Let's be clear, and I have said this many times, current-drive cannot lower driver distortion. The key to current-drive is that there is less amplifier distortion.
Once again I will be misinterpreted, but it goes with the job, so-to-speak.
I am not talking about voltage distortion of the amplifier, I am talking about the current distortion of the amplifier.
For some this seems such a strange thing to contemplate, I know. In most minds talking about distortion, then we are talking about voltage distortion. It is burned into our minds, it is the default position without even thinking about it. Then somebody like me comes along and blabber about distortion and associate that with current, and it's like I am speaking Chinese to them.
Let's get back to the driver. It has distortion. The quality of the driver must come first. I can't stress that enough. For one, look at the inductance of the driver, it is the first thing I look at. I have a Vifa P17WJ00-8 driver right next to me and this moment I am measuring the impedance...
What are we looking at?
Let's add the DC resistance of the voice coil which is near bang-on 6 Ohm:
Everything below the Red line is resistance and it stays constant 6 Ohm with frequency.
So it is part of the impedance, but what about the rest?
Yes, everything that is not resistance is back-EMF.
There are two obvious back-EMF and I have shown them, at LF is motional back-EMF and above 200 Hertz it is the inductive back-EMF.
(The third one is microphonic back-EMF and can be noticed by that bump at around 900 Hertz, which cannot be explained any other way, most likely a surround issue.)
We apply a 2KHz signal into an amplifier which is a voltage source and it will see a total impedance of 11 Ohm. But what it is seeing is 6 Ohm of the DC resistance and in series with 5 Ohm of back-EMF impedance.
Now I have gotten into trouble to equate back-EMF with impedance, but I tell you what, if the voltage across the voice coil is 11V, then the amplifier will produce 1 Amp of current. That means 5V across the back-EMF (inductive) and 6V across the DC resistance.
The back-EMF is an impedance, it impedes current.
And the back-EMF produces 5V which means that the back-EMF is in itself also a voltage source.
Yes, in the above scenario you have two voltage sources.
Before I go on. to see if you have followed so far, what wattage/heat is dissipated in the above scenario.
I leave it there for now as there is a lot to think about.
For example, next consider how that back-EMF behaves and what effect it has on the current of the amplifier. The DC resistance stabilises it to a degree. The back-EMF de-stabilises the current.
And at this point the microphonic back-EMF comes in, and that is a pure voltage source when analysed because it wants the driver to behave as a microphone and a microphone (with coil and magnet) is a pure voltage source, in this instance it will have a source impedance of 6 Ohm - the DC resistance of the coil in a microphone largely defines its source impedance. Reverse the action of the driver. This action cannot also enacted by the mechanical aspects of the driver. Remember that 900 Hertz bump most likely being attributed to the surround issues (very common) and it a bump because it produces a voltage that results in a bump in the impedance.
I am a good student of driver behaviour.
A lot to think about. Goodnight...
Once again I will be misinterpreted, but it goes with the job, so-to-speak.
I am not talking about voltage distortion of the amplifier, I am talking about the current distortion of the amplifier.
For some this seems such a strange thing to contemplate, I know. In most minds talking about distortion, then we are talking about voltage distortion. It is burned into our minds, it is the default position without even thinking about it. Then somebody like me comes along and blabber about distortion and associate that with current, and it's like I am speaking Chinese to them.
Let's get back to the driver. It has distortion. The quality of the driver must come first. I can't stress that enough. For one, look at the inductance of the driver, it is the first thing I look at. I have a Vifa P17WJ00-8 driver right next to me and this moment I am measuring the impedance...
What are we looking at?
Let's add the DC resistance of the voice coil which is near bang-on 6 Ohm:
Everything below the Red line is resistance and it stays constant 6 Ohm with frequency.
So it is part of the impedance, but what about the rest?
Yes, everything that is not resistance is back-EMF.
There are two obvious back-EMF and I have shown them, at LF is motional back-EMF and above 200 Hertz it is the inductive back-EMF.
(The third one is microphonic back-EMF and can be noticed by that bump at around 900 Hertz, which cannot be explained any other way, most likely a surround issue.)
We apply a 2KHz signal into an amplifier which is a voltage source and it will see a total impedance of 11 Ohm. But what it is seeing is 6 Ohm of the DC resistance and in series with 5 Ohm of back-EMF impedance.
Now I have gotten into trouble to equate back-EMF with impedance, but I tell you what, if the voltage across the voice coil is 11V, then the amplifier will produce 1 Amp of current. That means 5V across the back-EMF (inductive) and 6V across the DC resistance.
The back-EMF is an impedance, it impedes current.
And the back-EMF produces 5V which means that the back-EMF is in itself also a voltage source.
