Woofertester,
One of the first things I would be looking at would be a full length Faraday or shorting ring in the gap, this would lower the back emf value significantly to start your process. Any modern driver that is not using the advantage of the shorting ring this way, especially a long excursion driver would not be on my list of drivers for that type of application. Just something to keep in mind. no recommendations given on particular drivers.
One of the first things I would be looking at would be a full length Faraday or shorting ring in the gap, this would lower the back emf value significantly to start your process. Any modern driver that is not using the advantage of the shorting ring this way, especially a long excursion driver would not be on my list of drivers for that type of application. Just something to keep in mind. no recommendations given on particular drivers.
I was tired to be fired in an aggressive way at each word i printed and have to argue about everything. As it happens each time you try to talk about some stuff not in school books and share your experience with it.
I will write an article on this subject in my web site. it will be faster and easier to read. I will PM-you when done. For personal reasons, i wait for the contribution of the guy which had make-me discovered-it, but he is busy.
Yes, you are right, Kindhornman, it is directly correlated and even requested with current source amplifiers.
But you are obliged, with current source amp, if you want to keep a flat curve response, don't you ?
Well, i think we begin to be seriously off-topic, here.
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Christophe,
Thank you for the reply. I have a friend who insists that I do just what you are talking about. He has done what you are talking about for years now but trying to get all the details from him is not the easiest thing to do! I have had many long conversations with him about conjugate networks and he has it rather easy as he has decade boxes for all the components so can quickly zoom in on the solution for each speaker system he works on. I will look forward to your paper on these tank circuits and how to mathematically or electrically find all three LCR values.
ps. One of the major factors besides the EMF reduction is that he says the feedback circuit in the amplifier is directly impacted by this flattening of the impedance curve and the elimination of any reactive component hanging on the outside of the feedback loop. Basically a node outside the feedback circuit that can not be controlled by negative feedback causing a hidden distortion factor that is most often overlooked or unknown by most.
Thank you for the reply. I have a friend who insists that I do just what you are talking about. He has done what you are talking about for years now but trying to get all the details from him is not the easiest thing to do! I have had many long conversations with him about conjugate networks and he has it rather easy as he has decade boxes for all the components so can quickly zoom in on the solution for each speaker system he works on. I will look forward to your paper on these tank circuits and how to mathematically or electrically find all three LCR values.
ps. One of the major factors besides the EMF reduction is that he says the feedback circuit in the amplifier is directly impacted by this flattening of the impedance curve and the elimination of any reactive component hanging on the outside of the feedback loop. Basically a node outside the feedback circuit that can not be controlled by negative feedback causing a hidden distortion factor that is most often overlooked or unknown by most.
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Only the main resonances of the drivers are subject to be electrically damped by voltage driving. This relates to very basic theory.
Otherwise, current driving have beneficial effects on non-linear distortions whatever are their origin.
Anybody interested by details should read :
Current-Drive - The Natural Way of Loudspeaker Operation
There are definite advantages of using current drive to speakers Distortion Reduction in Moving-Coil Loudspeaker Systems Using Current-Drive Technology .. but also disadvantages.
The biggest is that with present materials, there is no sensible way to design current drive speaker units with flat response.
There are good reasons why Rice & Kellog's invention Notes on the Development of a New Type of Hornless Loudspeaker is pre-eminent today ... from the cheapest to the very highest quality.
It relies on voltage drive and mass control ... and with it, they invented bass response. The TS pseudo gurus would do well to read this as it has insights which escape most of these wannabes.
No present commercial speaker unit is designed to work well with current drive.
A minor (??) disadvantage is that alas "Only the main resonances of the drivers are subject to be electrically damped by voltage driving" isn't true. Current drive exaggerates practically all resonances including those introduced by cabinet walls vibrating .. in fact every little wriggle that appears on an impedance curve. eg all the stuff John Atkinson measures in Stereophile.
However, in da 21st century we can do stuff that Rice & Kellog couldn't. I posted this in another forum when I emerged from the bush in 2006
The biggest is that with present materials, there is no sensible way to design current drive speaker units with flat response.
There are good reasons why Rice & Kellog's invention Notes on the Development of a New Type of Hornless Loudspeaker is pre-eminent today ... from the cheapest to the very highest quality.
It relies on voltage drive and mass control ... and with it, they invented bass response. The TS pseudo gurus would do well to read this as it has insights which escape most of these wannabes.
