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Simple MFB woofer project
Simple MFB woofer project
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Old 9th September 2003, 08:17 PM   #1
MarkMcK is offline MarkMcK  United States
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Default Simple MFB woofer project

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.

Click the image to open in full size.

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:

Click the image to open in full size.

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.

Click the image to open in full size.

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.
Click the image to open in full size.

Click the image to open in full size.

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.

Click the image to open in full size.

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.

Click the image to open in full size.


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.

Click the image to open in full size.
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Old 9th September 2003, 08:24 PM   #2
SY is offline SY  United States
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Simple MFB woofer project
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.
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Old 11th September 2003, 06:03 AM   #3
JoeBob is offline JoeBob  Canada
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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.
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Old 11th September 2003, 07:52 AM   #4
MBK is offline MBK  Singapore
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Default 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
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Old 11th September 2003, 07:53 AM   #5
MBK is offline MBK  Singapore
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Join Date: Apr 2002
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Default 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 ?
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Old 11th September 2003, 09:05 AM   #6
phase_accurate is offline phase_accurate
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Quote:
- 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?
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
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Old 11th September 2003, 09:16 AM   #7
MBK is offline MBK  Singapore
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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.
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Old 11th September 2003, 12:32 PM   #8
SY is offline SY  United States
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Simple MFB woofer project
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.
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Old 11th September 2003, 12:46 PM   #9
MBK is offline MBK  Singapore
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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...
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Old 11th September 2003, 01:00 PM   #10
phase_accurate is offline phase_accurate
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Quote:
The advantages are that one can sense from any part of the cone, not just the voice coil, and one can get position measurement
I think one source of failure for MFB systems is the assumption that the radiated sound is accurately described by the motion of ONE PART ONLY of the driver's cone.


Regards

Charles
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