Motional Feedback Design
The expression for the complete system is;-
Gain = 1/(Rs/Re - B Rs/Re + B )
If Rs = 0, then Gain = 1/B as before
If Rs = Re, then gain = 1,
The current feedbackand voltage feedback cancel because their effects are opposing. Current feedback damps the series resonant circuit by effectively increasing the series resistance, while voltage feedback damps the parallel resonant circuit by effectively reducing the parallel resistance. So the response is as though there is no feedback at all.
These results show that the expression is correct.
A flat response is obtained if the gain at Fs is the same as the gain at Fm. So the question is, what value of Rs in the above expression fould give the same gain for the measured values ofB at Fs and Fm?
In my system it came out at 0.43 ohms, I used 0.47.
Having measured it with what little equipment I had at the time, I think that the errors and assumptions err on the side of too much current feedback. but it's not far off, considering. My system has been in use for 5 years.
BTW, this is only part of the system, you need an integrator to give an output proportional to acceleration. If the input is double integrated you get an output proportional to excursion, I thought of using that to trigger the protection relay but never got round to it.
The idea was to get a computer and CAD stuff to make more accurate measurements, but in the past few years I've got bogged down with computers and neglected the audio. So thanks for the inspiration to work at a subject I love.
Hope all this makes sense, any questions?
Re is voice coil resistance
Rs is R8 in the diagram
The expression for the complete system is;-
Gain = 1/(Rs/Re - B Rs/Re + B )
If Rs = 0, then Gain = 1/B as before
If Rs = Re, then gain = 1,
The current feedbackand voltage feedback cancel because their effects are opposing. Current feedback damps the series resonant circuit by effectively increasing the series resistance, while voltage feedback damps the parallel resonant circuit by effectively reducing the parallel resistance. So the response is as though there is no feedback at all.
These results show that the expression is correct.
A flat response is obtained if the gain at Fs is the same as the gain at Fm. So the question is, what value of Rs in the above expression fould give the same gain for the measured values ofB at Fs and Fm?
In my system it came out at 0.43 ohms, I used 0.47.
Having measured it with what little equipment I had at the time, I think that the errors and assumptions err on the side of too much current feedback. but it's not far off, considering. My system has been in use for 5 years.
BTW, this is only part of the system, you need an integrator to give an output proportional to acceleration. If the input is double integrated you get an output proportional to excursion, I thought of using that to trigger the protection relay but never got round to it.
The idea was to get a computer and CAD stuff to make more accurate measurements, but in the past few years I've got bogged down with computers and neglected the audio. So thanks for the inspiration to work at a subject I love.
Hope all this makes sense, any questions?
Re is voice coil resistance
Rs is R8 in the diagram
Attachments
May I jump in here. I might have missed something earlier in the thread, but what are the general thoughts when using a second voice coil for feedback? Is it that it somehow measures the cone velocity, and that the feedback loop would correct the input to the speaker such that the output (presumably proportional in some way to the cone velocity) closer to the audio signal?
If so, how is the mutual inductance between the two windings accounted for? I mean, even if the cone is forced to stand still, there will still come a signal back from the second winding, the system will act as a transformer.
Or did I miss something?
If so, how is the mutual inductance between the two windings accounted for? I mean, even if the cone is forced to stand still, there will still come a signal back from the second winding, the system will act as a transformer.
Or did I miss something?
Mutual Inductance
Yes the mutual inductance between both coils is an error term.
I would like to find a way to either cancel it out, or to include a term in the gain expression to account for it. When I get my new measuring system maybe I could find a way to do both, but I couldn't do it when I was designing the system.
The questions are, how big is the error? how does it affect the results?
To find out, I built it. The error is frequency dependant, as you would expect, and affects the feedback at Fm much more than at Fs.
Even so, the system works. As I said previously, the errors result in a bit too much current feedback, when calculated as shown previously.
Yes the mutual inductance between both coils is an error term.
I would like to find a way to either cancel it out, or to include a term in the gain expression to account for it. When I get my new measuring system maybe I could find a way to do both, but I couldn't do it when I was designing the system.
The questions are, how big is the error? how does it affect the results?
To find out, I built it. The error is frequency dependant, as you would expect, and affects the feedback at Fm much more than at Fs.
Even so, the system works. As I said previously, the errors result in a bit too much current feedback, when calculated as shown previously.
Frequency Response of MFB Sub
Maybe I should add that the frequency response of the system behaves like a low-pass filter, the corner frequency being Fm.
It's second order, 12dB/octave.
Fm is higher for smaller woofers.
This could be an advantage, no crossover required.🙂
Maybe I should add that the frequency response of the system behaves like a low-pass filter, the corner frequency being Fm.
It's second order, 12dB/octave.
Fm is higher for smaller woofers.