No present commercial speaker unit is designed to work well with current drive.
A minor (??) disadvantage is that alas "Only the main resonances of the drivers are subject to be electrically damped by voltage driving" isn't true. Current drive exaggerates practically all resonances including those introduced by cabinet walls vibrating .. in fact every little wriggle that appears on an impedance curve. eg all the stuff John Atkinson measures in Stereophile.
However, in da 21st century we can do stuff that Rice & Kellog couldn't. I posted this in another forum when I emerged from the bush in 2006
>For what it is worth, I think this is impossible. It seems to violate the cause and effect principle. How could the amplifier "know" what the speaker is up to? Short of sensing the actual motion of the drivers or the actual sound... this has to be sort of by guess and by golly, it seems to me.
It is entirely possible for an "amplifier" to know what the speaker is doing.
It needs to sense the speaker current. An amp which twiddled its Output Z using both current & voltage feedback does this. Speakers act as accurate microphones (sense the actual sound) if operated into Low Z.
See "Loudspeakers as Microphones" - Peter Baxandall special lecture London AES (early 80s, late 70s?)
If operated into High Z, then the voltage at the terminals is a measure of cone velocity.
Both these mechanisms obey superposition & Thevenin so if you're clever, you can look at this while the amp is giving zillion volts and amps to the speaker. But non est tantum facile.
There are several tried & tested methods of using this "controlled output Z" or "current + voltage feedback" or "actual sound & motion feedback" (different descriptions of the same thing) if you incorporate the amplifier design in the speaker. Some of these are in the zanier incarnations of my Powered Integrated Super Sub technology.
The simplest is the negative output R that Fons mentions.
More sophisticated but similar (??!) is ACE technology by Erik Stahl which was used by Audio Pro, Sweden for subs. Unfortunately, since he left, there isn't anyone there who understands it. Anyone have a contact for Erik? Or a clean copy of his original AES preprint?
These methods have the distortion reduction and dynamic overload protection features discussed in Mills & Hawksford. However, they are badly affected by heating of the voice coil.
David Birt (?) did an excellent IoA paper at Windermere where he arranged speaker and amp in a bridge so he could measure and compensate for heating on the fly. Anyone have an email for him?
These are the most elegant methods and they can be analysed from many viewpoints. Some of these viewpoints don't show up the distortion reduction advantages clearly.
I'm contemptous of methods which rely on extra transducers or extra windings (like Mills) or zillion point DSP EQ especially if they don't give ALL the advantages of the elegant methods.
A brute force zillion point approach possible today is measure accurately speaker Z (not that easy) and tailor the output of a High Z amp to suit. This would give some but not all the advantages of the above systems cos it wouldn't "know" what the speaker is up to.
It is entirely possible for an "amplifier" to know what the speaker is doing.
It needs to sense the speaker current. An amp which twiddled its Output Z using both current & voltage feedback does this. Speakers act as accurate microphones (sense the actual sound) if operated into Low Z.
See "Loudspeakers as Microphones" - Peter Baxandall special lecture London AES (early 80s, late 70s?)
If operated into High Z, then the voltage at the terminals is a measure of cone velocity.
Both these mechanisms obey superposition & Thevenin so if you're clever, you can look at this while the amp is giving zillion volts and amps to the speaker. But non est tantum facile.
There are several tried & tested methods of using this "controlled output Z" or "current + voltage feedback" or "actual sound & motion feedback" (different descriptions of the same thing) if you incorporate the amplifier design in the speaker. Some of these are in the zanier incarnations of my Powered Integrated Super Sub technology.
The simplest is the negative output R that Fons mentions.
More sophisticated but similar (??!) is ACE technology by Erik Stahl which was used by Audio Pro, Sweden for subs. Unfortunately, since he left, there isn't anyone there who understands it. Anyone have a contact for Erik? Or a clean copy of his original AES preprint?
These methods have the distortion reduction and dynamic overload protection features discussed in Mills & Hawksford. However, they are badly affected by heating of the voice coil.
David Birt (?) did an excellent IoA paper at Windermere where he arranged speaker and amp in a bridge so he could measure and compensate for heating on the fly. Anyone have an email for him?
These are the most elegant methods and they can be analysed from many viewpoints. Some of these viewpoints don't show up the distortion reduction advantages clearly.
