Getting conflicting answers on the following question.
Will two output tubes in parallel deliver as much power as the same two tubes in push pull.
Some say the PP delivers twice as much power PSE some say the same.
I understand the PP is more efficient just want to know about the power.
Will two output tubes in parallel deliver as much power as the same two tubes in push pull.
Some say the PP delivers twice as much power PSE some say the same.
I understand the PP is more efficient just want to know about the power.
Apples and oranges ...
Two different mode of operation, they do not resemble each other.
Therefore only one point of view (power) is completely uninteresting in terms of choice.
Two different mode of operation, they do not resemble each other.
Therefore only one point of view (power) is completely uninteresting in terms of choice.
Thanks for the (un)response
I understand the differences.
And I understand power is just one criteria.
Anyone else care to answer the question?
I understand the differences.
And I understand power is just one criteria.
Anyone else care to answer the question?
If the amplifier is running Class A the power should be similar, but a PP amplifier can be run in Class AB which has one side turn off and one can get more power at teh sacrifice (usually) of GM droop in the AB amplifier.
dave
dave
Look to a textbook for the efficiency limit derivations.
SE must, for linearity's sake, operate in Class "A", whose max. theoretical efficiently is 50%. Class "A" PP has the 50% theoretical limit too.
The theoretical limit for Class "B", with an exactly 50% duty cycle, is Π/4 = 78.54%. For several reasons, including crossover/notching distortion, we (at most) use near Class "B" conditions whose duty cycle exceeds 50% and the theoretical limit is reduced.
Aside from the possibility of increased efficiency, PP operation leads to the cancellation of PP pair generated even order harmonic distortion products and certain other artifacts, in the O/P "iron". The cancellation effect allows AC heated PP DHTs to exhibit quite satisfactory residual hum levels.
SE must, for linearity's sake, operate in Class "A", whose max. theoretical efficiently is 50%. Class "A" PP has the 50% theoretical limit too.
The theoretical limit for Class "B", with an exactly 50% duty cycle, is Π/4 = 78.54%. For several reasons, including crossover/notching distortion, we (at most) use near Class "B" conditions whose duty cycle exceeds 50% and the theoretical limit is reduced.
Aside from the possibility of increased efficiency, PP operation leads to the cancellation of PP pair generated even order harmonic distortion products and certain other artifacts, in the O/P "iron". The cancellation effect allows AC heated PP DHTs to exhibit quite satisfactory residual hum levels.
That's the max theoretical efficiency for a transformer coupled Class-A amplifier!
(the efficiency of a "normal" Class-A amp is lower, less then 30%)
In practice that will be around 40-45% (because of losses)
For a Class-AB it will be around 50-60%, or like 70% when you have a very cold biased amp.
Transformers for PP amps are also much cheaper/smaller.
I am curious were do you get those conflicted answers from?
This is just simple text book calculations.
here some info if you want to read on the subject;
Transformer Coupled Class A Power Amplifier - Tutorialspoint
(the efficiency of a "normal" Class-A amp is lower, less then 30%)
In practice that will be around 40-45% (because of losses)
For a Class-AB it will be around 50-60%, or like 70% when you have a very cold biased amp.
Transformers for PP amps are also much cheaper/smaller.
I am curious were do you get those conflicted answers from?
This is just simple text book calculations.
here some info if you want to read on the subject;
Transformer Coupled Class A Power Amplifier - Tutorialspoint
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IMHO, the question is incomplete. If want to do a power comparison you need to establish a distortion level. So for a given total distortion, class A amplifier, a PP will typically give more power.
In practice the benefit of such higher power will be hardly heard in terms of sound pressure. The class A PP amplifier is also a bit more flexible in terms of mating to loudspeakers.
If you have good speakers for SE amp then it's just a choice of what you like more really...
For an amplifier with no negative feedback, and Class A operation, one difference is . . .
A parallel triode single ended amp has a non-symmetrical damping factor.
A push pull triode amp has a symmetrical damping factor.
A parallel triode single ended amp has a non-symmetrical damping factor.
A push pull triode amp has a symmetrical damping factor.
PSE or parallel single ended is just the "same" as single ended. Keep this in mind. SE runs in class A where it is biased so the current is always flowing so you get the full sine wave, think peak of the crest AND the very bottom of the trough for a sine wave.
