelson is not ignorant, you just don't understand what Nelson wrote.
Right here Nelson is saying the same thing I'm saying.
"Higher bias doesn't just move the Class A transition to higher ground - it has a profound influence on the amplifier at all power levels. It lowers the distortion at low levels as well as high levels, as seen in the distortion vs power curves for an amplifier with the bias set at different levels."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/28/18 07/28/18
(nt)
Obviously, Pass is not a class A only snob.
Nelson Pass makes Class A amps too, in his First Watt lineup.
What Tre' is saying is that although Class AB amps can technically operate in Class A mode within the constraints of bias, they will not be operating optimally in Class A.
Say you compare a Class AB amp that is Class A to 10W to a Class A amp rated at 10W. The Class A amp will have superior distortion specifications because it would be optimized for Class A, with higher bias as needed.
So a Class AB amp operating in Class A mode will have inferior performance compared to a Class A amp. If quality Class A performance is demanded, then a Class A amp is required.
First off, what are we optimizing for? Maximum Class A output? Minimum distortion at low input signal levels? Minimum distortion at higher input signal levels? Thermal stability? Bandwidth? Something else?
Biasing the device so that the Q point is in the middle of load line results in the maximum Class A output, but that isn't necessarily the optimum for anything else.
Suppose you want to pick a different Q point, one that gives you a little better linearity at low signal levels. If this happens to be at a higher bias than the midpoint, you're increasing quiescent current dissipation and reducing maximum output. Some designers might make that trade, but most probably won't. On the other hand, suppose this optimum Q point is lower than the midpoint. If the circuit is single-ended, you're reducing maximum output. If the circuit is push-pull, you're reducing maximum output in Class A , but potentially increasing maximum output overall because one device can still amplify when the other hits cutoff.
That's just one possible rationale for choosing a Q point. Other factors that might play into the decision are thermal stability, limits on quiescent current dissipation imposed by the thermal design and operating environment, longevity, distortion at higher signal levels, and in the case of transistors, bandwidth/response time. And most of all, what sounds the best to the designer.
Class A and Class AB are modes of operation, not types of amplifiers. You can't categorically say that one amp which is operating up to 10W in Class A is better than another amp operating up to 10W in Class A.
I do understand what you're getting at.All I will say is picking the Q that gives the least harmonic distortion would mean placing the idle point in the dead center of the most linear part of the dynamic curve.
That's what I would call a Class A design.
I'm not a transistor guy but the above is how Class A is described in the tube text books and WRT designing for the least inherent (before applying NFB) harmonic distortion, transistors should be no different.
This part of this thread started when I perceived that people thought that the so called "Class A" part of the operation of a Class A/B design would be the same as true Class A. It isn't and I did my best to show that. Along the way someone linked an article by Nelson Pass where he, in fact, was saying the same thing as me.
The point is, with the "Class A" part of an A/B output stage, the transistors are not biased in the dead center of the most linear part of the dynamic curve (they are instead biased and therefore operating in a less linear part of the dynamic curve) so the performance can not, will not be up to the same standard as a circuit where the transistors are biased in the dead center of the most linear part of the dynamic curve. (all other things being equal)
The concept of the above is what I was trying to get across.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/30/18
Amplifiers are not my strength, but here's how I look at it.
Class AB is a superset that includes class A and class B.
Single-ended amplifier circuits are inherently class A.
Push-pull amplifier circuits are inherently class AB, but their operation can be confined to class A or class B.
Biasing to the center seems like an obvious starting point for a single-ended circuit. It may not be optimal in some respects, but it does maximize the usable output, which is usually one of the goals.
But I think there is more flexibility with push-pull. If it's acceptable to operate in class AB when high output is required, then the designer is free to choose the Q point based on other criteria. Hypothetically, if you take a push-pull amplifier circuit that is configured to run almost exclusively in class A, you could double the supply voltage while adjusting component values to keep the Q point the same. Now you have an amplifier that can run in class AB up to 4x the class A output power (provided the power supply and devices are up to it).
When it comes to solid state amplifiers, I think it would be hard to find a "Class A" amplifier that doesn't cross into class AB before reaching the clipping point when driving lower impedance loads.
I think that people who buy Class AB amps have speakers that require a bit of power for good sound, so they compromise a bit on distortion for power.
Whereas people who buy low power Class A amps have speakers that are quite efficient with benign impedance. These people are after the best amplifier performance possible, taking advantage of the sonic superiority of Class A in spite of the power consumption and inefficiencies.
So the theoretical limits of Class A are not an issue if the amps are not pushed to their limits. I have low power Class A single ended and push-pull transistor amps and they are paired with efficient speakers that have benign impedance.
It's horses for courses. If you have efficient speakers with benign impedance, power them with a low power Class A amp for best sound. If you have inefficient speakers, power them with a Class AB (or B or D) amp, unless of course you can justify a high power Class A amp that runs very hot, burns a large amount of electricity, weighs hundreds of pounds, and costs a big bundle of money.
Got it.
