A Cheap and Super Simple Motional Feedback (or Servo) Woofer
Two caveats:
One, I reserve the right to publish any or all of the following in other venues.
Two, if you do not feel comfortable working inside the covers of an amplifier or are unwilling to face the consequences if something goes wrong or are unwilling to void a manufacturer’s warranty, then do not begin the motional feedback woofer project described herein.
A Really Simple Motional Feedback Woofer Design
This is a really simple design that I hope will provide listening pleasure to any “diy”er, regardless of ascribed theory of enclosure design.
An Introduction to Transient Thinking
When you input a transient signal to a woofer, the motor of the woofer drives the cone producing what we call sound. When the transient input signal ends, the suspension of the woofer acts to return the cone toward its origination position producing more sound and often overshooting the original pre signal position of the cone. This undriven movement of the cone then decays into the natural resonant frequency of the driver and its enclosure alignment. Again, this signal looks similar to a decaying sinusoidal waveform, but never achieves the regularity or consistency over time of a sinusoidal waveform. This decay signal is the sound of the woofer. This decay signal is not the sound of the inputted signal. Since woofers in particular and loudspeakers in general are sound reproducers and not producers of sound, this sound production is a bad thing.
While controversial, when you examine the acoustic performance in a transient defined world, you can separate the various modalities of sound reproduction across time. When excited by a broadband impulse, the low order stop band filters define the onset system output. At later periods, the driver and system resonance modes dominate. When we speak of driver output at system resonance, the late period response is where the majority of the sound is located. Because of this, output at resonance bears little resemblance to the input. Since we usually define distortion as a difference between the shape of the input and output signal, it seems logical that the sound at resonance is highly distorted. It also seems reasonable to say that this is a bad thing.
Feedback
Feedback does wonderful things, but most of the time it does them by simply changing the closed loop gain of the amplifier. When the signal at the output of the amplifier varies from the input by being less than the input signal plus the theoretical gain of the amplifier, feedback decreases and closed loop gain increases. When the signal at the output of the amplifier varies from the input by being more than the input signal plus the gain of the amplifier, feedback increases and the closed loop gain decreases.
When we use motional feedback from a woofer, we are doing nothing different than the amplifier is already doing to itself. With the woofer we must assume some range of linear frequency and some limits of linear operational magnitude as a set point. When cone motion exceeds what is consistent with the input to the driven coil, the sensing coil generates a voltage greater than that on the drive coil (as long as the motion is within our operational set points), and this signal fed back to the amplifier reduces the closed loop gain. When the cone motion is less, the opposite occurs.
Both feedback in electronic circuits and motional feedback in loudspeakers have limits. You can do a lot with feedback, but try to use too much feedback and things can go wrong. Instead of making signal amplification or signal transformation better, too much feedback can make it worse. “All things in moderation” is a wonderful proverb. In this MFB woofer project, a moderate target of 6 db of feedback is a good thing.
Parts for the Project
This is a sealed box design requiring an extra amplifier channel utilizing global feedback, a box to use as an enclosure, acoustic damping material to stuff the box, a dual voice coil woofer, one extra lead from the speaker to the amplifier and one or two resistors (and the various assorted screws, terminals, and gaskets needed to put together any loudspeaker, and maybe a couple of potentiometers if you want to be able to dial in the performance).
Preparing the Amplifier
The feedback circuit of a global feedback amplifier consists of two resistors connected as a voltage divider. There may be other parts, such as capacitors, but they are not important for the servo. Hopefully the graphic of the simplified schematic will appear about here:
R1 and R2 form the existing feedback voltage divider of a polarity-conserving amplifier. The amplified signal is fed through R1 from the amplifier output to the inverting input of our global feedback amplifier at the junction of R1 and R2. We add R3, connecting one lead of the resistor at the junction point of R1 and R2. We connect the other lead of R3 to the wire coming from the second woofer voice coil. R3 inputs the signal from our sensing coil to the amplifier and determines the amount of change in the closed loop gain of the amplifier. If amplifier is polarity conserving and we have wired the powered voice coil of our woofer as polarity-conserving, then we connect the positive lead of the second voice coil to R3 and the negative lead to ground. The ground wire to the voice coils may be separate wires or one shared wire. If we make the value of R3 equal the value of R1, then we will provide about 6db of feedback from our sensor coil. The actual amount of feedback at resonance will vary depending upon the cabinet/woofer tuning and Qts of the driver used.
You can make the circuit tunable by adding to adjustable controls.
Testing
One of the advantages of motional feedback is the control it gives you over the alignment of the driver in the box. With motional feedback it is not as critical to choose just the right woofer and fit it into the perfect box. The woofer I am using in these tests is the MCM Audio Select woofer, model number 55-1460 from MCM Electronics. They list a price of $26.75 each in small quantities of this 10-inch driver. They claim a “Vas” of 4.5 cubic feet, an “fs” of 29 Hz and a “Qts” of .49. Whether in a sealed box or in free air, the driver exhibits rising output as you approach resonance that belies it “Qts” rating. The driver also shows a huge bell mode resonance above 2 kHz.
The graphs here show the before and after MFB response of the driver in a 3.5 cubic foot box with about 3 lbs of medium density fiber fill. As described in the graph annotations, we are controlling the resonance peak and we getting viable feedback signal off the second voice coil until about 1.6 kHz. The higher frequency bell mode resonance is not affected by the application of feedback.