Next... Single ended is where one tube does all the work. With a PSE, a number of tubes act alike, but with the power tubes each handling the same cycle working in unison. Like rowing a boat in time. To keep things simple for this discussion, the output power of tube working in unison is the same as a single tube in SE. SE is like one guy with two oars in the water working both oars.
In PP, two tubes work in unison together BUT in alternative cycles. Like two guys opposite sides of a boat (we are talking amps here so no marine talk, just left and right side). Great thing is two guys each using one ear on each side.
In PP operation, two tubes are fed a signal in opposite phase to each other. This is why phase splitters exist in PP and not in SE. The phase splitter copies and splits the signal, inverts one copy 180 degrees so it is inverted. Each is then fed into a power tube, remember out of phase, then fed into the output transformer which drives the speaker. It is the out of phase where one pushes, the other one pulls.
Get into it too deeply and you will start to discuss biasing, distortion, and feedback.
Just know that PP gives you more power. PP can reach down to bass levels SE can't plumb and is detailed, but SE sounds so much better with it's warmth and harmonics. But SE you are running those tubes harder than PP does... and it goes on and on... but that wasn't the question.
How much more power? For our sanity sake, theoretically double in PP than PSE. Plain old SE and plain old PP because we can start talking ultralinear operation, triode, pentode, PPP (parallel push pull). In reality, I don't think it would be a perfect double though.
edit: I over simplified it because I am not that technical and well, I am sure we could fill a hundred pages exploring all parallel and SE and PP topologies.
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A PP can run (and generally does) the tubes much harder than SE. What consumes the tubes faster is the current they deliver to the load. In PP amps the tubes slip in AB1 (or even worse AB2 if they are designed to go into positive grid) even if they are designed as Class A into a nominal load (because loudspeakers are not nominal loads) delivering currents that will never happen in SE. In fact if one has used tubes and want to put them use, a SE amp is a better idea in my experience.
I think the power the tube dissipates is that burdens the tube, not the power it delivers to the load.
Can you elaborate on this idea of damping factor symmetry, please? I didn't know there is symmetry of damping factor in a transformer-coupled amp.
The damping factor is based on the power tube plate resistance. At maximum current, plate resistance will be lowest, at the other end of that signal, when the tube is nearly cut off, the plate resistance is highest. The damping factor therefore varies depending on the instantaneous value of plate resistance across the waveform.
cheers,
Douglas
It's not the most relevant if the tube is run within specs. The quiescent conditions are not that different between class A and AB to see a relevant difference due to average power consumption. Maybe after 10 years of use...or you might see more difference if you run the tubes close to true class B like in 60-70W EL34 PP....isn't it?
Tubes die when the filament will have no more emission, the more current you pull out the quicker will die. The class AB push-pull is the champion.
Of course this is relative to the average power one uses but the PP can run the tubes harder. No question about that.
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What is consuming the tube... another rabbit hole.
Talking about Class A PP topology, but better throw in Class AB. AND WARNING! Don't roast me. I am not an expert here, so correct me and forgive me if I get something wrong. This is what I have gleaned over the many years reading what the designers and experts (George at Tubelab, Nelson Pass, and Pete Millett) here at diyaudio and the nuggets they have shared over the years. The monumental arguments that existed over the craziest things (anyone recall the RH84 fire fight?) is something I don't want.
This is just an info share to the community and I am no where near those legends I named up there, but this is the point of the forum. Share knowledge and be nice.
Class A is so NOT efficient (12% as a maximum was a number I seem to recall floated in a discussion way back). But if the PP is run at Class A going halfway between cutroff and saturation, then each tube in the PP pair only needs to handle half the waveform (180 degrees using two tubes in PP makes 360) as the current goes up in one, the current in the other tube of the pair goes down, so they cancel out in the OPT. These amps as Class A PP in theory will have double the output power as a single tube in SE. The neat thing is PP OPTs can be smaller (and tend to run cooler) because you don't need the air gap as there is little to no core saturation as the net current difference is zero (each side cancels the other side out since they are out of phase).