Mine was more a concept piece.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
although Class AB amps can technically operate in Class A mode within the constraints of bias, they will not be operating optimally in Class A.
Fair enough. I see that higher bias levels continue to reduce distortion even after the stage "reaches" class A (fig 3).
I don't know what to say . I feel like you will think I'm picking on you.
I am not. I am only trying to help.
A circuit is either Class A or it's not. It doesn't "reach" Class A.
** A side note to help clarify. It could be said that a Class A/B amplifier operates in "Class A mode" until it "reaches" the "Class B mode" and that would be an easy way to explain it to someone who's not technically inclined.
That being said, A Class A amplifier is Class A from the smallest output to the largest output it is capable of and then it totally falls apart. So there is no "reaching" Class A with a Class A amp. It just is Class A until it's over driven.
Figures 1 through 3 are showing the output of one of the two transistors in a push pull circuit when those transistors are driven by a sine wave.
Figure 1 shows the output wave form of a transistor that has no bias current (Class B) when driven by a sine wave. The bottom half the wave form is missing.
Figure 2 shows the output wave form of a transistor that has some idle (bias) current (Class A/B). The bottom half of the wave form is mis-shapen but at least there is some bottom half wave form.
Figure 3 shows the output wave form of a transistor that has full idle (bias) current (Class A). This is the idle current that it takes to get the transistor to give a bottom half wave form that looks as much like the top half wave form as the transistor is capable of. This is spoken of as running the device in as "linear a fashion as possible". Some books use the term that Class A is when you run the device only in the "most linear part" of the operating curve.
Another note for clarity, The more inherently linear the device is to start with, the closer one gets to the goal. With a triode vacuum tube the bottom half can look exactly like the top half (within limits). With a transistor, not so much.
P.S. Missing or mis-shapen wave forms or parts of wave forms = harmonic distortion.
I hope you take this in the spirit of good will. The spirit in which I intend it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I feel like you will think I'm picking on you.
You made statements not supported by fact. To wit:
If it can "leave Class A" to produce more power then it's not a Class A amplifier.
That's a direct quote from Pass.
I hope you take this in the spirit of good will. The spirit in which I intend it.
What I now understand is there are degrees of performance once you enter the state where the devices no longer switch - and that continues to improve with higher levels of bias.
"What I now understand is there are degrees of performance once you enter the state where the devices no longer switch - and that continues to improve with higher levels of bias."
Yes, but only to a point. The bias level can only go as far as full Class A otherwise the device will run into saturation (ie; non-linear behavior much like cutoff but at the other end of the dynamic operating curve).
Only the center part of the of the operating curve is linear. It becomes non-linear at each end. The idea is to bias in the middle of the linear part, ie; Class A.
Please note that I have made no attempt to address trick circuits such as the one Nelson mentions in the link.
"In 1991 Pass Labs developed a hybrid class topology which paralleled a push-pull Class A output stage with a current source which biased it into single-ended Class A. The Aleph 0 amplifier operated as a single-ended Class A amplifier to its output rating of 75 watts into 8 ohms, and at currents beyond that it continued to deliver current as a push-pull Class A circuit."
My comments only address the behavior of a basic single ended or push pull amplifier not a trick circuit that somehow combines the two as described above by Nelson.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Have Fun and Enjoy the Music
"Still Working the Problem"
There's so much audiophile entrenched "understanding" on this topic that the goal post has been moved over the decades. It seems you've caught yourself in a similar predicament other times as well. :)Any amplifier with a push-pull output stage is not Class-A. Only single-ended amplifiers are (or can be) Class-A...or "true" Class-A, if that floats your boat.
Anything else is some sort of "hybrid" configuration that doesn't fit the classic definition.Dave.
Edits: 07/28/18
Neither am I.I was just trying to explain what Nelson explained in the link.
A Class A/B amp operating in the so called Class A Mode* (*Nelsons word, not mine) is not the same as true Class A.
"Higher bias doesn't just move the Class A transition to higher ground - it has a profound influence on the amplifier at all power levels. It lowers the distortion at low levels as well as high levels, as seen in the distortion vs power curves for an amplifier with the bias set at different levels."
Nelson said it and you don't question him, I say it and you think I don't know what I'm talking about.
Whatever!
Like I said in another post on this thread, I entered this thread trying to explain these things to you all. No one what's to believe me. Then someone posts a link to Nelson Pass saying the same thing.
If you don't want to understand what Nelson and I are trying to explain to you, then neither Nelson or I can help you.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/28/18 07/28/18
What Nelson Pass says:
That link proves my point.
"Higher bias doesn't just move the Class A transition to higher ground - it has a profound influence on the amplifier at all power levels. It lowers the distortion at low levels as well as high levels, as seen in the distortion vs power curves for an amplifier with the bias set at different levels."
Nelson Pass is using the term "Class A" loosely when he talks about the "Class A" transition in a Class A/B amplifier.
When the transistors are biased to full Class A, then we are talking about true Class A and it makes a difference as Nelson says.
That is the difference I was trying to point out.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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