For the next set of tests I mounted a 55-1460 driver in a 3 cubic foot box, densely packed with poly fill and driven by an old Dick Smith Kit ETI 480 amplifier module. I have found the ETI 480 kit to be poor performers and be less than unconditionally stable. If this modification is stable with this amp kit, then it should work with just about anything else out there.
This 3 cubic foot (internal volume) box is too small for this driver, yet with MFB, it works. As shown in the impulse graphs, the resonant decay is quickly damped with MFB.
This combination of a high Q woofer in too small of a box and driven by a low quality power amplifier should not sound nearly as good as it does. Because the high Q of the woofer is being compensated for by decreased closed loop gain, the amplifier is loafing and plays as if it were a much more powerful amplifier. The bass is tight, it is clean, it sounds powerful, and it is at times startling in its detailed reproduction of bass sounds. This power-conserving feature would be a perfect set-up for to use one of Nelson Pass’ Zen amplifiers.
Limitations
I see two limitations of this incredibly cheap and easy MFB woofer system. Because sensor output depends upon velocity and below system resonance velocity is falling, little feedback is available. Unlike the accelerometer systems, the speaker does not “harden up” when you attempt to manually move the cone. I know some people who are very impressed (wowed) by this. The second limitation is that because of this loss of sensor sensitivity, you cannot push the cut-off frequency super low. This MFB system is not the answer for really big woofers in tiny boxes.
Saving the Electronics Just in Case
Because circuit boards are stuffed with components and often placed in difficult to reach places, it is possible you may not attach the MFB feedback resistor or lead in the correct position. Depending on where you put it, things could go very wrong. It is nice to be able to have a fail safe just in case. If you have a high idle current draw amp, the series light bulb will work, it just needs to be a high wattage bulb and the visual indication that something is wrong is not as obvious.
Two caveats:
One, I reserve the right to publish any or all of the following in other venues.
Two, if you do not feel comfortable working inside the covers of an amplifier or are unwilling to face the consequences if something goes wrong or are unwilling to void a manufacturer’s warranty, then do not begin the motional feedback woofer project described herein.
A Really Simple Motional Feedback Woofer Design
This is a really simple design that I hope will provide listening pleasure to any “diy”er, regardless of ascribed theory of enclosure design.
An Introduction to Transient Thinking
When you input a transient signal to a woofer, the motor of the woofer drives the cone producing what we call sound. When the transient input signal ends, the suspension of the woofer acts to return the cone toward its origination position producing more sound and often overshooting the original pre signal position of the cone. This undriven movement of the cone then decays into the natural resonant frequency of the driver and its enclosure alignment. Again, this signal looks similar to a decaying sinusoidal waveform, but never achieves the regularity or consistency over time of a sinusoidal waveform. This decay signal is the sound of the woofer. This decay signal is not the sound of the inputted signal. Since woofers in particular and loudspeakers in general are sound reproducers and not producers of sound, this sound production is a bad thing.
While controversial, when you examine the acoustic performance in a transient defined world, you can separate the various modalities of sound reproduction across time. When excited by a broadband impulse, the low order stop band filters define the onset system output. At later periods, the driver and system resonance modes dominate. When we speak of driver output at system resonance, the late period response is where the majority of the sound is located. Because of this, output at resonance bears little resemblance to the input. Since we usually define distortion as a difference between the shape of the input and output signal, it seems logical that the sound at resonance is highly distorted. It also seems reasonable to say that this is a bad thing.
An externally hosted image should be here but it was not working when we last tested it.
Feedback
Feedback does wonderful things, but most of the time it does them by simply changing the closed loop gain of the amplifier. When the signal at the output of the amplifier varies from the input by being less than the input signal plus the theoretical gain of the amplifier, feedback decreases and closed loop gain increases. When the signal at the output of the amplifier varies from the input by being more than the input signal plus the gain of the amplifier, feedback increases and the closed loop gain decreases.
When we use motional feedback from a woofer, we are doing nothing different than the amplifier is already doing to itself. With the woofer we must assume some range of linear frequency and some limits of linear operational magnitude as a set point. When cone motion exceeds what is consistent with the input to the driven coil, the sensing coil generates a voltage greater than that on the drive coil (as long as the motion is within our operational set points), and this signal fed back to the amplifier reduces the closed loop gain. When the cone motion is less, the opposite occurs.
Both feedback in electronic circuits and motional feedback in loudspeakers have limits. You can do a lot with feedback, but try to use too much feedback and things can go wrong. Instead of making signal amplification or signal transformation better, too much feedback can make it worse. “All things in moderation” is a wonderful proverb. In this MFB woofer project, a moderate target of 6 db of feedback is a good thing.
Parts for the Project
This is a sealed box design requiring an extra amplifier channel utilizing global feedback, a box to use as an enclosure, acoustic damping material to stuff the box, a dual voice coil woofer, one extra lead from the speaker to the amplifier and one or two resistors (and the various assorted screws, terminals, and gaskets needed to put together any loudspeaker, and maybe a couple of potentiometers if you want to be able to dial in the performance).
Preparing the Amplifier
The feedback circuit of a global feedback amplifier consists of two resistors connected as a voltage divider. There may be other parts, such as capacitors, but they are not important for the servo. Hopefully the graphic of the simplified schematic will appear about here:
An externally hosted image should be here but it was not working when we last tested it.