To get more power, we can choose to run PP in Class AB because it is more efficient use of power because two halves of the signal are on either side of the operating point. Class AB needs a little more than the 180 degrees but less than the full 360 degrees so we don't get distortion at the "top of the range" of the tube (what is the terminology I am looking for?). The tubes are run at lower than the max dissipation and the the full current only is seen at full peak load. This increased idle bias keeps the tube "running at all times, but not like it would be in Class A. So the bias is lower (and colder because less current until demanded). However, less plate current means more distortion because operation is now in a less linear region as you approach the the zero line ("halfway" mark of each pair of PP tubes, where as Class A runs the full sine wave above the zero line but below the max dissipation because distortion). The zero line being crossed over is where the distortion in a PP amp is found as the signal crosses over (crossover distortion).
So if you go for more power, we reach up into distortion in tubes. We lower that power, we get lower distortion in tubes. Somewhere is the best middle ground.
Then there is discussion about biasing methods... and that is where I am kind of lost other than you can use cathode or plate biasing with PP and neither is an indication of the amp being in Class A or AB.
Which one is better? WELL... that depends. Depends on the music, the room, the listener, the speakers. Say all things are equal in the system using Kilpsch Cornwalls (very efficient speakers commercially available) with a CD player (a plain old source as an example so I am not bringing in turntables and then MM or MC carts) through a buffer pre-amp (so we are injecting as little as possible into the signal path, so no DAC, no optical cables, no XLR cables, just plain old RCA cables), in the same room, in the same house. Pick your tube, but the same one for each PP and PSE/SE amp. So, EL84, EL34, KT88, and 6L6 are all good and readily available choices that are still being produced.
So gooberpat, there you have it. More power in PP Class AB. Less power in PSE. I hope that answers your question.
It is left up to the listener and with someone with my own pair of tin ears... Jazz and vocals I like through SE. If I am playing Led Zepplin and want to hear Jimmy Page's drumming or a full symphony orchestra playing classical music, I like that through PP because the details come out. Now if I am out of luck and my EL34 Baby Huey is being borrowed by a friend, I will run my Tubelab Simple SE in UL for that extra kick in volume.
My ears will bleed and the windows will shake because that is a lot of power out of either system through a pair of highly efficient speakers so I had better turn down the volume.
Which is why I will probably in the end just grab my Audio Technica headphones instead and use a CMoy headphone amp running a Burr Brown OpAmp.
Talking about Class A PP topology, but better throw in Class AB. AND WARNING! Don't roast me. I am not an expert here, so correct me and forgive me if I get something wrong. This is what I have gleaned over the many years reading what the designers and experts (George at Tubelab, Nelson Pass, and Pete Millett) here at diyaudio and the nuggets they have shared over the years. The monumental arguments that existed over the craziest things (anyone recall the RH84 fire fight?) is something I don't want.
This is just an info share to the community and I am no where near those legends I named up there, but this is the point of the forum. Share knowledge and be nice.
Class A is so NOT efficient (12% as a maximum was a number I seem to recall floated in a discussion way back). But if the PP is run at Class A going halfway between cutroff and saturation, then each tube in the PP pair only needs to handle half the waveform (180 degrees using two tubes in PP makes 360) as the current goes up in one, the current in the other tube of the pair goes down, so they cancel out in the OPT. These amps as Class A PP in theory will have double the output power as a single tube in SE. The neat thing is PP OPTs can be smaller (and tend to run cooler) because you don't need the air gap as there is little to no core saturation as the net current difference is zero (each side cancels the other side out since they are out of phase).
To get more power, we can choose to run PP in Class AB because it is more efficient use of power because two halves of the signal are on either side of the operating point. Class AB needs a little more than the 180 degrees but less than the full 360 degrees so we don't get distortion at the "top of the range" of the tube (what is the terminology I am looking for?). The tubes are run at lower than the max dissipation and the the full current only is seen at full peak load. This increased idle bias keeps the tube "running at all times, but not like it would be in Class A. So the bias is lower (and colder because less current until demanded). However, less plate current means more distortion because operation is now in a less linear region as you approach the the zero line ("halfway" mark of each pair of PP tubes, where as Class A runs the full sine wave above the zero line but below the max dissipation because distortion). The zero line being crossed over is where the distortion in a PP amp is found as the signal crosses over (crossover distortion).
So if you go for more power, we reach up into distortion in tubes. We lower that power, we get lower distortion in tubes. Somewhere is the best middle ground.
Then there is discussion about biasing methods... and that is where I am kind of lost other than you can use cathode or plate biasing with PP and neither is an indication of the amp being in Class A or AB.
