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Motional feedback in speakers



 
 
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  #1  
Old November 4th 19, 09:08 PM posted to rec.audio.high-end
Trevor Wilson[_3_]
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Default Motional feedback in speakers

In another forum I inhabit, someone posted details about a National
Panasonic amplifier and speakers that employs a Motional FeedBack system:

https://www.hifiengine.com/manual_li...l/sa-52h.shtml

I was more than a little surprised, as I assumed that Philips invented
the system for their famous range of speakers, released in the 1970s.
Anyway, I did a little digging and found that the National system was
based on one invented by Luxman in the 1960s:

http://www.proaudiodesignforum.com/f...php?f=12&t=707

Impressive technology for a domestic product for the time.

--
Trevor Wilson
www.rageaudio.com.au

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  #2  
Old November 5th 19, 01:16 PM posted to rec.audio.high-end
Peter Wieck[_2_]
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Default Motional feedback in speakers

No surprise at all, back in the day when tube-based amplifier damping factors were somewhere between 10 and 15 on a good day with a tailwind, and acoustic suspension was uncommon. The 'cure' for tubby bass would be some choice between a stiffer spider, stiffer surround, smaller magnet, shorter excursion, any or all.

Seems overly complex - and I can see where it did not last long, or penetrate past a very few makers. By the way, Philips speakers were "active" speakers, needing only a pre-amp input.

Peter Wieck
Melrose Park, PA
  #3  
Old November 6th 19, 09:55 PM posted to rec.audio.high-end
Trevor Wilson[_3_]
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Posts: 116
Default Motional feedback in speakers

On 5/11/2019 11:16 pm, Peter Wieck wrote:
> No surprise at all


**Well, it was a surprise to me. In 45 years of servicing equipment,
I've never seen that technology is a valve based product. Perhaps it was
more common in the US.

, back in the day when tube-based amplifier damping factors were
somewhere between 10 and 15 on a good day with a tailwind, and acoustic
suspension was uncommon.

**The output impedance of modern valve amps is no better. In fact, the
real figures are frequently much poorer than the (inferred) figure of
0.8 ~ 0.5 Ohms you suggest (so-called 'Damping Factor' is a misleading
and rather poor specification to quote in technical matters). Here are
some figures from Stereophile:

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

Fundamentally, the output transformer is a major limitation in most
valve amplifiers. It leads to serious problems with frequency/phase
response figures for most loudspeakers.



The 'cure' for tubby bass would be some choice between a stiffer
spider, stiffer surround, smaller magnet, shorter excursion, any or all.

**Or, of course, a decent amplifier, with a sensible output impedance
figure (which excludes most valve amps).


>
> Seems overly complex - and I can see where it did not last long, or penetrate past a very few makers. By the way, Philips speakers were "active" speakers, needing only a pre-amp input.
>


**Well, yes, but the Philips MFB speakers also employed a closed loop
system, which measured the output signal from the speakers, thus
negating much of the influence of issues with the amplifier used (and
the limitations of the enclosure). The Luxman/Panasonic system appears
to employ a similar, closed loop approach. And, as I stated before, I
was surprised to see such a system in a domestic system way back then,
due to the cost/complexity of such a system. I'd like to see one in the
flesh.


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Trevor Wilson
www.rageaudio.com.au

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  #4  
Old November 7th 19, 02:56 PM posted to rec.audio.high-end
[email protected]
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Default Motional feedback in speakers

On Wednesday, November 6, 2019 at 11:12:22 AM UTC-5, Peter Wieck wrote:
> No surprise at all, back in the day when tube-based amplifier
> damping factors were somewhere between 10 and 15 on a good day
> with a tailwind, and acoustic suspension was uncommon. The 'cure'
> for tubby bass would be some choice between a stiffer spider,
> stiffer surround, smaller magnet, shorter excursion, any or all.


Uhm, no, not anywhere near a "cure", in fact, demonstrably feeds the
disease. Let's look at it piece by piece. Assume everything else
remains the same, and by "tubby bass", you mean excessive Q at resonance:

1. Stiffer spider, stiffer surround, either or both will raise the
resonant frequency of the driver, thus a higher Q at resonance.
Result: more "tubby bass".

2. Smaller magnet: presumably what youare in effect saying is reduce
the BL product of the motor system. Well, since the Q at resonance
is an inverse function of the BL product, that raises the Q of the
system. Result: more "tubby bass."

3. Shorter excursion: that, by itself will have little effect on
"tubby bass" per what it will do is cause more distortion at
lower levels. Result: more distorted "tubby bass."