This could be an advantage, no crossover required.🙂
Johnnyx:
Have you measured your system acoustically? If so, it would be interesting to see the graphs. If not, may I suggest a measurement with the microphone *inside* the box. The resulting curve will be tilted by -12dB/octave, but a double differentiator will fix this. I could point you to some software that does this too if you want.
Have you measured your system acoustically? If so, it would be interesting to see the graphs. If not, may I suggest a measurement with the microphone *inside* the box. The resulting curve will be tilted by -12dB/octave, but a double differentiator will fix this. I could point you to some software that does this too if you want.
And, yes, forgot:
Have you had a look at Audio Pro's ACE-bass. It does feedback on the current of the voice coil and manages to extend the lower cutoff of the loudspeaker. It is however not best understood as a feedback system in the traditional sense.
I found this link, but there might be better ones.
http://www.1388.com/articles/tech_Ace-Bass/
Have you had a look at Audio Pro's ACE-bass. It does feedback on the current of the voice coil and manages to extend the lower cutoff of the loudspeaker. It is however not best understood as a feedback system in the traditional sense.
I found this link, but there might be better ones.
http://www.1388.com/articles/tech_Ace-Bass/
I have a 3rd octave RTA graph of the sub together with my cheap satellite speakers. I have a LAUD measuring system, but had problems with the computer. It uses an ISA sound-card by Turtle Beach, and is good, but I've had no luck getting the computer working. I had so many audio projects on at the time, I didn't take many of the MFB sub. I've just ordered Praxis by the same company, Liberty Instruments, which should be less dependant on the computer. So watch this space!😀
Regarding the ace bass, it is just the same as Yamaha's Active Servo Technology system. I think, if I remember correctly, Jeff Macauley uses the same principle.
Negative current feedback increases output impedence, and positive current feedback reduces it.
I accidentally used positive current feedback in my earlier experiments. These first attempts were published in Hi-Fi World DIY letters, August 1996. I used a tapped , dual impedence woofer for my first attempts, because they were cheap. I used a differential amp, -ve input to one half of the winding, +ve input to ground, and I used remote sensing; ie the inputs connected at the speaker. The centre tap of the voice coil was grounded, and the amp drove the other half. The +ve input then had +ve current feedback from the voltage drop along the speaker wire. I couldn't understand why the offset voltage was so high, and drifted so much.
There lies the problem; the sensing resistor has to have the same thermal characteristics as the voice-coil resistance.
Yamaha used to sell thermally matched resistors for some woofers, to promote their idea.
My system seems relatively immune to these problems, because it is not dependant on just one parameter, but ideally Rs should have the same thermal characteristics as Re.
Regarding the ace bass, it is just the same as Yamaha's Active Servo Technology system. I think, if I remember correctly, Jeff Macauley uses the same principle.
Negative current feedback increases output impedence, and positive current feedback reduces it.
I accidentally used positive current feedback in my earlier experiments. These first attempts were published in Hi-Fi World DIY letters, August 1996. I used a tapped , dual impedence woofer for my first attempts, because they were cheap. I used a differential amp, -ve input to one half of the winding, +ve input to ground, and I used remote sensing; ie the inputs connected at the speaker. The centre tap of the voice coil was grounded, and the amp drove the other half. The +ve input then had +ve current feedback from the voltage drop along the speaker wire. I couldn't understand why the offset voltage was so high, and drifted so much.
There lies the problem; the sensing resistor has to have the same thermal characteristics as the voice-coil resistance.
Yamaha used to sell thermally matched resistors for some woofers, to promote their idea.
My system seems relatively immune to these problems, because it is not dependant on just one parameter, but ideally Rs should have the same thermal characteristics as Re.
johnnyx said:
I will... 😀
[/B]
Yes, Yamaha bought the rights to use ACE-bass, because it was so similar, and used it in their products. There is however more to it than positive current feedback, as you may know. The extra loop generates an output impedance that in effect modifies the mechanical parameters by electrical means. Pretty neat to be able to choose fs, Vas, Qts, Mms etc by just changing capacitors and resistors.
Altering parameters
I did many experiments including components within and without the feedback loop, like conjugate filters, capacitors and resistors, coils you name it, and I've seen the parameters change. Its strange to alter the resonant frequency or damping factor in such a way. I wanted to increase the bandwidth of the system to higher frequencies. In the end I decided to leave it as it is.
I feel frustrated when I see some design, but no method for calculating the effects of component values. This is the reason I've given the equations above; so you can calculate your own design, and verify (or maybe not) the results. Putting numbers and equations to something says so much more about it.
I did many experiments including components within and without the feedback loop, like conjugate filters, capacitors and resistors, coils you name it, and I've seen the parameters change. Its strange to alter the resonant frequency or damping factor in such a way. I wanted to increase the bandwidth of the system to higher frequencies. In the end I decided to leave it as it is.