I'm contemptous of methods which rely on extra transducers or extra windings (like Mills) or zillion point DSP EQ especially if they don't give ALL the advantages of the elegant methods.
A brute force zillion point approach possible today is measure accurately speaker Z (not that easy) and tailor the output of a High Z amp to suit. This would give some but not all the advantages of the above systems cos it wouldn't "know" what the speaker is up to.
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... Including membrane fractioning. Thanks for confirming this, Richard
Any waterfall we can do, playing with serial resistances will show this at evidence.
This is obvious and *basic* (i can be impolite too ;-) on an electrical point of view, as P=V²/R. When impedance (R) increase at some frequency, with constant V (voltage source), the electrical power decrease. While, with current sources, P=RI², with constant I, the electrical power increase with R. ("exaggerates practically all resonances")
In the same time, any unwanted movement of the moving coil (even due to cabinet walls vibrations) will produce EMF. Shorting the voice coil across a low impedance voltage amp output will produce a current absorbed by the amp in opposition to this move. hence damping.
Easy to verify: hit your speaker with your finger with no cable connected or a current source, and do the same with a short circuit at their connector or any voltage amp connected.
About distortions, it seems people is too much focused on this, nowadays, we have to look at all the other side points as well.
Thanks too, Richard, to had enlighten impedance curve, the easiest way to know near everything from a given speaker at first sight: where and how it resonates, where it begin to fractionate and how much it will bring accidents to the response curve.
On my side, i had played a lot with all this long time ago, at the morning of my professional life, and gave-it up about all exotic attempts. It seems loudspeakers have a lot of ego and too much personality. They don't like much to be forced (servos) while we don't like them to impose their own personality. The only authority they accept with grace seems 0 impedance sources and acoustic charges
Again, we are far from the subject of this topic: VSSA is a voltage low impedance source, like near all amplifiers, it is well as it, and I'm sure our speakers and our ears will love it a lot.
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Kgrlee
There are definite advantages of using current drive to speakers Distortion Reduction in Moving-Coil Loudspeaker Systems Using Current-Drive Technology .. but also disadvantages.
Current drive is not easy at all and is reserved to some diyers who perfectly understand the operating mechanisms of loudspeakers. But, bar the lack of damping of the main resonance, it has many advantages.
First of all, you get rid of the "cables and connectors sounds".
I discover the comfort of current driving in the lab when trying to measure thermal data of loudspeakers. I understand why WooferTester (BTW, I own a WTpro) is interested in having a current driving amp.
With current drive, the high frequency response is extended, the effects of the voice-coil inductance, which are not benign even at low frequencies, vanish (I do not think that thermal effects in voice coils are very important at least in domestic loudspeakers).
More sophisticated but similar (??!) is ACE technology by Erik Stahl which was used by Audio Pro, Sweden for subs. Unfortunately, since he left, there isn't anyone there who understands it. Anyone have a contact for Erik? Or a clean copy of his original AES preprint?
Erik Stahl's technlogy is used by Yamaha since very long.
Some people here have experimented it. Just as current driving or servoing, it demands real skills in loudspeakers and electronics.
Esperado,
any unwanted movement of the moving coil (even due to cabinet walls vibrations) will produce EMF.
Any unwanted movement generating EMF will conjugate with the initial amp output voltage and then will modulate the current through the voice-coil, and then modulate the force on the moving part of the driver. This does not happen with current driving.
Shorting the voice coil across a low impedance voltage amp output will produce a current absorbed by the amp in opposition to this move. hence damping.
Easy to verify: hit your speaker with your finger with no cable connected or a current source, and do the same with a short circuit at their connector or any voltage amp connected.
This is very basic and only partially true as the test is only valid in the resonance region. If the hit could be very short, voltage or current drive, it won't change anything for the damping at high frequencies. But the current through the voice coil is affected by the induced EMF with voltage drive.
Distorsion
I have long been puzzled by the reason for which it has been often measured that current driving decreases non-linear distortions of loudspeaker.
I think one argument is that in current drive, the force f applied by the voice-coil on the cone if proportional to B
Fcd = B.l.i
as in voltage drive, the force is proportional to B squared
Fvd = B².l².i
As B is not linear with the voice-coil position, the non-linearity of the force is higher with voltage drive.
