Why use a FET when a 6922 can push pull 30 watts with a pair of mje13007 bjt transistors
Triode curves are outstanding to 200 ma current.
Triode curves are outstanding to 200 ma current.
Took a little google fu, but I got https://www.diyaudio.com/community/threads/3-x-3-tube-amp-30-watts.268043/
LF distortion has nothing to do with radiator size relative to the wavelength. it’s solely determined by the linearity of the volume displacement of the air moved.
Saturation : yes it reduces hysteresis but only painfully little for steel. however, a fully charged permanent magnet is so saturated that hysteresis is practically gone. Hence esoteric iron free drivers with magnet only motors.
The flux modulation in a conventional motor will, however, drive the steel out of saturation when current goes in one direction in the coil which leads to gross distortion. This does not happen for the PTT motor with strongly suppressed flux modulation.
Field coil: beyond driving the flux it acts as a giant shorting ring as explained by Cunningham in 1949 and this reduces the hysteresis by shorting the AC field and n the steel
All distortion mechanisms get worse for long stroke but are fundamental for all drivers. the solution can be optimised for and performance gained at any stroke.
Saturation : yes it reduces hysteresis but only painfully little for steel. however, a fully charged permanent magnet is so saturated that hysteresis is practically gone. Hence esoteric iron free drivers with magnet only motors.
The flux modulation in a conventional motor will, however, drive the steel out of saturation when current goes in one direction in the coil which leads to gross distortion. This does not happen for the PTT motor with strongly suppressed flux modulation.
Field coil: beyond driving the flux it acts as a giant shorting ring as explained by Cunningham in 1949 and this reduces the hysteresis by shorting the AC field and n the steel
All distortion mechanisms get worse for long stroke but are fundamental for all drivers. the solution can be optimised for and performance gained at any stroke.
Your proposing is hazy and clear as mud. What are you proposing - besides theoretical, isolated from reality, fairy-tales about current drive no-feedback pentode/tetrode amplifier? How will work all that amplifier+loudspeaker system? Please answer these questions:
1. What type of loudspeaker will be connected to that current drive amplifier: single fullrange driver, two-way or three/four way?
2. If the loudspeaker is two or three way, it must use passive or active crossover. I have never heard of passive crossover which will work with a current drive amplifier - can you post here schematic of such two-way passive crossover, with values of L , C and R components adequate for some crossover frequency, for example at 2 kHz?
3. What kind of speaker/driver will be connected to that current drive amplifier: subwoofer, woofer, mid-bass speaker, midrange or tweeter? Can you point to some examples (brand and model) of speakers/drivers which will work nice with current drive amplifier? Can you post here distortion measurements of complete system comprising current drive amplifier plus loudspeaker, for example at SPL=95dB/1m (or 100dB/1m)?
4. If the speaker/driver is subwoofer, woofer or mid-bass, what kind of enclosure are you proposing - sealed, vented (including TL), horn or open baffle? Which one will work best with the current drive amplifier? Can you post measurements of low frequency response of such loudspeaker (box) optimized for current drive amplifier?
So? It is the same with any other loudspeaker optimized for current drive, isn't it?
Wrong. Physically it is not possible for speaker effective radiator size to be comparable to the wavelength which it reproduces - for example, for F=40Hz wavelength is 860cm (slightly over 338 inches). How it is comparable to a big 18-inch woofer? 18-inch woofer is almost 19 times smaller (magnitude smaller!) than that wavelength!
In reality, woofer ability to handle low frequencies loud is determined only by the displacement volume = Sd x Xmax (effective driver area x maximum linear cone displacement).
No, it is confirmed by science - human hearing is much less sensitive to distortion at low frequencies compared to midrange frequencies.
You forget the importance of Hoffman's Iron Law - if you want both high efficiency and ability to reproduce deep bass, than the loudspeaker enclosure volume must be huge. Not everyone want huge loudspeaker boxes in a room. It is not a problem of Purifi drivers only, it is a problem which relates equally to loudspeakers optimized for current drive.
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I see a lot of work has been done to accommodate that portion of the speaker market.
The philosophy is important. In speaker building orthodoxy, amplifiers are fungible as long as they have the same low distortion and high damping factor. If measured distortion is low, then perceived sound quality MUST be high. Flat frequency response is the most important measure of sound quality. Capacitors and inductors do not affect the quality of sound.