Yes, in the above scenario you have two voltage sources.
Before I go on. to see if you have followed so far, what wattage/heat is dissipated in the above scenario.
I leave it there for now as there is a lot to think about.
For example, next consider how that back-EMF behaves and what effect it has on the current of the amplifier. The DC resistance stabilises it to a degree. The back-EMF de-stabilises the current.
And at this point the microphonic back-EMF comes in, and that is a pure voltage source when analysed because it wants the driver to behave as a microphone and a microphone (with coil and magnet) is a pure voltage source, in this instance it will have a source impedance of 6 Ohm - the DC resistance of the coil in a microphone largely defines its source impedance. Reverse the action of the driver. This action cannot also enacted by the mechanical aspects of the driver. Remember that 900 Hertz bump most likely being attributed to the surround issues (very common) and it a bump because it produces a voltage that results in a bump in the impedance.
I am a good student of driver behaviour.
A lot to think about. Goodnight...
There are about three ways I can tackle this, but please look at the next three graphics below:
You can measure the current this way
This is the tool that I am using, except that the resistor is 0.1R.
You want Two Tone JMD tests. here you go.
The load is the same load resistors in series with a 1.0 mH inductor.
The amplifier and voltage feedback loop still look mostly innocent.
Next time it will be plots of the ScanSpeak reference driver.
Two Tone current distortion plot.
YES!!! Definitely use 0.1R and I always thought that 1R is too crude, but even with that value he [Pavel] got results, but with one/tenth the value, the resolution should logically go up by a 10x factor. At least a lot better.
As you would expect. The driver should have almost no change in results here. But the current side...?
With a perfect driver both the voltage and current results would be the same. I bet they won't be. No driver is perfect.
Is this an older Scan-Speak driver? Take a look at the rated inductance, usually rated at 1KHz, but sometimes they will tell you at 10KHz. It does have a well-behaved polycone (soft) cone.
The Vifa P17WJ08 driver I have here is a classic but the inductance is around 0.5mH @ 1KHz and that is not great. Indeed using my Clio setup I got:
Not great and for testing purposes this is good. Your Scan-Speak is likely to be better than that. What does Scan-Speak rate the inductance as?
If there was no inductance, then at 1KHz and above you would be left with just pure resistance and your results are going to be the same as the amp. What I am trying to tell you is that any aberrations that show up is linked to the inductive back-EMF.
Let us see how you go. I may suggest different frequency combination and yes IMD (not JMD). lol
PS: I have a pair of drivers out a pair of B&W DM303 speakers - real el-cheapo drivers. Inductance at 1KHz around 0.7mH and 10KHz is 0.4378mH measured. This is not great.
I have some tests in mind for this driver, in fact will be using two speakers with these 6.5" driver. I am not yet revealing these tests (how), but they will be very telling because I can measure the back-EMF of the driver as a voltage source using this test. It's complex, but I am getting stuff together to make it possible.
PPS: The driver used in the standard MFC Elsinores is astoundingly low and SOA: 1KHz 0.189Mh and 10KHz is 67uH, measured with my LCR meter. This driver, SB17MFC35-08, is so incredibly underrated because it is not expensive.
The ScanSpeak driver that I am using is the current production of the old Vifa P17WJ-00-08 made in Denmark purchased from Madisound.
If you know the amplifier output voltage, voice coil DC resistance, and the voltage across the current sensing resistor you can calculate the Back EMF as you graphically show in your plot that I pasted here.
I agree that for testing purposes you do not want a perfect driver, you want one that is capable of showing the effects of the test variables.
First off I probably will pick frequency combinations as used on the Purify driver data sheets. Your suggestions so we can be on the same page?
Thanks DT
Purifi is good, that will create excursions. But also try 150/1250Hz and 300/2500Hz and see what happens. These frequencies are suggested by Esa from a paper he wrote in 2016, I wonder if you know about that one. I have it in PDF format.
@Joe Rasmussen
With regards to your attached picture (attachment 1157559) quoted above, are you saying that the 'back-EMF impedance' is only represented by the pink colored part, and the rest below, the green part, is the actual DC resistance seen in that impedance graph?
edit: and just so I don't misunderstand here, is the so called 'back-EMF impedance' the same as the generic name 'impedance', just your fancy way calling it, or?
Last edited:
Basically the answer is yes. Appreciate the question. It is important to understand that they are in series.
Like this:
Em is the inductive back-EMF and Ei is the motional back-EMF.
Impedance is anything that impedes current (not voltage) and hence it has a value in Ohm. Impedance is not just limited to resistance.