Which one is better? WELL... that depends. Depends on the music, the room, the listener, the speakers. Say all things are equal in the system using Kilpsch Cornwalls (very efficient speakers commercially available) with a CD player (a plain old source as an example so I am not bringing in turntables and then MM or MC carts) through a buffer pre-amp (so we are injecting as little as possible into the signal path, so no DAC, no optical cables, no XLR cables, just plain old RCA cables), in the same room, in the same house. Pick your tube, but the same one for each PP and PSE/SE amp. So, EL84, EL34, KT88, and 6L6 are all good and readily available choices that are still being produced.
So gooberpat, there you have it. More power in PP Class AB. Less power in PSE. I hope that answers your question.
It is left up to the listener and with someone with my own pair of tin ears... Jazz and vocals I like through SE. If I am playing Led Zepplin and want to hear Jimmy Page's drumming or a full symphony orchestra playing classical music, I like that through PP because the details come out. Now if I am out of luck and my EL34 Baby Huey is being borrowed by a friend, I will run my Tubelab Simple SE in UL for that extra kick in volume.
My ears will bleed and the windows will shake because that is a lot of power out of either system through a pair of highly efficient speakers so I had better turn down the volume.
Which is why I will probably in the end just grab my Audio Technica headphones instead and use a CMoy headphone amp running a Burr Brown OpAmp.
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12%? Typical?
Well no, unless you consider the tubes on the low end of efficiency (they are not all the same) and add the filament consumption. You can guess that from mutual conductance roughly speaking. Tubes with lower mutual conductance tend to be less efficient. Then also the type of filament/heater and original application plays a role. Let me say that 12% is not really the average....
The comparison was PP vs PSE. If you stay in class A with the PP, the quiescent condition will be very close if not identical. The output power too. Main difference will be distortion.
Example? Tube 45: let's take 275V/36mA max operation in class A1 (datasheet numbers):
1) you get 4W at 5% THD from PSE using 2.3K load (4.6K per tube),
2) you get 5.5W at 1% from PP but it's marginal class A as the load is just 3.9K plate-to-plate. If run in true class A with 9.2K (4.6K per tube) you will get...4W but distortion can be even lower.
Plate efficiency for 4W power is just over 20% (which is what tube articles and books normally refer to). If throw in filament consumption it becomes 14.65%.
This is low gm tube.....and of course in this case tubes last the same!
If you do the same for 300B in class A you'll get way better efficiency. Better than typical class A solid state, I think.
EL34....SE amplifier...meh...whatever might be the choice. De de gustibus non est disputandum...
In PP, IME, less than 50 mA are bad regardless of class A/class AB or pentode/triode/UL. Bad sound, bad everything except they might just measure ok on the bench (with substantial feedback).
The EL 34 is one of my favourite pentodes to run in triode mode Class A PP: 400V/50mA with a good 10K plate-to-plate transformer, giving a healthy 14-15W at 1% THD without any external feedback. If add feedback, THD up 5-8W can be even better than more powerful EL34 UL or pentode amplifiers in class AB.
Unlike most pentodes, the EL 34 triode Class A PP has rather good plate efficiency at 35-37% and this probably due to it's very good mutual conductance and general construction. It becomes 24-25% considering the heater power. I mean that one gets such efficiency with low distortion too. No compromise.
We always end up more or less there....the only real difference is distortion. For pentodes the difference between PP and PSE is generally bigger, including triode mode. But there are some exceptions there too...just to confirm the rule! Example? 4P1L. In triode mode SE or PSE this is pretty good.
Well no, unless you consider the tubes on the low end of efficiency (they are not all the same) and add the filament consumption. You can guess that from mutual conductance roughly speaking. Tubes with lower mutual conductance tend to be less efficient. Then also the type of filament/heater and original application plays a role. Let me say that 12% is not really the average....
The comparison was PP vs PSE. If you stay in class A with the PP, the quiescent condition will be very close if not identical. The output power too. Main difference will be distortion.
Example? Tube 45: let's take 275V/36mA max operation in class A1 (datasheet numbers):
1) you get 4W at 5% THD from PSE using 2.3K load (4.6K per tube),
2) you get 5.5W at 1% from PP but it's marginal class A as the load is just 3.9K plate-to-plate. If run in true class A with 9.2K (4.6K per tube) you will get...4W but distortion can be even lower.