On this forum, most participants have different views. We know that different amplifiers sound different. We know that amplifiers that don't measure well may nevertheless sound better than those that do measure well. We know that capacitors affect sound. In Lynn Olson's blog, a single capacitor in the signal path makes such notable coloration that the amplifier topology has to be abandoned in favor of another topology that would exclude the capacitor. When I look at capacitor pileups in speaker level crossovers, my hair rise in horror.
I don't barge in with my philosophy into a speaker forum and bomb it by declaring that what they are doing is nonsense. Let's agree that we have disagreements and move on in pursuit of our different paths.
On this forum, most participants have different views. We know that different amplifiers sound different. We know that amplifiers that don't measure well may nevertheless sound better than those that do measure well. We know that capacitors affect sound. In Lynn Olson's blog, a single capacitor in the signal path makes such notable coloration that the amplifier topology has to be abandoned in favor of another topology that would exclude the capacitor. When I look at capacitor pileups in speaker level crossovers, my hair rise in horror.
I don't barge in with my philosophy into a speaker forum and bomb it by declaring that what they are doing is nonsense. Let's agree that we have disagreements and move on in pursuit of our different paths.
How about using two speakers (for LF end), front to front, with a thin spacer ring between. Set in a cabinet. Then drive the rear speaker with a little more amplitude (inverted so as to be additive) so as to lower the back pressure on the front speaker. The front speaker mostly working easily, so as to lower distortion, just putting the final touch on the radiated air.
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that is close what is known as isobaric configuration. Normally two identical woofers are used. It is a thing of the past when motors and magnets were not so strong: the two motors work in parallel because the two drivers are connected with a small air volume. Consequently the isobaric compound acts as a single driver with twice the moving mass and twice the suspension stiffness. The motor strength doubles as well but due to the doubled mass and unchanged cone are Sd we get the same sensitivity. The isobaric compound is just more suited to work in a smaller box. Again, a modern driver choice would simply use a stronger and heavier motor to achieve the same with lower cost and complexity
From June 8 to June 15 (8 days) this very new thread with 150 Posts, has covered a lot of ground and a lot of subjects.
. . . All Very interesting.
An experiment for all of you:
How many of you have taken either a medium to high damping factor tube amplifier (or a solid state amplifier), and connected a resistor between the amplifier hot (+) tap to the speaker hot (+) tap. And just connect the amplifier Common (-) tap to the speaker Common (-) tap to each other.
(it does not matter which polarity connection that you use to connect the resistor in series with the speaker lead).
For example, with an "8" Ohm speaker, first try using a 4 or 5 Ohm resistor, and then try a 15 or 16 Ohm resistor, and then a 30 or 33 Ohm resistor.
Use similar speaker Z to resistor ratios, if your speaker is "4", "6", "12", or "16" Ohm rated.
Start with a moderate sound level with no series resistor.
Turn the up the volume when you use series resistors, to keep the listeng level about the same (according to the series resistor power loss).
Then, for each series resistance, . . . Listen . . . Listen . . . Listen.
Hint:
Do Not be critical, and do not be analytical, just sit back, listen, and enjoy.
After those first listens, then repeat with more critical and analytical listening.
You may be surprised.
. . . All Very interesting.
An experiment for all of you:
How many of you have taken either a medium to high damping factor tube amplifier (or a solid state amplifier), and connected a resistor between the amplifier hot (+) tap to the speaker hot (+) tap. And just connect the amplifier Common (-) tap to the speaker Common (-) tap to each other.
(it does not matter which polarity connection that you use to connect the resistor in series with the speaker lead).
For example, with an "8" Ohm speaker, first try using a 4 or 5 Ohm resistor, and then try a 15 or 16 Ohm resistor, and then a 30 or 33 Ohm resistor.
Use similar speaker Z to resistor ratios, if your speaker is "4", "6", "12", or "16" Ohm rated.
Start with a moderate sound level with no series resistor.
Turn the up the volume when you use series resistors, to keep the listeng level about the same (according to the series resistor power loss).
Then, for each series resistance, . . . Listen . . . Listen . . . Listen.
Hint:
Do Not be critical, and do not be analytical, just sit back, listen, and enjoy.
After those first listens, then repeat with more critical and analytical listening.
You may be surprised.
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Yes, you did exactly that - just remind you what you wrote about Purifi speakers:
You may say this:
but we didn't understand at all - what is your path? You didn't explain how current drive will work with real-world loudspeakers?