Low frequencies are dominated by motional back-EMF, so at those frequencies that motional back-EMF. Above a few Hertz they start becoming dominated by inductive back-EMF. So yes, the pink can be seen as sitting on top of the DC resistance of the voice coil, which is the only part that dissipates heat. The back-EMF does not dissipate heat.
Being both back-EMF, they are also voltage sources. You may or may not have gotten your mind around that.
___________________
Let me point out something, that I am not pro current-drive, but one has to point out that the main advantage of current-drive is that it ignores those voltage sources in a way that they no longer impede current. This is the result of the large source impedance of a current source. The current now becomes fixed at 1 Amp no matter what the back-EMF impedance is. The 5V will still show up, but since 5 Ohm is only tiny in relation to the source impedance (again in series) that is so large that 5 Ohm becomes rather noting. It no longer impedes current when we have current drive.
A current source amplifier cannot be put off supplying 1 Amp of current. This means that the back-EMF, which is not stable and affected by all kinds of issues because that is where all the driver problems will show up (that is why low driver inductance is always a positive thing).
The back-EMF's unstable impedance means it suffers from impedance modulations. And that means that with the use of current source amplifiers, impedance modulations lead to current modulations of the amplifier!
A current source amplifier is impervious to these current modulations/distortion.
Hence the current source amplifier will not distort its current, no matter what the back-EMF does. The current will not change. With voltage sources (amplifier) it will because the driver back-EMF impedance will boss the amplifier around and cause the amplifier to produce distortion.
Again I stress, this is now about lowering driver distortion (the driver designer has to do that), it can only reduce distortion (relative to voltage sources).
But despite the known advantages of current sources, I am not promoting current drive.
Get the speaker right first and use crossover techniques to fix the problem.
Voltage sources are here to stay. The world is not going to get converted to current drive.
The Elsinores are a design that goes a long way to sound like current-drive even when voltage sources are used.
That is my position - and I make/design stuff that deals with these issues and I make it available to everybody to build.
Like this:
Em is the inductive back-EMF and Ei is the motional back-EMF.
Impedance is anything that impedes current (not voltage) and hence it has a value in Ohm. Impedance is not just limited to resistance.
Low frequencies are dominated by motional back-EMF, so at those frequencies that motional back-EMF. Above a few Hertz they start becoming dominated by inductive back-EMF. So yes, the pink can be seen as sitting on top of the DC resistance of the voice coil, which is the only part that dissipates heat. The back-EMF does not dissipate heat.
Being both back-EMF, they are also voltage sources. You may or may not have gotten your mind around that.
___________________
Let me point out something, that I am not pro current-drive, but one has to point out that the main advantage of current-drive is that it ignores those voltage sources in a way that they no longer impede current. This is the result of the large source impedance of a current source. The current now becomes fixed at 1 Amp no matter what the back-EMF impedance is. The 5V will still show up, but since 5 Ohm is only tiny in relation to the source impedance (again in series) that is so large that 5 Ohm becomes rather noting. It no longer impedes current when we have current drive.
A current source amplifier cannot be put off supplying 1 Amp of current. This means that the back-EMF, which is not stable and affected by all kinds of issues because that is where all the driver problems will show up (that is why low driver inductance is always a positive thing).
The back-EMF's unstable impedance means it suffers from impedance modulations. And that means that with the use of current source amplifiers, impedance modulations lead to current modulations of the amplifier!
A current source amplifier is impervious to these current modulations/distortion.
Hence the current source amplifier will not distort its current, no matter what the back-EMF does. The current will not change. With voltage sources (amplifier) it will because the driver back-EMF impedance will boss the amplifier around and cause the amplifier to produce distortion.
Again I stress, this is now about lowering driver distortion (the driver designer has to do that), it can only reduce distortion (relative to voltage sources).
But despite the known advantages of current sources, I am not promoting current drive.
Get the speaker right first and use crossover techniques to fix the problem.
Voltage sources are here to stay. The world is not going to get converted to current drive.
The Elsinores are a design that goes a long way to sound like current-drive even when voltage sources are used.
That is my position - and I make/design stuff that deals with these issues and I make it available to everybody to build.
I=U*R
If current decreases, voltage decreases if impedance is constant. Let's not forget that when we try to mystify ohms law. You are trying to separate two endlessly entangled entities - its futile. Master of the Obvious much? Like telling the audience here that a speaker impedance consists of a real (R) and and a reactive (imaginary) part - but baking a layered cake out of that impedance like done above I'm not sure its a perfect way of describing it as the y-axis is Impedance and not resistance - its probably not truly correct. Savour that!
//
Ohms law is only part of it.
A voltage source amplifier has a different feedback loop than a current source amplifier.
A current source amplifier compensates for increased Back EMF by increasing the voltage output, the output current tends to remain constant as impedance varies.