A second fact is that, when the voice-coil goes away from its central position, B diminished, and also the instantaneous back-EMF, leading to an increase in the instantaneous current i.
The diminishing B and the increasing i do not compensate each other, this can be at the origin of what is sometimes called the "loudspeaker instability".
With current drive, the effect of variations of B are less important.
Some solutions using servos have been proposed to circumvent the lack of the damping of the low frequency resonance with current drive (Greiner and Sims, Hawksford) but they do not seem to have exceeded the experimental stage. I fear that a good simple electrical solution is not to emerge soon. It belongs to manufacturers to linearize the drivers under voltage drive as far as they can.
There are definite advantages of using current drive to speakers Distortion Reduction in Moving-Coil Loudspeaker Systems Using Current-Drive Technology .. but also disadvantages.
Current drive is not easy at all and is reserved to some diyers who perfectly understand the operating mechanisms of loudspeakers. But, bar the lack of damping of the main resonance, it has many advantages.
First of all, you get rid of the "cables and connectors sounds".
I discover the comfort of current driving in the lab when trying to measure thermal data of loudspeakers. I understand why WooferTester (BTW, I own a WTpro) is interested in having a current driving amp.
With current drive, the high frequency response is extended, the effects of the voice-coil inductance, which are not benign even at low frequencies, vanish (I do not think that thermal effects in voice coils are very important at least in domestic loudspeakers).
More sophisticated but similar (??!) is ACE technology by Erik Stahl which was used by Audio Pro, Sweden for subs. Unfortunately, since he left, there isn't anyone there who understands it. Anyone have a contact for Erik? Or a clean copy of his original AES preprint?
Erik Stahl's technlogy is used by Yamaha since very long.
Some people here have experimented it. Just as current driving or servoing, it demands real skills in loudspeakers and electronics.
Esperado,
any unwanted movement of the moving coil (even due to cabinet walls vibrations) will produce EMF.
Any unwanted movement generating EMF will conjugate with the initial amp output voltage and then will modulate the current through the voice-coil, and then modulate the force on the moving part of the driver. This does not happen with current driving.
Shorting the voice coil across a low impedance voltage amp output will produce a current absorbed by the amp in opposition to this move. hence damping.
Easy to verify: hit your speaker with your finger with no cable connected or a current source, and do the same with a short circuit at their connector or any voltage amp connected.
This is very basic and only partially true as the test is only valid in the resonance region. If the hit could be very short, voltage or current drive, it won't change anything for the damping at high frequencies. But the current through the voice coil is affected by the induced EMF with voltage drive.
Distorsion
I have long been puzzled by the reason for which it has been often measured that current driving decreases non-linear distortions of loudspeaker.
I think one argument is that in current drive, the force f applied by the voice-coil on the cone if proportional to B
Fcd = B.l.i
as in voltage drive, the force is proportional to B squared
Fvd = B².l².i
As B is not linear with the voice-coil position, the non-linearity of the force is higher with voltage drive.
A second fact is that, when the voice-coil goes away from its central position, B diminished, and also the instantaneous back-EMF, leading to an increase in the instantaneous current i.
The diminishing B and the increasing i do not compensate each other, this can be at the origin of what is sometimes called the "loudspeaker instability".
With current drive, the effect of variations of B are less important.
Some solutions using servos have been proposed to circumvent the lack of the damping of the low frequency resonance with current drive (Greiner and Sims, Hawksford) but they do not seem to have exceeded the experimental stage. I fear that a good simple electrical solution is not to emerge soon. It belongs to manufacturers to linearize the drivers under voltage drive as far as they can.
I can confirm that he has, because today I received one in the mailbox
.His packing skills are marvelous BTW., thank you LC.
Regards
Marko
Hello forr. The current source I am trying to make will be driving a speaker connected to a WTPro. I hope to get better BL vs Xm curves with a current source power amp. The current source WT2 produces extremely accurate results for BL, Qt etc at 3mA. I am investigating essentially making the WTPro into a WT2 but at 4A to 5A.
The VSSA is attractive as a platform because of its simplicity, high bandwidth and the design is open source so DIY WTPro users can make their own amp if they desire. I have a channel of VSSA working on a breadboard and will be making the modification to current source and measuring some speakers soon.
Bandwidth is not terribly important. At 4A there is not much of interest above 100 Hz.