I posted (#145) several logical and practical questions about that, but you didn't answer. Too hot to handle?
I will ask again:
I tried that many years ago, and I wasn't surprised at all: ugly, bloated, filthy, imprecise, one-note bass... plus disgustingly loud, exaggerated portion of high midrange spectra.
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I am thinking of a 3-band system, the bands approximately covering 3 decades, 20-100, 100-1,000, and 1,000-20,000 Hz. The boundaries are not firmly fixed; they will be dictated by driver characteristics.
For the top band, my choice is Bohlender Graebener Neo-8 or Neo 10. Their advantage as a load for no-feedback pentode amplifier is constant and purely resistive impedance across the band. They are good for open baffle dipoles, the kind of speakers that I prefer. They have low measured distortion. Like electrostats, they have very low moving mass, and they sound to me like electrostats - in comparison with my electrostatic Stax SR3. After trying a number of different high frequency drivers, I came to these ones and stopped looking any further. They fully satisfy me. I'd like to have a driver like Neo-8 covering both mid and high frequencies, but, alas, such drivers do not exist. So, the mid driver is to be a conventional speaker.
The mid driver should be something with the lowest moving mass, lowest voice coil inductance, and lowest mechanical damping factor that I could find.
The low driver I would like to discuss elsewhere.
For the top band, my choice is Bohlender Graebener Neo-8 or Neo 10. Their advantage as a load for no-feedback pentode amplifier is constant and purely resistive impedance across the band. They are good for open baffle dipoles, the kind of speakers that I prefer. They have low measured distortion. Like electrostats, they have very low moving mass, and they sound to me like electrostats - in comparison with my electrostatic Stax SR3. After trying a number of different high frequency drivers, I came to these ones and stopped looking any further. They fully satisfy me. I'd like to have a driver like Neo-8 covering both mid and high frequencies, but, alas, such drivers do not exist. So, the mid driver is to be a conventional speaker.
The mid driver should be something with the lowest moving mass, lowest voice coil inductance, and lowest mechanical damping factor that I could find.
The low driver I would like to discuss elsewhere.
Mid driver size
The mid driver should be preferably operated between its two resonant frequencies, Fs and F1 (first cone breakup). The latter is the limit of piston range. In this area driver's distortion is at its lowest.
For lightweight paper cone drivers (surround one piece with cone), Fs and F1 are largely determined by speaker diameter (although deviations are possible). The smaller diameter, the higher these frequencies, with other things equal. For a 12" driver, the two frequencies are around 50 and 1,000 Hz. I believe 12" is the optimal size for the band of 120-800 Hz. The lower and the upper limits are away from resonances. In a 8" driver, piston range is more extended, but low crossover point must be raised. 12" is the best match for Neo-8 if one wants to take full advantage of Neo-8's low frequency capability.
The mid driver should be preferably operated between its two resonant frequencies, Fs and F1 (first cone breakup). The latter is the limit of piston range. In this area driver's distortion is at its lowest.
For lightweight paper cone drivers (surround one piece with cone), Fs and F1 are largely determined by speaker diameter (although deviations are possible). The smaller diameter, the higher these frequencies, with other things equal. For a 12" driver, the two frequencies are around 50 and 1,000 Hz. I believe 12" is the optimal size for the band of 120-800 Hz. The lower and the upper limits are away from resonances. In a 8" driver, piston range is more extended, but low crossover point must be raised. 12" is the best match for Neo-8 if one wants to take full advantage of Neo-8's low frequency capability.
OK, thanks for your reply to some of my questions from my posts #145.
I concur Bohlender-Graebener Neo-8 is a very good driver, I used it in a loudspeaker which I made for my friend: Neo-8PDR in a small open baffle, crossed at 800Hz to ScanSpeak 6.5" in a vented box.
But what is the benefit of current drive (no-feedback pentode) amplifier for Neo-8, over using conventional voltage-drive solid-state amp with negative feedback for the same driver? In what way current drive will be better? I can't see why it will be better (or different), because Neo-8 impedance is flat as a ruler!
But we know what will be worse: H3= 0.8% distortion coming from the pentode amp alone, versus THD=0.001% distortion coming from the solid-state amplifier. Distortion form the Neo-8 alone will be the same with both amplifiers!
And for the 12" midrange driver - did you make distortion measurements for such driver connected to current drive pentode amplifier, at SPL=95dB/1m or similar SPL level? Is this 12" midrange going to have H3 distortion lower than 0.1% at SPL=95dB/1m? Just to remind you, Purifi (and ScanSpeak) have H3=0.1% at SPL=95dB/1m!