Plate efficiency for 4W power is just over 20% (which is what tube articles and books normally refer to). If throw in filament consumption it becomes 14.65%.
This is low gm tube.....and of course in this case tubes last the same!
If you do the same for 300B in class A you'll get way better efficiency. Better than typical class A solid state, I think.
EL34....SE amplifier...meh...whatever might be the choice. De de gustibus non est disputandum...
In PP, IME, less than 50 mA are bad regardless of class A/class AB or pentode/triode/UL. Bad sound, bad everything except they might just measure ok on the bench (with substantial feedback).
The EL 34 is one of my favourite pentodes to run in triode mode Class A PP: 400V/50mA with a good 10K plate-to-plate transformer, giving a healthy 14-15W at 1% THD without any external feedback. If add feedback, THD up 5-8W can be even better than more powerful EL34 UL or pentode amplifiers in class AB.
Unlike most pentodes, the EL 34 triode Class A PP has rather good plate efficiency at 35-37% and this probably due to it's very good mutual conductance and general construction. It becomes 24-25% considering the heater power. I mean that one gets such efficiency with low distortion too. No compromise.
We always end up more or less there....the only real difference is distortion. For pentodes the difference between PP and PSE is generally bigger, including triode mode. But there are some exceptions there too...just to confirm the rule! Example? 4P1L. In triode mode SE or PSE this is pretty good.
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directdriver,
Bandersnatch gave a good start to answering your question regarding damping.
He said the tubes plate resistance, rp, varies versus plate current.
So I give more detail:
For an output stage that has no negative feedback, the damping factor is dependent on the rp of the output tube(s), and the output transformer.
Single Ended:
Because rp varies versus plate current, the damping factor is larger for one alternation (one polarity), versus the other alternation (the opposite polarity).
This controls the woofer differently for when the cone is driven out of the box (no pun intended), versus when the cone is driven into the box.
Or another way to put it is the output impedance is non-symmetrical from the positive voltage out, versus the negative voltage out.
Push Pull, Class A:
Output tubes . . . Push tube #1; Pull tube #2.
During positive polarity of the signal, the rp of # 1 tube Increases, while the rp of # 2 tube Decreases.
Then for the negative signal polarity, the rp of # 1 tube Decreases, while the
rp of # 2 tube increases.
With push pull, they are in opposite phase, so as one rp increases, the other tube rp decreases.
And for the positive voltage out, versus the negative voltage out, the total effective rp is Symmetrical.
It varies versus the voltage; but varies the same from a given positive voltage to the same amount of negative voltage.
An example, just for illustration (not literal values),
Output voltage versus total rp:
+1V rp=2k, -1V rp=2k;
+5V rp=1.9k, -5V rp=1.9k;
+10V rp=1.8k, -10V rp =1.8k
Now, if we drive out of Class A, to Class AB:
+15V rp= 2.7k, -15V rp=2.7k
(one output tube is off during those large + and - signal excursions, so we only have the rp of one tube; not both tubes).
But that is also Symmetrical.
Since damping factor is dependent on rp, then damping factor is also symmetrical.
I hope that explains it.
Bandersnatch gave a good start to answering your question regarding damping.
He said the tubes plate resistance, rp, varies versus plate current.
So I give more detail:
For an output stage that has no negative feedback, the damping factor is dependent on the rp of the output tube(s), and the output transformer.
Single Ended:
Because rp varies versus plate current, the damping factor is larger for one alternation (one polarity), versus the other alternation (the opposite polarity).
This controls the woofer differently for when the cone is driven out of the box (no pun intended), versus when the cone is driven into the box.
Or another way to put it is the output impedance is non-symmetrical from the positive voltage out, versus the negative voltage out.
Push Pull, Class A:
Output tubes . . . Push tube #1; Pull tube #2.
During positive polarity of the signal, the rp of # 1 tube Increases, while the rp of # 2 tube Decreases.
Then for the negative signal polarity, the rp of # 1 tube Decreases, while the
rp of # 2 tube increases.
With push pull, they are in opposite phase, so as one rp increases, the other tube rp decreases.
And for the positive voltage out, versus the negative voltage out, the total effective rp is Symmetrical.
It varies versus the voltage; but varies the same from a given positive voltage to the same amount of negative voltage.