The original WT was a DIY endeavor. I made the first one for me in the early 1990s to replace a pile of test equipment. It is not surprising that most WT customers are DIY people.
The VSSA is attractive as a platform because of its simplicity, high bandwidth and the design is open source so DIY WTPro users can make their own amp if they desire. I have a channel of VSSA working on a breadboard and will be making the modification to current source and measuring some speakers soon.
Bandwidth is not terribly important. At 4A there is not much of interest above 100 Hz.
The original WT was a DIY endeavor. I made the first one for me in the early 1990s to replace a pile of test equipment. It is not surprising that most WT customers are DIY people.
Thanks for the suggestions. I am stuck measuring what is made by others. I am agnostic about construction. I just have to be able to test. I am investigating using VSSA to solve a problem with back emf as per my earlier post of the loudspeaker current scope shot.
I hold forth that most DIY speaker builders would be appalled if they saw a scope trace of the current in the voice coil of their favorite driver at Xmax using a voltage amplifier.
Wharfedale made a speaker for Yamaha based on this technology. They were surprised to find that we knew more about their system than they themselves.
This was the most satisfying OEM project I was ever involved in. Most OEM projects are about getting another 2c from the cost. Yamaha were as concerned as we were to get the best sound.
They brought Nakamura (spelling?), the designer of the NS1000, as head of the team. By this time he was very old and didn't really understand this yucky electronic stuff. But the rest of the young team deferred to him in all things.
One of the proudest moments in my previous life was when we showed them our treble test booth which was the ancestor of the tech. which later went into ClioQC. It showed the day's production as a real time 2 sigma band around the 'Reference Unit' response.
They obviously didn't believe the consistency and there was a big discussion in Japanese. Then their spokesman said to me, "Mr. Lee, please confirm this is 2 standard deviations." When he relayed my answer to the team, Nakamura led his team to bow.
___________
Woofertester, have you got a link to WT?
From what you say, it appears this obtains da evil TS parameters at high level. At Wharfedale and Celestion, we were quite concerned to do this at very small levels to avoid the inaccuracies at high level. Julian Wright did a lot of work on this.
What's your opinion of TS at high & low level?
Maybe move this to another thread.
kgrlee,
I have often wondered about all the testing being done at 1 watt@ 8 ohms and 1 meter myself. I happen to have a Clio unit and if you had something to do with that thank you, though I would really like to get one of the newer units that now have waterfall graphing capabilities. If you start another thread on testing at higher input power levels I would be very interested in following that. Working on long excursion small diameter speakers that would fit in well with what I am trying to accomplish. I already have working units and even some that have been in the field now for many years and I am now working on the next generation. Please let me know if you do decide to go down this path.
Steven
I have often wondered about all the testing being done at 1 watt@ 8 ohms and 1 meter myself. I happen to have a Clio unit and if you had something to do with that thank you, though I would really like to get one of the newer units that now have waterfall graphing capabilities. If you start another thread on testing at higher input power levels I would be very interested in following that. Working on long excursion small diameter speakers that would fit in well with what I am trying to accomplish. I already have working units and even some that have been in the field now for many years and I am now working on the next generation. Please let me know if you do decide to go down this path.
Steven
Yes, please start a thread concerning loudspeaker testing at high excursion and let me know where it is. I will invite my partner to join in.
The link to WT is woofertester.com
I'm not really the right person to start this thread as I've been a beach bum since before the end of the previous Millenium. I can only contribute Jurassic anecdote.
AFAIK, you can download all Windoze versions of the Clio software from Mauro's website. The data files are backward compatible so you might like to try this. Or just ask Mauro.
The reason Julian and I chose low level is cos confirmation was always 2.83V at 1m. Celestion's anechoic was better than Wharfedale's. For precise measurements, I had MLLSA and a hydraulic lift in an empty warehouse bay that was known as the Chinese Space Rocket.
My anechoic at Calrec was good for microphones but not for speakers.
BTW, Julian Wright of Celestion developed the Voice Coil inductance model that's used in all advanced TS programmes today.
Apologies to LC for dis off topic stuff
Yes I did. Mosfets are here, VSSA PCB-s (last 50 pcs) to complete the orders now in production and will be delivered hopefully next week. So all orders will be concluded soon.
TR1, TR2 - 5 k
TR3 - 1 k
From 260 sets already sent out there were app. 2-3 cases with trimming problems.