I concur Bohlender-Graebener Neo-8 is a very good driver, I used it in a loudspeaker which I made for my friend: Neo-8PDR in a small open baffle, crossed at 800Hz to ScanSpeak 6.5" in a vented box.
But what is the benefit of current drive (no-feedback pentode) amplifier for Neo-8, over using conventional voltage-drive solid-state amp with negative feedback for the same driver? In what way current drive will be better? I can't see why it will be better (or different), because Neo-8 impedance is flat as a ruler!
But we know what will be worse: H3= 0.8% distortion coming from the pentode amp alone, versus THD=0.001% distortion coming from the solid-state amplifier. Distortion form the Neo-8 alone will be the same with both amplifiers!
And for the 12" midrange driver - did you make distortion measurements for such driver connected to current drive pentode amplifier, at SPL=95dB/1m or similar SPL level? Is this 12" midrange going to have H3 distortion lower than 0.1% at SPL=95dB/1m? Just to remind you, Purifi (and ScanSpeak) have H3=0.1% at SPL=95dB/1m!
Because Neo-8 is purely resistive, it can be driven equally well with current or voltage drive. But for the conventional driver paired with it, current drive will be beneficial.
I have neither equipment, nor intention to measure harmonic distortion of the 12" driver. What I am going to use sounds much better than the 6.5" Scan Speak midbass {to my ears, of course}, so I don't care if it has higher measured HD. If it does, current drive will be helpful.
I don't subscribe to the belief that low THD equals good sound. At least not low THD achieved by applying negative feedback to an amplifier that has high THD in open loop, like most transistor amplifiers. But achieving low open loop THD is a worthwhile goal.
I have neither equipment, nor intention to measure harmonic distortion of the 12" driver. What I am going to use sounds much better than the 6.5" Scan Speak midbass {to my ears, of course}, so I don't care if it has higher measured HD. If it does, current drive will be helpful.
I don't subscribe to the belief that low THD equals good sound. At least not low THD achieved by applying negative feedback to an amplifier that has high THD in open loop, like most transistor amplifiers. But achieving low open loop THD is a worthwhile goal.
I did something similar actually. The sound is very nice on the first impression. Then I realized that the bass is "boomy" and unbearable on the long run.
Output impedance measurements
Reason for applying GNFB
Initially I was against global NFB, but I am converted after listening a couple of LPs. Loudspeakers are designed to run from constant voltage, anyway.
Even back in the early '80s, I heard that it's better to design a good open loop response, use a little feedback to make it even better, than it is to have a design with a not-so good open loop response and use a lot of feedback to make it acceptable. This was in the context of how "musical" an amplifier sounded.
Along a similar vein, I can appreciate the effects of lesser amplifier damping on a speaker that's only playing well above its resonant frequency. It's something to do with grip; the action of the global negative feedback in the first case, the action of the speaker damping factor against the back emf of the driver in the second. Which interacts with the negative feedback part.
So, if you had both a minimally fedback amplifier along with current output.... Each driver has its own amplifier and DSP generated FR bandwidth... I can understand how this would be a fun thing to try! Less interaction between the driver's back emf and the amplifier feedback behavior.
The Dynaco A-25 had a ported enclosure with acoustically resistive damping in the port.
The KLH 33 had resistive damping over the port.
There are speakers that do not need amplifiers with high damping factors to keep the bass from booming.
And there are speakers that have fairly constant impedance throughout the audio frequency range.
The trick in designing a good system is the compatibility and synergy of the various components.
CD player; or phono cartridge, tone arm, and turntable;
Preamp
Power amp
Speakers
Room
etc.
There are perhaps more bad combinations of the above, than there are good combinations of the above.
All generalizations have exceptions.
The KLH 33 had resistive damping over the port.
There are speakers that do not need amplifiers with high damping factors to keep the bass from booming.
And there are speakers that have fairly constant impedance throughout the audio frequency range.
The trick in designing a good system is the compatibility and synergy of the various components.
CD player; or phono cartridge, tone arm, and turntable;
Preamp
Power amp
Speakers
Room
etc.
There are perhaps more bad combinations of the above, than there are good combinations of the above.
All generalizations have exceptions.
Many are - including the Purifi drivers (see post #129). Others may not be designed with low output impedance in mind and might benefit from current drive.