R1 and R2 form the existing feedback voltage divider of a polarity-conserving amplifier. The amplified signal is fed through R1 from the amplifier output to the inverting input of our global feedback amplifier at the junction of R1 and R2. We add R3, connecting one lead of the resistor at the junction point of R1 and R2. We connect the other lead of R3 to the wire coming from the second woofer voice coil. R3 inputs the signal from our sensing coil to the amplifier and determines the amount of change in the closed loop gain of the amplifier. If amplifier is polarity conserving and we have wired the powered voice coil of our woofer as polarity-conserving, then we connect the positive lead of the second voice coil to R3 and the negative lead to ground. The ground wire to the voice coils may be separate wires or one shared wire. If we make the value of R3 equal the value of R1, then we will provide about 6db of feedback from our sensor coil. The actual amount of feedback at resonance will vary depending upon the cabinet/woofer tuning and Qts of the driver used.
You can make the circuit tunable by adding to adjustable controls.
An externally hosted image should be here but it was not working when we last tested it.
Testing
One of the advantages of motional feedback is the control it gives you over the alignment of the driver in the box. With motional feedback it is not as critical to choose just the right woofer and fit it into the perfect box. The woofer I am using in these tests is the MCM Audio Select woofer, model number 55-1460 from MCM Electronics. They list a price of $26.75 each in small quantities of this 10-inch driver. They claim a “Vas” of 4.5 cubic feet, an “fs” of 29 Hz and a “Qts” of .49. Whether in a sealed box or in free air, the driver exhibits rising output as you approach resonance that belies it “Qts” rating. The driver also shows a huge bell mode resonance above 2 kHz.
The graphs here show the before and after MFB response of the driver in a 3.5 cubic foot box with about 3 lbs of medium density fiber fill. As described in the graph annotations, we are controlling the resonance peak and we getting viable feedback signal off the second voice coil until about 1.6 kHz. The higher frequency bell mode resonance is not affected by the application of feedback.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
For the next set of tests I mounted a 55-1460 driver in a 3 cubic foot box, densely packed with poly fill and driven by an old Dick Smith Kit ETI 480 amplifier module. I have found the ETI 480 kit to be poor performers and be less than unconditionally stable. If this modification is stable with this amp kit, then it should work with just about anything else out there.
This 3 cubic foot (internal volume) box is too small for this driver, yet with MFB, it works. As shown in the impulse graphs, the resonant decay is quickly damped with MFB.
An externally hosted image should be here but it was not working when we last tested it.
This combination of a high Q woofer in too small of a box and driven by a low quality power amplifier should not sound nearly as good as it does. Because the high Q of the woofer is being compensated for by decreased closed loop gain, the amplifier is loafing and plays as if it were a much more powerful amplifier. The bass is tight, it is clean, it sounds powerful, and it is at times startling in its detailed reproduction of bass sounds. This power-conserving feature would be a perfect set-up for to use one of Nelson Pass’ Zen amplifiers.
An externally hosted image should be here but it was not working when we last tested it.
Limitations
I see two limitations of this incredibly cheap and easy MFB woofer system. Because sensor output depends upon velocity and below system resonance velocity is falling, little feedback is available. Unlike the accelerometer systems, the speaker does not “harden up” when you attempt to manually move the cone. I know some people who are very impressed (wowed) by this. The second limitation is that because of this loss of sensor sensitivity, you cannot push the cut-off frequency super low. This MFB system is not the answer for really big woofers in tiny boxes.
Saving the Electronics Just in Case
Because circuit boards are stuffed with components and often placed in difficult to reach places, it is possible you may not attach the MFB feedback resistor or lead in the correct position. Depending on where you put it, things could go very wrong. It is nice to be able to have a fail safe just in case. If you have a high idle current draw amp, the series light bulb will work, it just needs to be a high wattage bulb and the visual indication that something is wrong is not as obvious.
An externally hosted image should be here but it was not working when we last tested it.
I experimented with dual VC systems like this some years back. On the one hand, it's a convenient way of deriving the feedback signal. On the other hand, you're tossing out half of the motor strength to get the signal. That's why I ended up biting the bullet and using an accelerometer.
As you noted, the feedback is pretty limited as you go down in frequency- I wonder if this explains your observation about the behavior when you manually move the cone.
One other thought- at a low feedback level like 6 dB, you're trading off 2nd HD for 3rd HD at levels that could potentially have a negative impact on sound quality.
As you noted, the feedback is pretty limited as you go down in frequency- I wonder if this explains your observation about the behavior when you manually move the cone.
One other thought- at a low feedback level like 6 dB, you're trading off 2nd HD for 3rd HD at levels that could potentially have a negative impact on sound quality.
I applied the exact same thing to simmilar woofers (MCM 55-1465, 12" version of the 10" driver Mark spoke off). I emailed Mark asking him about his results with this before his post. And I must agree with most of what he says.
Except I did notice one thing, the cone does stiffin up when tapped. It doesn't become completely hard, but there is much resistance compared to when the amp is turned off. I have my woofers mounted on an open baffle.
My results were this. When comparing this type of MFB to no motion feedback at all, I noticed an improvement in sound with the MFB. The bass sounded more "right" and dipole bass sounds mighty great to begin with. Given, this was with a cheap driver, but it really cleaned things up, the bass was already punchy, this just made it sounds more clean, sorry for the horrible explanation. Long story short, it made things better.
In my book a drop in efficiency is alright when weighing how much it sounded better.
Give it a try, you'll see, much cheaper and simpler way to experiment than using an accelerometer.