I feel frustrated when I see some design, but no method for calculating the effects of component values. This is the reason I've given the equations above; so you can calculate your own design, and verify (or maybe not) the results. Putting numbers and equations to something says so much more about it.
It is my understanding that the signal from the voice coil is proportional to velocity, hence it needs a DIFFERENTIATOR not an INTEGRATOR to get an output proportional to acceleration??
With my first attempts at MFB, I had no idea what to expect. Once I had a stable system, I measured the frequency response with a sig. gen., mic, and 'scope. Crude but effective, and requiring a great deal of patience. I noticed an overlying +6dB/octave rise in the frequency response, despite the presence of the peaks I described earlier. (I had not applied current feedback at the time). I had no idea why, but thought that a preceding integrator would combine with this to give a flat overall frequency responce. Later I learned that acoustic output is proportional to the acceleration of the diaphragm.
Tannoy had a commercial model using the principles we are discussing here, the ALF625. It was while reading a review of this in Hi - Fi World sometime in 1994 that I was inspired to try doing something similar. Later that year, the ALFie 625 was reviewed in What Hi - Fi? who described it as "slow" 😕
They also ridiculed the acronym, "Advanced Low Frequency improved electronics", saying; "More Arnie and less Alfie would be better". Maybe such a review in the popular hi - fi press can stifle a product in the marketplace, I think it ceased production in 1996.
In their system they differentiate the voltage feedback which is, I think, what you are suggesting, this would give a flat response by itself. They too use current feedback to raise the output impedence of the amplifier, so maybe the current feedback provides sufficient stability to allow such a thing.
Here's a block diagram of their system, hope it's readable🙂
They also ridiculed the acronym, "Advanced Low Frequency improved electronics", saying; "More Arnie and less Alfie would be better". Maybe such a review in the popular hi - fi press can stifle a product in the marketplace, I think it ceased production in 1996.
In their system they differentiate the voltage feedback which is, I think, what you are suggesting, this would give a flat response by itself. They too use current feedback to raise the output impedence of the amplifier, so maybe the current feedback provides sufficient stability to allow such a thing.
Here's a block diagram of their system, hope it's readable🙂
Attachments
System RTA
Here's a 3rd octave RTA response of the system. The bass is my MFB system, the rest is some small speakers I made. I was trying out an active crossover I bought, intended for in - car use, but it was after I was burgled and lost my 4 channel amp with active crossovers, so I just needed something quick. The LAUD system allowed me to average several mic positions, but I don't think it eliminated room nodes. I remember setting the mic in position for a near - field measurement of the subs but I cann't remember what happened.
Here's a 3rd octave RTA response of the system. The bass is my MFB system, the rest is some small speakers I made. I was trying out an active crossover I bought, intended for in - car use, but it was after I was burgled and lost my 4 channel amp with active crossovers, so I just needed something quick. The LAUD system allowed me to average several mic positions, but I don't think it eliminated room nodes. I remember setting the mic in position for a near - field measurement of the subs but I cann't remember what happened.
Attachments
ACE-Bass is a really effective alternative to MFB.
I've built a few sub boxes over the past 10 years using ACE-Bass, from twin 6.5" to quad 10", with 3db points as low as 20Hz.
The degree of control the electronics has over the cone is quite staggering. If you try to manipulate the cone with the system on, it actually feels heavier, stiffer, more damped, with a lower Fs (assuming you choose to increase all of these terms).
You just have to listen to one of those little Yamaha subs to realise that for such a small driver, being driven hard in a small box, the LF extension is quite impressive. Applying the technology to less compromised drivers running a much larger headroom results in very impressive performance, especially if dual subs are arranged in push-pull fomat.
If you look at Stahl's original AES paper on the technology, he prints some fine examples of measured improvement in performance and range extension, comparable to the best commercial MFB systems.
No messing around with accellerometers, and the system works great with a vented box, giving plenty of efficiency down low (if you choose that avenue).
For the DIYer, I think it would be much harder to get better performance from a MFB system compared to an ACE-Bass system design with the same effort.
I've built a few sub boxes over the past 10 years using ACE-Bass, from twin 6.5" to quad 10", with 3db points as low as 20Hz.
The degree of control the electronics has over the cone is quite staggering. If you try to manipulate the cone with the system on, it actually feels heavier, stiffer, more damped, with a lower Fs (assuming you choose to increase all of these terms).
You just have to listen to one of those little Yamaha subs to realise that for such a small driver, being driven hard in a small box, the LF extension is quite impressive. Applying the technology to less compromised drivers running a much larger headroom results in very impressive performance, especially if dual subs are arranged in push-pull fomat.
If you look at Stahl's original AES paper on the technology, he prints some fine examples of measured improvement in performance and range extension, comparable to the best commercial MFB systems.