Now, if the issue is speakers with "tubby bass" and low damping factor
amplifiers, motional feedback IS NOT THE CURE AT ALL. The entire point
of motional feedback was to attempt to linearize the speaker system,
i.e. reduce distortion. It does NOTHING to reduce Q, reduce "tubby bass."

One of the big problems with drivers is that the suspension stiffness
and the magnet's BL product is, for the most part, highly non-linear:
the stiffness increases with excursion, the BL product decreases
with excursion. And the greater the excursion, the greater the
deviation of both. And the greatest excursion occurs at low frequencies
(all other things being equal) and, ultimately, at resonance.

Now, IF the difference between sensor (the sense coil, in at least on
case) says where the woofer is vs where the amplifier thinks the
woofer should be, the feedback SHOULD provide a complementary non-linear
change in its output voltage to compensate.

There are several different ways of doing this: a sense coil
will provide an output that's a function of velocity, while
things like the piezo sensor in things like the Velodyne output
a signal that's proportional to acceleration. In the former
case, you integrate once to get position, in the later, you
integrate twice to get position.

But here's the rub: there IS NOT a simple functional relationship
between either the position, velocity or acceleration of the cone
and its acoustic output. In other words: the sensor providing the
feedback signal CANNOT be used to correct the frequency response
of the system, i.e., it's not fixing "tubby bass", it's trying to fix
"distorted bass".

Let me give one example that shows why this is true: look at ANY
bass reflex system: let's assume, for the moment, that it is
"optimally" tuned, i.e., that its frequency response is that of a
"perfect" lossless B4 alignment: it's dead flat down to cutoff,
and it rolls off at 24 dB/octave below that. Now, look at what
the cone is doing as it goes down in frequency through the region
of cutoff:

Acceleration: Constant with frequency as you go lower until
you approach the enclosure tuning frequency, at which point
is approaches 0, ad blow which it increases back up to
the same constant as above tuning.
Velocity: Increases as inverse of frequency until you approach
the enclosure tuning frequency, then goes to 0, goes back up for
a bit then starts decreasing with decreasing frequency.
Position: Increases as the inverse square of frequency until
you approach the enclosure tuning, at which point it approaches
0, then below it starts increasing with decreasing frequency.

And, oh by the way, where all the mechanical stuff goes to 0 at
the enclosure tuning frequency, the output of the port is increasing
and the bulk of the speaker's acoustic output comes NOT from the driver,
but from the port.

In other words, as far as frequency response is concerned, neither the
acceleration, nor the velocity, nor the position of the cone is a
useful predictor of the acoustic output in such a system.

> Seems overly complex - and I can see where it did not last long,
> or penetrate past a very few makers. By the way, Philips speakers
> were "active" speakers, needing only a pre-amp input.


Well, I "was there" when the the phillips was being marketed, and at
the same time there was an actually reasonably well-implemented B6
system from EV, designed by, I believe D. B. Keele. Neither speaker was
"outstanding", but both were quite reasonable systems, performance-wise.

Both died in the market for a variety of reasons, but, especially with
the Philips, customers resisted it because they wanted to use it with
THEIR amplifier. Often times, they already had a system and wanted to
upgrade speakers, and looked at their existing amplifier as now being
a "waste". And, with the EV, that little EQ box, no matter how technically
sound the approach was, was just an obstacle for most people. To many,
it reminded them of the Bose 901, and for many, that was just too much.

It had NOTHING to do with performance, it was all about perception,
rational or otherwise. Regardless, systems like this withered on the
vine having little to do with their technical merit.
  #5  
Old November 7th 19, 08:31 PM posted to rec.audio.high-end
Peter Wieck[_2_]
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Posts: 104
Default Motional feedback in speakers

Take the (permanent magnet) speaker as a linear motor.

Now, for illustrative purposes, obtain a small DC brush-type motor. Spin it with your fingers. Now, short the leads to the motor. Now, try spinning it again with your fingers.

Not so easy.

That is the function of "damping factor" - As a way to prevent a PM speaker from wobbling like a spring when it is released.

Peter Wieck
Melrose Park, PA
  #6  
Old November 7th 19, 09:39 PM posted to rec.audio.high-end
Trevor Wilson[_3_]
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Posts: 116
Default Motional feedback in speakers

On 8/11/2019 6:31 am, Peter Wieck wrote:
> Take the (permanent magnet) speaker as a linear motor.
>
> Now, for illustrative purposes, obtain a small DC brush-type motor. Spin it with your fingers. Now, short the leads to the motor. Now, try spinning it again with your fingers.
>
> Not so easy.
>
> That is the function of "damping factor" - As a way to prevent a PM speaker from wobbling like a spring when it is released.
>


**No. It doesn't work like that. In a PROPERLY designed enclosure,
so-called "damping" is not dependent on amplifier output impedance.
Damping is supplied by the enclosure itself.