Except I did notice one thing, the cone does stiffin up when tapped. It doesn't become completely hard, but there is much resistance compared to when the amp is turned off. I have my woofers mounted on an open baffle.
My results were this. When comparing this type of MFB to no motion feedback at all, I noticed an improvement in sound with the MFB. The bass sounded more "right" and dipole bass sounds mighty great to begin with. Given, this was with a cheap driver, but it really cleaned things up, the bass was already punchy, this just made it sounds more clean, sorry for the horrible explanation. Long story short, it made things better.
In my book a drop in efficiency is alright when weighing how much it sounded better.
Give it a try, you'll see, much cheaper and simpler way to experiment than using an accelerometer.
Interesting
I thought about motional feedback as one possible way of ultimate total systems integration, but maybe not the most elegant one.
Ideally an amplifier should get feedback from the actual output device. Unfortunately I don't have a clear idea of the options. Here my thoughts and state of information:
- microphone feedback. Problem, hard to calibrate and not exactly direct.
- dual voice coil system discussed here. Problem, must use dual voice coil woofers. I would think the midrange should benefit as well from this.
- accelerometer: SY, how do you do that?
- I could imagine an optical system that measures woofer position by reflected light, say, from a white spot on the dustcap (as done in camera autofocus). Does anybody know of implementations of such a thing?
- Ideally I could imagine using power as feedback, by sensing amp power output (not voltage or current which both depend on device impedance). This would have the advantage of staying in the purely electrical domain and to avoid any power compression at the same time. Do any of you know a way how to electrically implement power feedback ?
If you don't
I thought about motional feedback as one possible way of ultimate total systems integration, but maybe not the most elegant one.
Ideally an amplifier should get feedback from the actual output device. Unfortunately I don't have a clear idea of the options. Here my thoughts and state of information:
- microphone feedback. Problem, hard to calibrate and not exactly direct.
- dual voice coil system discussed here. Problem, must use dual voice coil woofers. I would think the midrange should benefit as well from this.
- accelerometer: SY, how do you do that?
- I could imagine an optical system that measures woofer position by reflected light, say, from a white spot on the dustcap (as done in camera autofocus). Does anybody know of implementations of such a thing?
- Ideally I could imagine using power as feedback, by sensing amp power output (not voltage or current which both depend on device impedance). This would have the advantage of staying in the purely electrical domain and to avoid any power compression at the same time. Do any of you know a way how to electrically implement power feedback ?
If you don't
Interesting
I thought about motional feedback as one possible way of ultimate total systems integration, but maybe not the most elegant one.
Ideally an amplifier should get feedback from the actual output device. Unfortunately I don't have a clear idea of the options. Here my thoughts and state of information:
- microphone feedback. Problem, hard to calibrate and not exactly direct.
- dual voice coil system discussed here. Problem, must use dual voice coil woofers. I would think the midrange should benefit as well from this.
- accelerometer: SY, how do you do that?
- I could imagine an optical system that measures woofer position by reflected light, say, from a white spot on the dustcap (as done in camera autofocus). Does anybody know of implementations of such a thing?
- Ideally I could imagine using power as feedback, by sensing amp power output (not voltage or current which both depend on device impedance). This would have the advantage of staying in the purely electrical domain and to avoid any power compression at the same time. Do any of you know a way how to electrically implement power feedback ?
I thought about motional feedback as one possible way of ultimate total systems integration, but maybe not the most elegant one.
Ideally an amplifier should get feedback from the actual output device. Unfortunately I don't have a clear idea of the options. Here my thoughts and state of information:
- microphone feedback. Problem, hard to calibrate and not exactly direct.
- dual voice coil system discussed here. Problem, must use dual voice coil woofers. I would think the midrange should benefit as well from this.
- accelerometer: SY, how do you do that?
- I could imagine an optical system that measures woofer position by reflected light, say, from a white spot on the dustcap (as done in camera autofocus). Does anybody know of implementations of such a thing?
- Ideally I could imagine using power as feedback, by sensing amp power output (not voltage or current which both depend on device impedance). This would have the advantage of staying in the purely electrical domain and to avoid any power compression at the same time. Do any of you know a way how to electrically implement power feedback ?
Not the way you describe it. But the German manufacturer T&A once made a thingie consisting of a long cone shaped piece that was mounted to the back-side of the dustcap and protruding out of the back of the magnet assembly through the pole-piece venting hole. On the back of the magnet the motion (i.e. coil POSITION to be exact) was then detected by y light barrier (which was more or less masked depending upon cone position).
Regarding the "electrical MFB" there are a lot of resources including the EW&WW article by Jeff Macaulay that was discussed here recently. There is still some discussion whether you can call this MFB or not.
Another related technique is the one called ACE (Amplifier Controlled Euphonic) which influences the TSP electrically by the use of frequency-dependant current feedback.
Regards
Charles
Thank you Charles.
The motion sensing I thought of could be done with say, a laser distance metering system for instance. That would have the advantage of not being mechanically or electrically coupled to the driver.
Electrical power feedback, I haven't really found anything so far in that respect, but I'll keep on searching... People talk about current drive or current feedback, yes, but I don't see how this differs from voltage feedback in that in both cases the amplifier stays unaware of the nonlinearity of the driver's response.
The motion sensing I thought of could be done with say, a laser distance metering system for instance. That would have the advantage of not being mechanically or electrically coupled to the driver.
Electrical power feedback, I haven't really found anything so far in that respect, but I'll keep on searching... People talk about current drive or current feedback, yes, but I don't see how this differs from voltage feedback in that in both cases the amplifier stays unaware of the nonlinearity of the driver's response.