No messing around with accellerometers, and the system works great with a vented box, giving plenty of efficiency down low (if you choose that avenue).
For the DIYer, I think it would be much harder to get better performance from a MFB system compared to an ACE-Bass system design with the same effort.
Hi ChrisChris Lockwood said:
My first experiment with MFB had both voltage feedback and an accidental negative output impedence, as I described above. So my first attempt combined both techniques
When I pressed the cone, it hardly moved, very slowly it would move inwards. It felt very stiff. I was impressed🙂
With this set-up I was concerned by the large offset voltage that was present across the speaker, and how the value drifted over time. How did you solve the dc stability issue and compensate for the temperature coefficient of the voice - coil resistance in your systems?
I never bothered with VC temp stabilisation. Since the VC resistance has a +ve temp coefficient, the worst thing that would happen was having slightly less than optimal -ve resistance in the amplifier, which meant a less tightly controlled system.
The biggest problem I saw was the non-linear voice-coil inductance.
This alone required trimming of the negative resistance to be less (in magnitude) than optimal, or else at high excursions the thing would go into +ve feedback mode and clip hard. Its a pretty horrible sound when that happens 🙂
As far as DC offsets go, it never became a serious issue at the levels I was listening. I did notice some rather large LF displacements going on, so I assume this is similar to your observations. Unfortunately I sold or scrapped all of the ACE-Bass systems that I built and have lost track of them.
The DC offset problem only happens when you allow the current feedback to dominate the voltage feedback. The loop should be tailored for increasing voltage feedback below the cutoff frequency of the system. AC coupling the current feedback can help here.
I'm all keen to make another system now 🙂 Its way easier to measure the driver parameters these days, using soundcard-based test software.
Chris
The biggest problem I saw was the non-linear voice-coil inductance.
This alone required trimming of the negative resistance to be less (in magnitude) than optimal, or else at high excursions the thing would go into +ve feedback mode and clip hard. Its a pretty horrible sound when that happens 🙂
As far as DC offsets go, it never became a serious issue at the levels I was listening. I did notice some rather large LF displacements going on, so I assume this is similar to your observations. Unfortunately I sold or scrapped all of the ACE-Bass systems that I built and have lost track of them.
The DC offset problem only happens when you allow the current feedback to dominate the voltage feedback. The loop should be tailored for increasing voltage feedback below the cutoff frequency of the system. AC coupling the current feedback can help here.
I'm all keen to make another system now 🙂 Its way easier to measure the driver parameters these days, using soundcard-based test software.
Chris
Chris Lockwood said:
Hi Chris
Yes I have ordered a Praxis system and hope to do more measurements. I had problems with the computer that had the Laud measuring system on it, but the new system will be so much more flexible. Even the free version can measure driver parameters accurately, I'm waiting so that all my test leads can be calibrated together. With it, I will be able to see the nature and magnitude of the errors in my design method as described above, I'm really excited about it😀
None of my systems is set in stone.
The speakers are Seas CA21 RE4X/DC. I have 4, 2 per enclosure. each driver has about 20 litres of box, with a shelf brace between them. There is a partition at the base of the box containing the amplifier and other electronics. I could try different designs by changing the electronics in the box. I will have a go at Ace bass and LT equalization, and any other technique that is suggested, then compare the results. I have a coupled cavity design using these speakers, which I find interesting, but is not so easy to implement.
More than 10 years ago I built an ACE sub as well. It was designed for 30 Hz (-3dB) cutoff only but it was quite impressive. Even at very low volumes almost everything within the appartement shook ! The downside however was this strange clipping. I didn't have the measurement equipment to see where the effect came from back then, so I gave up on that one.
The DC-offset problem was eliminated by a single-pole highpass, with adequately low cutoff frequency, in the forward-path of the circuit.
It was also quite impressive to hear the differenc in sound, generated by tapping the cones, with the circuit activated or deactivated !
The efficiency of the principle will be in the same ballpark as other electronically assisted ones. It's advantage is that it can be used with reflex enclosures, needing reasonable amounts of Vd only.
Regards
Charles
Re ACE bass
Okay okay, I'll try it
I have a 12" Peerless unit that I used in a 125l enclosure. That was a reflex design, but it was just too big. Enormous even. I wanted to build a smaller enclosure for it, and maybe it would be an ideal candidate to try the ace bass technique.
I was also planning a reflex enclosure for it.
Do you have any links to the design procedures?
Okay okay, I'll try it
I have a 12" Peerless unit that I used in a 125l enclosure. That was a reflex design, but it was just too big. Enormous even. I wanted to build a smaller enclosure for it, and maybe it would be an ideal candidate to try the ace bass technique.
phase_accurate said:
I was also planning a reflex enclosure for it.
Do you have any links to the design procedures?
You can mail me your address and I can send you the according JAES article by snail-mail.
Regards
Charles
Regards
Charles
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