If you are interested, google for transconductance amplifier. The MATLAB site shows all the formulas and shows the current source amplifier feedback loop.
Thanks DT
Thanks, I will toy around with those frequency combinations as well. I have the Esa book , I do not have the 2016 paper in total recall. Will you post it or PM it to me please.
Thanks DT
You are welcome. I just don't want the acrimony.
I am not doubting the accuracy of the information you offer, just the way you approach it and make it personal like 'I know better than you' and that is a complete turn-off.
See what I mean? That turns me right off, just there!
BTW, "Ohm's Law" if you must know how to spell it correctly, but I resist to play games.
How ironic that I am actually trying to de-mystify Ohm's Law and you make it sound exclusive because you want to use engineering language. Big deal!
I just don't want to get into ideological battles about who is smarter and who knows the difference real and imaginary numbers of an impedance. That just completely sidesteps the issue I am highlighting. It can be understood even without understanding complex impedance terms. In this case just not necessary.
What it does reveal however, is not having a single inclination to understand what I am talking about!
And that's sad.
Maybe I should just ignore a ghost from Sverige?
TNT, are you related to Alfred Nobel?
(I hope others here gets the joke? He does.)
BINGO!
I did some searching and luckily I found somebody online who has parked the article by Esa:
Link to PDF: Comparative Measurements on Loudspeaker Distortion: Current vs. Voltage Control
I find it interesting that here, in 2016, he using Current Control rather that current-drive.
I have always been dubious about the term current-drive. Maybe current-control is an improvement? The interesting thing (and I know this will get a rise from some people) is that Esa says that the 'control' of current should be done by the amplifier and I say no! Ultimately the cause is the speaker and hence that is where it should be fixed. That is causal right there, the speaker. But this is about controlling the current, no doubt about that.
As for voltage control term being an improvement over voltage-drive? I don't think so. The best a voltage source can do is indirectly guide the current, that is all. And yes, that is a fraught situation.
Voltage-drive as a descriptor is just plain wrong. About 70% of a voice coil is outside the magnetic gap, so only 30% of the voltage inside - OTOH, the voice coil sees 100% of the current. It also means that 70% of the length of the voice coil is nothing more than (seen visually) two wire-wound resistors, one in front of the gap and one behind. That's makes the output impedance of the amplifier look silly.
Just my opinions being expressed, nobody needs to react negatively.
Cheers, Joe
So did also I so no reason for you to react negatively. 🙂
Like... "It is burned into our minds, it is the default position without even thinking about it."
It goes so much both ways Sir.
//
Sometimes, but not always. I was 'brought up' to think like you. Then something struck me as odd, that something that should be questioned, yet it is rather not.
Thankfully, that is slowly changing, but the Titanic is slow in turning.
Maybe if you stopped for a moment, take a deep breath, have an open mind (but not too open I admit) and maybe you will see it as well.
Why are we always measuring the distortion of amplifiers that makes least sense? Are you of the view that all amplfiers sound much the same, like Eral Geddes does, maybe 5% is all they contribute? I think it is fair for you to disclose that. Or do you agree with me that they do sound different and much more than 5%?
Look at those mags that do reviews and proudly spout that they also do measurements. Nothing wrong with that, but we all know the distortion figures are not going to change too much. Hey, I have done heaps of those measurements myself. But there is one measurement that is conspicuously absent. Note when I said 'measuring distortion' that you immediately defaulted to 'voltage distortion' being measured. I am talking about a different measurement - I am talking about what the amplifier does on the current side.
Indeed I am so conservative in my thinking that I am pushing back against current-drive. This despite the fact that there is increasing support in that area, but I believe they should be limited to active speakers where they are inside sealed boxes, not what we normally used called 'separates.'
So I am curious (always), what is your view when it comes to voltage-drive versus current-drive? As I said, I am on the side of voltage-drive, we can make it work if we have the right strategies in place.
Over to you, but please let us not be demeaning each other. OK?
You realised that Alfred Nobel was Swedish, right? 😉
Is that a Swedish flag before me?
The hospital that I was born in, hop in a car and I would be in Sweden thirty minutes later. So yeah, I have many friends there. I asked one of them, from up Göteborg (Gothenhurg) way, is it true that Danes know how to have fun and the Swedes don't? You could have knocked me over with a feather when he said 'yes, it's true.' The Swedes can be a bit more serious and less party-going it seems. But they are still my friends. Norwegians? Better not say, I have family there, play it safe. But a famous loudspeaker designer who spent many years in Aarhus, Denmark, said about his countrymen 'they don't understand irony' and he loved that about his Danish friends. Danes can be very ironic!