I used an AD accelerometer, the 100g version. It's attached with epoxy glue to the voice coil in my JBL 2245H (the dust cap has to be removed and then replaced). A couple of very fine wires swiped out of an old tone arm bring the signal out to a preamp module glued to the driver frame. This conditions the signal and sends it down a shielded cable back to the amplifier mounted a few feet away. Very simple, and if I'd bought some plate amps when I put this together, it would have been even simpler.
I haven't seen an optical method yet, but there's a really, really cool way to do that. If I can hook up with someone who can do the microcontroller end of the project, I'll go optical. The advantages are that one can sense from any part of the cone, not just the voice coil, and one can get position measurement, which allows derivation of both velocity and position. With all these in hand, some rather nice signal processing/feedback can be implemented.
I haven't seen an optical method yet, but there's a really, really cool way to do that. If I can hook up with someone who can do the microcontroller end of the project, I'll go optical. The advantages are that one can sense from any part of the cone, not just the voice coil, and one can get position measurement, which allows derivation of both velocity and position. With all these in hand, some rather nice signal processing/feedback can be implemented.
Wow, sounds pretty nifty already...
Yes the optical method should get superb accuracy and zero interference with the acting driver. The signal processing would need some tuning I guess but it looks very elegant to me. Sadly I have no experience with microcontrollers.
This kind of approach could really cut speaker driver distortion down to 1/10 or 1/100 of typical (gross) levels...
Yes the optical method should get superb accuracy and zero interference with the acting driver. The signal processing would need some tuning I guess but it looks very elegant to me. Sadly I have no experience with microcontrollers.
This kind of approach could really cut speaker driver distortion down to 1/10 or 1/100 of typical (gross) levels...
Is more information better?
Design, particularly in the concept phase, is always fascinating and enjoyable. Implementation is where things can get a little frustrating. The maximum frustration is always found the first time you implement a concept/design. For example, radar or ultrasonic range detection would also conceptually work to identify absolute cone position. Yet, off the shelf range detectors have sampling rates too low to work with audio spectrum MF woofers. Substantial work is needed for implementation. Still, it is a really cool idea. Just wanted to add this to the others suggested.
If you will allow, I would like to be just a little philosophical for a moment or two.
One value of sharing is to reduce the frustration of implementation for other first timers. When sharing, maybe more information is better.
I love all the alternative ideas presented here. I would just like to see more details about their real world implementation. I would like to know what you were attempting to achieve (concept) and how close did your implementation came to the design?
When you "looped" the feedback, where did you insert it into the amplification block? Did you sum at the inverting input? Did you construct a summing network and add this to the input? Did you try to use integrators or in any way process the feedback signal? And so on?
This is not meant as a criticism, just a request for details I would value and appreciate. My own post was just an offer I thought might be helpful to members just getting started and maybe a little intimidated by designs requiring more parts, more modifications of the drivers (or woofers costing over $800 each), and sometimes the assistance of expertise few people have access to.
I have been building these simple servo woofer systems for 21 years. I know a lot about what they can do and what they cannot do. I searched the site and could not find other detailed project descriptions. I thought having one might be beneficial to a couple of other newbies. For others at a more intermediate level, maybe more intermediate design projects would be valuable. For the really sophisticated, we could have really sophisticated projects. I do not see any of these as being exclusive of any of the others.
Thanks to all for the comments and please keep on thinking about new ways to tackle old problems,
Mark
Design, particularly in the concept phase, is always fascinating and enjoyable. Implementation is where things can get a little frustrating. The maximum frustration is always found the first time you implement a concept/design. For example, radar or ultrasonic range detection would also conceptually work to identify absolute cone position. Yet, off the shelf range detectors have sampling rates too low to work with audio spectrum MF woofers. Substantial work is needed for implementation. Still, it is a really cool idea. Just wanted to add this to the others suggested.
If you will allow, I would like to be just a little philosophical for a moment or two.
One value of sharing is to reduce the frustration of implementation for other first timers. When sharing, maybe more information is better.
I love all the alternative ideas presented here. I would just like to see more details about their real world implementation. I would like to know what you were attempting to achieve (concept) and how close did your implementation came to the design?
When you "looped" the feedback, where did you insert it into the amplification block? Did you sum at the inverting input? Did you construct a summing network and add this to the input? Did you try to use integrators or in any way process the feedback signal? And so on?
This is not meant as a criticism, just a request for details I would value and appreciate. My own post was just an offer I thought might be helpful to members just getting started and maybe a little intimidated by designs requiring more parts, more modifications of the drivers (or woofers costing over $800 each), and sometimes the assistance of expertise few people have access to.
I have been building these simple servo woofer systems for 21 years. I know a lot about what they can do and what they cannot do. I searched the site and could not find other detailed project descriptions. I thought having one might be beneficial to a couple of other newbies. For others at a more intermediate level, maybe more intermediate design projects would be valuable. For the really sophisticated, we could have really sophisticated projects. I do not see any of these as being exclusive of any of the others.
Thanks to all for the comments and please keep on thinking about new ways to tackle old problems,
Mark
implementation details
Mark:
The accelerometer I used is the Analog Devices ADXL190 . I described the mounting a few posts back. The app info at analog.com show exactly how to hook the device up. Its response is quite flat to well beyond where I use it (my subs have a 4th order LP at 70 Hz).