However, the poor output impedance exhibited by the vast majority of
valve amplifiers and some SS amplifiers can lead to significant
frequency response aberrations in line with the impedance variations
exhibited by most speaker systems.


Again, I refer you to a couple of curves published by Stereophile:

https://www.stereophile.com/content/...r-measurements

Note the 3.5dB peaks at 65Hz and 1.5kHz.

Now here is the curve of a 'perfect' amplifier driving the same,
simulated, speaker load:

https://www.stereophile.com/content/...r-measurements
(fig.1)

Note the differences between resistive loads and reactive loads.

And, of course, here is a rather more modestly priced 'perfect' amplifier:

https://www.stereophile.com/content/...r-measurements

And, once mo "Damping factor" is a misnomer. It is a misleading and
incorrect term to use. Output impedance (preferably quoted from 20Hz ~
20kHz) is vastly more preferable. A PROPERLY designed speaker system
already has adequate damping. Amplifier damping is not relevant, BUT
output impedance is.


--
Trevor Wilson
www.rageaudio.com.au
  #7  
Old November 8th 19, 01:27 AM posted to rec.audio.high-end
[email protected]
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Posts: 332
Default Motional feedback in speakers

On Thursday, November 7, 2019 at 3:51:05 PM UTC-5, Trevor Wilson wrote:
>
> **No. It doesn't work like that. In a PROPERLY designed enclosure,
> so-called "damping" is not dependent on amplifier output impedance.
> Damping is supplied by the enclosure itself.


Nope, the enclosure supplies NO damping at all. Maybe the
internal fibrous fill provides a little, but the enclosure,
that is, the volume of air that goes through compression and
rarefaction as the cone moves, provides absolutely no damping
at all.

Unless someone has hijacked the meaning of the term, "damping"
quite unambiguously and precisely to the action by which energy is
removed from a system and does not return. In the case of loudspeakers,
there are three sources of such action (the permanent removal of
energy from a resonant system):

1. Electrical: motional or electrical kinetic energy is transformed
into heat by the electrically resistive elements in the circuit.
The single most significant such element is the DC resistance of
the voice coil: most of the energy will be converted by that DC
resistance into heat and removed from the system.

Changing the total loop resistance from, say, 6 ohms, connected
to an amplifier with infinite "damping factor" to, say, 6.3 ohms,
connected to an amplifier with a "damping factor" of 20, guess
what, the change in electrical damping is a STAGGERING ... 5%.

2. Mechanical, principally the frictional losses in the surround and
spider. These frictional losses are less significant than the
electrical. Typically, the mechanical damping is anywhere from
6 to 10 times less than the electrical damping.

3. Acoustical: this is the energy carried away by the sound the speaker
is making and, for the vast majority of direct-radiator speaker, this
damping comprises the tiniest part of the total dissapitive losses.

  #8  
Old November 8th 19, 01:12 AM posted to rec.audio.high-end
[email protected]
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Posts: 332
Default Motional feedback in speakers

On Thursday, November 7, 2019 at 2:41:04 PM UTC-5, Peter Wieck wrote:
> Take the (permanent magnet) speaker as a linear motor.
>
> Now, for illustrative purposes, obtain a small DC brush-type motor.
> Spin it with your fingers. Now, short the leads to the motor.
> Now, try spinning it again with your fingers.
>
> Not so easy.
>
> That is the function of "damping factor" - As a way to prevent a
> PM speaker from wobbling like a spring when it is released.


Cute analogy, but FAR from the physical reality of speakers.

Start by inserting a 6-ohm resistor in series with the
lead to the motor. Make sure it's PERMANENTLY connected
and can't be bypassed. Pretend that's the DC resistance
of the voice coil.

Try to spin it with your finger. Easy, right?

NOW short it (making sure that 6-ohm resistor is still in
series).

Try to spin it with your finger. Really, how much harder
to you think it might be (hint: not much).

Take your same motor, but pack the bearings with thick grease.
Now, try spinning it with your fingers. Not so easy.

So, if you want your little experiment to have ANY connection with
the reality of a speaker, that's what you have to do.
  #9  
Old November 8th 19, 08:50 PM posted to rec.audio.high-end
Peter Wieck[_2_]
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Posts: 104
Default Motional feedback in speakers

Ummmmmm.....