An example, just for illustration (not literal values),
Output voltage versus total rp:
+1V rp=2k, -1V rp=2k;
+5V rp=1.9k, -5V rp=1.9k;
+10V rp=1.8k, -10V rp =1.8k
Now, if we drive out of Class A, to Class AB:
+15V rp= 2.7k, -15V rp=2.7k
(one output tube is off during those large + and - signal excursions, so we only have the rp of one tube; not both tubes).
But that is also Symmetrical.
Since damping factor is dependent on rp, then damping factor is also symmetrical.
I hope that explains it.
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That might not be necessarily a bad thing. A sealed box loudspeaker, for example, also works asymmetrically. So, if the woofer is moves out and the amp has better damping is ok. When the woofer moves inwards for sure the air-spring is a better damping system than any amplifier.
45,
Good point.
Many woofers have 2nd Harmonic distortion (many midrange and tweeters do too).
If you like single ended 45 tube and single ended 2A3 tube amplifiers, you may like this experiment that my friend and I did about 40 years ago.
We had a 2A3 single ended amp, no feedback, and a Spica TC-50 speaker.
We wired the amp to Spica TC-50 using a Double Pole, Double Throw switch on the speaker cable.
We used a very low distortion oscillator (-70dBc), and applied a 100Hz signal from the oscillator to the 2A3 amp.
One of us would flip the switch back and forth, the other would listen for a difference in the 100Hz tone.
Then, we would switch places, switch operator, and listener; and listen for difference in the 100Hz tone.
We did not even pay attention to which switch position was 'in phase', and which switch position was 'out of phase'.
The result, yes, there was a difference in the tonality/timbre of the 100Hz tone, depending on the switch position.
This was not a music test, just a pure tone test, and with the resultant 2nd Harmonic distortions of the amplifier and the speaker.
As you can expect, the 2nd harmonic of the amp and speaker either added, or partially cancelled, depending on the switch setting.
You may want to try that test yourself, with a friend.
Oh, one more thing to think about, Air is one of the most linear springs in the world.
I would check the spider, and the cone's outer suspension for linearity; long before I would suspect the Air spring.
And, what if part of the voice coil was out of one end of the magnetic circuit. 2nd Harmonic Distortion.
Imperfectly aligned voice coils, whether over hung or underhung will create 2nd Harmonic Distortion.
So much for many real world speakers.
Good point.
Many woofers have 2nd Harmonic distortion (many midrange and tweeters do too).
If you like single ended 45 tube and single ended 2A3 tube amplifiers, you may like this experiment that my friend and I did about 40 years ago.
We had a 2A3 single ended amp, no feedback, and a Spica TC-50 speaker.
We wired the amp to Spica TC-50 using a Double Pole, Double Throw switch on the speaker cable.
We used a very low distortion oscillator (-70dBc), and applied a 100Hz signal from the oscillator to the 2A3 amp.
One of us would flip the switch back and forth, the other would listen for a difference in the 100Hz tone.
Then, we would switch places, switch operator, and listener; and listen for difference in the 100Hz tone.
We did not even pay attention to which switch position was 'in phase', and which switch position was 'out of phase'.
The result, yes, there was a difference in the tonality/timbre of the 100Hz tone, depending on the switch position.
This was not a music test, just a pure tone test, and with the resultant 2nd Harmonic distortions of the amplifier and the speaker.
As you can expect, the 2nd harmonic of the amp and speaker either added, or partially cancelled, depending on the switch setting.
You may want to try that test yourself, with a friend.
Oh, one more thing to think about, Air is one of the most linear springs in the world.
I would check the spider, and the cone's outer suspension for linearity; long before I would suspect the Air spring.
And, what if part of the voice coil was out of one end of the magnetic circuit. 2nd Harmonic Distortion.
Imperfectly aligned voice coils, whether over hung or underhung will create 2nd Harmonic Distortion.
So much for many real world speakers.
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Harmonic distortion from loudspeakers is not a big problem. At least it is in much lower league in comparison to amplifiers. More so if 2nd harmonic....GEEDLEE published an article on objective perception of distortion from loudspeakers.
However all the issues about loudspeakers are generally amplified with (symmetrical) high damping factor amplifiers. They are not diminished. The more worrying non-linear behaviour of loudspeakers gets worse with low Zout amps.
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