Once you have a good, clean acceleration signal, it's just a couple opamps in standard blocks to mix it with the input signal (analog adder) and adjust the feedback ratio (i.e., the proportion of input signal to feedback signal), adjust the delay (one stage all-pass), and then output to the power amp. In my particular implementation, I buffered the accelerometer output (using a plain old LM310) to send it down a cable to the rest of the circuitry, which I built on a perfboard and glued inside of an Adcom 555. A smarter guy would have bought a plate amp, installed that and the opamp board into the subwoofer cabinet, and eliminated the need for a buffer.
Each 2245H is mounted in an 8 cu ft sealed enclosure. The measured small-signal f3 is about 16 Hz with the feedback, about 40-45 Hz without (if memory serves). I don't have the capability to easily do woofer distortion measurements, so can't comment on any improvement. I note that the accelerometer has a 0.2% linearity spec.
The key in getting it to work is not trying to push the sub too high in frequency. I wouldn't use anything other than a sealed enclosure, and of course, the driver max SPL limits will not be improved.
Mark:
The accelerometer I used is the Analog Devices ADXL190 . I described the mounting a few posts back. The app info at analog.com show exactly how to hook the device up. Its response is quite flat to well beyond where I use it (my subs have a 4th order LP at 70 Hz).
Once you have a good, clean acceleration signal, it's just a couple opamps in standard blocks to mix it with the input signal (analog adder) and adjust the feedback ratio (i.e., the proportion of input signal to feedback signal), adjust the delay (one stage all-pass), and then output to the power amp. In my particular implementation, I buffered the accelerometer output (using a plain old LM310) to send it down a cable to the rest of the circuitry, which I built on a perfboard and glued inside of an Adcom 555. A smarter guy would have bought a plate amp, installed that and the opamp board into the subwoofer cabinet, and eliminated the need for a buffer.
Each 2245H is mounted in an 8 cu ft sealed enclosure. The measured small-signal f3 is about 16 Hz with the feedback, about 40-45 Hz without (if memory serves). I don't have the capability to easily do woofer distortion measurements, so can't comment on any improvement. I note that the accelerometer has a 0.2% linearity spec.
The key in getting it to work is not trying to push the sub too high in frequency. I wouldn't use anything other than a sealed enclosure, and of course, the driver max SPL limits will not be improved.
Setting up mfb sub (again). This time with acc 01-04-05 ( it has better noise performance than the adxl series from analog devices)
The opamp konfiguration is werry similar dual vice coil or accelerometer. Slowly applying feedback, tapping the element until it you hear the element beeing harder and harder, until it resonates or start to resonate, then redusing feedback or open loop gain abt 9 dB makes a verry stable system.
After all my strugling with servo's it has become more and more easy to understand what is going on. I am realy supriced that there is so few servo subs out there!!!
feedback node - ref 200k, innput-ref 33k, ref output 100k, ref output 33n, and ref connected to negative innput of opamp
I'l make a sketch and post it after x-mas. It's nice to have a nice pair of sub's.
The opamp konfiguration is werry similar dual vice coil or accelerometer. Slowly applying feedback, tapping the element until it you hear the element beeing harder and harder, until it resonates or start to resonate, then redusing feedback or open loop gain abt 9 dB makes a verry stable system.
After all my strugling with servo's it has become more and more easy to understand what is going on. I am realy supriced that there is so few servo subs out there!!!
feedback node - ref 200k, innput-ref 33k, ref output 100k, ref output 33n, and ref connected to negative innput of opamp
I'l make a sketch and post it after x-mas. It's nice to have a nice pair of sub's.
I've yet to see a documented MFB project that achieves better than 6dB distortion reduction. Check out the Analog accelerometer implementation, they achieved only 38% average reduction, primarily due to phase problems. This simplistic implementation looks about the same, but it isn't clear exactly what the perpetrator is.
Now, the images accompanying the recent AX article looked to be more dramatic, but no measurements were given, especially of distortion reduction over a range. I fear that only the "best working" images were submitted with the article.
MarkMcK, I noted that you tried to implement this and didn't find much success as yet. This is on the back burner for me at least until early next year, but I'm following any research with interest.
I'm going to throw out the notion that these second-voicecoil based techniques may be the easiest to successfully implement due to the lack of a phase problem. The AX article may or may not prove this; hopefully someone can duplicate this and definitively measure the performance.
Now, the images accompanying the recent AX article looked to be more dramatic, but no measurements were given, especially of distortion reduction over a range. I fear that only the "best working" images were submitted with the article.
MarkMcK, I noted that you tried to implement this and didn't find much success as yet. This is on the back burner for me at least until early next year, but I'm following any research with interest.
I'm going to throw out the notion that these second-voicecoil based techniques may be the easiest to successfully implement due to the lack of a phase problem. The AX article may or may not prove this; hopefully someone can duplicate this and definitively measure the performance.
Mark,
Perhaps part of the reason your design didn't work to very low frequencies is because it didn't use an integrator. The coil voltage can be seen as "differentiated" because it's sensitive to the "rate of change" of movement as opposed to the "amount of movement". It's along the same principle that grammophones need to have a "phono" stage in order not to sound extremely tinny.
IME amplifiers are able to "stiffen the cone" when they're switched on anyway. Any tapping/pushing of the speaker's cone will attempt to induce a voltage across the voicecoil. However, the amplifier is a voltage "source" and large currents will flow in order to stop the cone from vibrating in any way that it's not meant to. The effect is practically identical to shortcircuiting the speaker's terminals (when it's not plugged in, AKA output impedance). At very low frequencies though, the acceleration of the coil is very small, therefore the voltage induced across it is also small, so the amplifier can't do much about it.