A speaker is a linear motor with a magnet, and a commutator (voice coil). Just as in a PM Motor, when current is applied, the motor spins. DC motors spin according to the polarity of the power applied. Speakers move in or out depending on the polarity of the current applied. And, PM motors do, also, have a fixed resistance across the commutator just like a voice coil.

Now, when current stops being applied, the motor generates current - acts as a generator as it spins down. If it is unloaded, that current goes nowhere and does not add additional resistance to the motor spinning than normal bearing friction. However, if the motor is loaded, there will be additional friction.

Similarly the (conventional) speaker. Try it some time with a sensitive VOM.. The bigger the driver, the more easily this is observed. Just a few taps on the speaker cone will show you.

All and at the same time, DF is only one (1) single factor in how amplifiers interact with speakers. And, today in 2019, the issues that drove speaker design in the era after field-coil speakers were dominant up until the development of acoustic suspension are not particularly relevant as much evolution is taken for granted (and usually is granted). However, as one who spends as much time with electronics from the 1930s as from the the 1970s and up, I see all sorts of variations on how to control large speaker overshoot, sagging, and similar problems. A 15" Zenith speaker driven by a single-ended 6F6 is an entirely different animal than a 12" Long-throw woofer from an AR3a.

Peter Wieck
Melrose Park, PA
  #10  
Old November 7th 19, 08:32 PM posted to rec.audio.high-end
Mat Nieuwenhoven
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Posts: 42
Default Motional feedback in speakers

On Thu, 7 Nov 2019 05:56:12 -0800 (PST),
wrote:

<snip>
>One of the big problems with drivers is that the suspension stiffness
>and the magnet's BL product is, for the most part, highly non-linear:
>the stiffness increases with excursion, the BL product decreases
>with excursion.


Is this BL change true also for underhang/overhang voice coils? The
german magazine HobbyHifi (very technical about homebuilding
speakersystems) stated several times that (paraphrased) some speaker
have a fair amount of excursion before the cross-section of voice
coil and magnetic field changes.

<snip>
>Let me give one example that shows why this is true: look at ANY
>bass reflex system: let's assume, for the moment, that it is
>"optimally" tuned, i.e., that its frequency response is that of a
>"perfect" lossless B4 alignment: it's dead flat down to cutoff,
>and it rolls off at 24 dB/octave below that. Now, look at what
>the cone is doing as it goes down in frequency through the region
>of cutoff:
>
> Acceleration: Constant with frequency as you go lower until
> you approach the enclosure tuning frequency, at which point
> is approaches 0, ad blow which it increases back up to
> the same constant as above tuning.
> Velocity: Increases as inverse of frequency until you approach
> the enclosure tuning frequency, then goes to 0, goes back up for
> a bit then starts decreasing with decreasing frequency.
> Position: Increases as the inverse square of frequency until
> you approach the enclosure tuning, at which point it approaches
> 0, then below it starts increasing with decreasing frequency.


I don't get this. If velocity is 0 at the tuning frequency, the
speaker is making no sound at all. I'm pretty sure it keeps moving.

I though bass-reflex boxes below the tuning frequency behaved as open
enclosure, more open the lower you get in frequency. If you keep
applying the same power, you can damage the speaker due to too large
excursions, because it has typically a flexible suspension and relies
on the air in the box to damp its movement.

>Well, I "was there" when the the phillips was being marketed, and at
>the same time there was an actually reasonably well-implemented B6
>system from EV, designed by, I believe D. B. Keele. Neither speaker was
>"outstanding", but both were quite reasonable systems, performance-wise.
>
>Both died in the market for a variety of reasons, but, especially with
>the Philips, customers resisted it because they wanted to use it with
>THEIR amplifier. Often times, they already had a system and wanted to
>upgrade speakers, and looked at their existing amplifier as now being
>a "waste". And, with the EV, that little EQ box, no matter how technically
>sound the approach was, was just an obstacle for most people. To many,
>it reminded them of the Bose 901, and for many, that was just too much.


Interesting story. I had two of the smallest Philips MFBs a few years
back, was not impressed with the sound.

Regarding feedback, I remember there was an hobby project long ago to
have a very small R between speaker and GND (GND also being the amp's
ground), and using the speaker's back EMF as feedback to correct
excursions. There are some later publications from W.Kippel about it.
The idea is that the speaker's voice coil itself is the sensor. Will
this feedback method work? And only for closed boxes, or for
bassreflex also?

Mat Nieuwenhoven


 




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