I suspect that the biggest effect of your design was actually equivalent to reducing the reactive component of the system's impedance. When the impedance is very flat and close to DC resistance, it is usually because the box is large and heavily damped. This would explain why the speaker sounded much "tighter", more expensive etc. Of course, it can't really change the impedance, it just reduces sensitivity where the impedance is highest (at resonance), which I intuitively think is quite similar.
I wouldn't worry much about the physical movement of the cone versus the voltage supplied by the amplifier, because it's possible for sounds to be virtually identical despite being visually unrecognizeable. The importance of phase coherence across a frequency range is often grossly overstated, and in many cases the efforts to compensate for phase differences are just a waste of time, no pun intended.
About accelerometers, they too measure the rate of change of movement (ie: "acceleration"), just like coils. The only real advantage they have over the voicecoil design is that they don't rely on the magnet system that powers the speaker. With magnet-and-coil design that you described you wouldn't be able to reduce distortion. The difficulty in getting an accelerometer design to work effectively would be integrating the frequency response (to make it flat rather than sloping), and making sure that phase differences don't adversely affect the performance. I suspect that this is why existing designs aren't that great at reducing distortion despite the promising concept.
CM
Perhaps part of the reason your design didn't work to very low frequencies is because it didn't use an integrator. The coil voltage can be seen as "differentiated" because it's sensitive to the "rate of change" of movement as opposed to the "amount of movement". It's along the same principle that grammophones need to have a "phono" stage in order not to sound extremely tinny.
IME amplifiers are able to "stiffen the cone" when they're switched on anyway. Any tapping/pushing of the speaker's cone will attempt to induce a voltage across the voicecoil. However, the amplifier is a voltage "source" and large currents will flow in order to stop the cone from vibrating in any way that it's not meant to. The effect is practically identical to shortcircuiting the speaker's terminals (when it's not plugged in, AKA output impedance). At very low frequencies though, the acceleration of the coil is very small, therefore the voltage induced across it is also small, so the amplifier can't do much about it.
I suspect that the biggest effect of your design was actually equivalent to reducing the reactive component of the system's impedance. When the impedance is very flat and close to DC resistance, it is usually because the box is large and heavily damped. This would explain why the speaker sounded much "tighter", more expensive etc. Of course, it can't really change the impedance, it just reduces sensitivity where the impedance is highest (at resonance), which I intuitively think is quite similar.
I wouldn't worry much about the physical movement of the cone versus the voltage supplied by the amplifier, because it's possible for sounds to be virtually identical despite being visually unrecognizeable. The importance of phase coherence across a frequency range is often grossly overstated, and in many cases the efforts to compensate for phase differences are just a waste of time, no pun intended.
About accelerometers, they too measure the rate of change of movement (ie: "acceleration"), just like coils. The only real advantage they have over the voicecoil design is that they don't rely on the magnet system that powers the speaker. With magnet-and-coil design that you described you wouldn't be able to reduce distortion. The difficulty in getting an accelerometer design to work effectively would be integrating the frequency response (to make it flat rather than sloping), and making sure that phase differences don't adversely affect the performance. I suspect that this is why existing designs aren't that great at reducing distortion despite the promising concept.
CM
CM,
Thanks for taking the time to read, think about, and respond to my simple little posting. I commend you for thinking about the idea of MFB in loudspeaker woofers.
I believe that this forum and others like it are much better for learning than they are for teaching. As a result I have tried to be careful about my responses. Nothing I say here is meant as criticism; it is just my thinking about your thinking.
First, for those interested in the basics of integrators and differentiators, this is a good Web site:
http://www.allaboutcircuits.com/vol_3/chpt_8/11.html
As I model the coherent transient world the use of integration in this application does not make any sense. I have designed an experiment to test my hypothesis, but it awaits the correspondence of free time and energy. These are two things that I don't seem to have at the same time lately.
Second, I chose to limit feedback to 6-db in this implementation. As a function of decay control, the test results included in the posting show a 6-db reduction continuing down to the low frequency limit of the test. There is a driver anomaly at about 20 Hz that appears in box or free air that cannot be controlled by MFB (from any source), but otherwise there is no drop off in feedback or correction. It is just that the output has fallen so far by these frequencies that it is not relevant. The application of MFB, however, is still working as intended even at very low frequencies.
Third, is the impedance rise at system resonance a cause or an effect?
Lastly, my comment about resistance to cone motion was meant as a relative comparison to a very expensive commercially available MFB woofer. With thousands of watts available and relatively large amounts of feedback, this system becomes extremely resistant to manipulation by human hands. Indeed, you don't move the cone by pushing on it; you have to lean your body weight into it. As I stated then, some are very impressed by this. Is it acoustically important? Not sure, but it seems doubtful. And yes, the application of MFB increases the apparent stiffness over what you have by turning on the amplifier without MFB.
Keeping learning and thinking, you seem to be doing well at it so far.
Mark
Thanks for taking the time to read, think about, and respond to my simple little posting. I commend you for thinking about the idea of MFB in loudspeaker woofers.
I believe that this forum and others like it are much better for learning than they are for teaching. As a result I have tried to be careful about my responses. Nothing I say here is meant as criticism; it is just my thinking about your thinking.
First, for those interested in the basics of integrators and differentiators, this is a good Web site:
http://www.allaboutcircuits.com/vol_3/chpt_8/11.html
As I model the coherent transient world the use of integration in this application does not make any sense. I have designed an experiment to test my hypothesis, but it awaits the correspondence of free time and energy. These are two things that I don't seem to have at the same time lately.
Second, I chose to limit feedback to 6-db in this implementation. As a function of decay control, the test results included in the posting show a 6-db reduction continuing down to the low frequency limit of the test. There is a driver anomaly at about 20 Hz that appears in box or free air that cannot be controlled by MFB (from any source), but otherwise there is no drop off in feedback or correction. It is just that the output has fallen so far by these frequencies that it is not relevant. The application of MFB, however, is still working as intended even at very low frequencies.
Third, is the impedance rise at system resonance a cause or an effect?
Lastly, my comment about resistance to cone motion was meant as a relative comparison to a very expensive commercially available MFB woofer. With thousands of watts available and relatively large amounts of feedback, this system becomes extremely resistant to manipulation by human hands. Indeed, you don't move the cone by pushing on it; you have to lean your body weight into it. As I stated then, some are very impressed by this. Is it acoustically important? Not sure, but it seems doubtful. And yes, the application of MFB increases the apparent stiffness over what you have by turning on the amplifier without MFB.
Keeping learning and thinking, you seem to be doing well at it so far.
Mark
Hello
I think one of the main things to do first, even before trying to apply feedback is to make the amplifier a current amplifier. The pahse is therfore changed 90* in our favor at high freq, and higher bandwith is possible. The problems getting feedback at resonanse is gone (if you have available plenty voltage out swing). At high frequensies the current drive does not suffer from coil inductance. As current is the force, the applyed voltage is not. The system easily becoms stable at a much higher freq using current drive. Making the amplifier current feedback is essential. The possibility to increase the amplifier gain below the upper corner freq also increases the amount of feedback possible. But then again i'm using accelerometer, thou the possibility to use the DVC MFB feedback is there ( dual voice coil elements ) .
I think one of the main things to do first, even before trying to apply feedback is to make the amplifier a current amplifier. The pahse is therfore changed 90* in our favor at high freq, and higher bandwith is possible. The problems getting feedback at resonanse is gone (if you have available plenty voltage out swing). At high frequensies the current drive does not suffer from coil inductance. As current is the force, the applyed voltage is not. The system easily becoms stable at a much higher freq using current drive. Making the amplifier current feedback is essential. The possibility to increase the amplifier gain below the upper corner freq also increases the amount of feedback possible. But then again i'm using accelerometer, thou the possibility to use the DVC MFB feedback is there ( dual voice coil elements ) .
Can it be simplified?
Mark, please forgive my ignorance but I don't know much about electronics. The MFB design really intriques me. I have a dual voice coil sub which I drive with an old stereo amp driven in mono. Would this system work on any amplifier? How do you determine the values of the resistors?
Finally, I can't quite follow the wiring schematics so perhaps I'm too ignorant to attempt this, however, it seems quite simple!
Mark, please forgive my ignorance but I don't know much about electronics. The MFB design really intriques me. I have a dual voice coil sub which I drive with an old stereo amp driven in mono. Would this system work on any amplifier? How do you determine the values of the resistors?
Finally, I can't quite follow the wiring schematics so perhaps I'm too ignorant to attempt this, however, it seems quite simple!
MarkMck,
I took a break from other work this weekend to check out your "super simple" MFB concept--and I must say I was impressed. (I am investigating solely for subwoofer use at the moment, so I can live with a very restricted operational bandwidth)
Unfortunately phase issues were a bigger problem than I had expected. Open-baffle, the phase differences between the input signal and the voicecoil measurement were fairly negligable, but in a sealed enclosure the phase shift was 90 degrees at the frequencies of interest. I chose to correct this about 30Hz via a capacitor and found that the voicecoil signal could be made to match the input EXACTLY. There is no reason this couldn't be done at 20Hz, and CM, note that this is without differentiation!
The only problem with this is that we introduce a low frequency pole into the feedback network...but this is somewhat inevitable anyway due to phase shift. Possibly it is even beneficial from a stability standpoint?
I'll post some scope traces in the next couple of days. I haven't taken distortion measurements yet but the servo responds as expected when I touch the cone...increases the input to sustain the voicecoil measurement.
I am now inclined to believe that a "simple" differentiator-free solution might be quite reasonable.
I took a break from other work this weekend to check out your "super simple" MFB concept--and I must say I was impressed. (I am investigating solely for subwoofer use at the moment, so I can live with a very restricted operational bandwidth)
Unfortunately phase issues were a bigger problem than I had expected. Open-baffle, the phase differences between the input signal and the voicecoil measurement were fairly negligable, but in a sealed enclosure the phase shift was 90 degrees at the frequencies of interest. I chose to correct this about 30Hz via a capacitor and found that the voicecoil signal could be made to match the input EXACTLY. There is no reason this couldn't be done at 20Hz, and CM, note that this is without differentiation!
The only problem with this is that we introduce a low frequency pole into the feedback network...but this is somewhat inevitable anyway due to phase shift. Possibly it is even beneficial from a stability standpoint?
I'll post some scope traces in the next couple of days. I haven't taken distortion measurements yet but the servo responds as expected when I touch the cone...increases the input to sustain the voicecoil measurement.
I am now inclined to believe that a "simple" differentiator-free solution might be quite reasonable.
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