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Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
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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.

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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
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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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www.rageaudio.com.au

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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.
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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


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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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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.


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www.rageaudio.com.au
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Default Motional feedback in speakers

On 8/11/2019 6:32 am, Mat Nieuwenhoven wrote:


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


**I've never lived with a pair, but was always impressed with the quite
decent sound quality from the Philips MFB systems.


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 first system I saw with that arrangement was the Infinity RS1. It
introduced as many problems as it solved. Amplifiers with 'floating'
output stages encountered some problems. Bridged amplifiers too. That
said, the bass extension available from a rather modestly sized, sealed
enclosure was impressive.

The idea is that the speaker's voice coil itself is the sensor. Will
this feedback method work?


**I can attest that it certainly works.

And only for closed boxes, or for
bassreflex also?


**I would have thought that such a system works best with the simple
roll-off available with a sealed enclosure. I guess with sufficient
modelling and appropriate filtering, it should be OK with more complex
enclosure types.


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www.rageaudio.com.au

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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.
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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.



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Default Motional feedback in speakers

On Thursday, November 7, 2019 at 4:32:20 PM UTC-5, Trevor Wilson wrote:

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 first system I saw with that arrangement was the Infinity RS1. It
introduced as many problems as it solved. Amplifiers with 'floating'
output stages encountered some problems. Bridged amplifiers too. That
said, the bass extension available from a rather modestly sized, sealed
enclosure was impressive.


May well be the case, but it wasn't because of feedback. If there
was anything done electronically, it was EQ which, itself, is a
completely legitimate way of getting bandwidth, if done properly*.

* Which, of course, is subject to Dick Pierce's First Law
of Acoustics: Anny idiot can design a loudspeaker and,
unfortunately, many do.


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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
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On Friday, November 8, 2019 at 3:52:03 PM UTC-5, Peter Wieck wrote:
Ummmmmm.....

A speaker is a linear motor with a magnet, and a commutator
(voice coil).


Sorry, no. The commutator in a motor IS NOT the equivalent of
the voice coil. The field windings of the motor are the equivalent
of the voice coil. The commutator, which is present only in motors
where the magnet is fixed and the windings are on the rotor, serves
two functions:

1. It's the way the current gets from the fixed input wires to the
spinning windings,

2. And it's the way that ensures that the polarity of the current
switches in synchrony with the relative position of the rotor
and the magnetic field.

And, mostly, it's what makes for DC generators.

Just as in a PM Motor, when current is applied,
the motor spins. DC motors spin according to the
polarity of the power applied.


And, yes, without the commutator, an applied DC motor would cause
the rotor to spin 180 degrees, at which point, the relative polarity
of the fixed magnetic field reverses (because the windings are now 180
degrees "backwards" mechanically) and the motor wants to spin in the
opposite direction.

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.


I'm sorry, you're absolutely wrong he the DC resistance
IS IN SERIES, not in parallel.

And forget the commutator, it's leading down the wrong path

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.


Peter, perhaps you've forgotten who I am: I've been doing this
stuff with speakers professionaly for a sizeable portion of
half a century at this point. And your analogy is STILL
inappropriate and flawed.

Be that as it may, you're omitting several VERY crucial points.
The most important one is that speakers are, first and foremost,
resonant systems. They are not motors that spin forever.

Secondly, damping is specifically the mechanism by which energy
is irretrievably removed form a resonant system: that is it's
fundamental definition.

And in ANY resonant system, the damping of that resonant system
is controlled by the total series resistance (be it electrical,
mechanical or acoustical).

Specific to drivers and speakers, the electrical portion of the damping
is the inverse function of the total series electrical resistance
in the electrical loop of the driver. And the single LARGEST series
electrical resistance in the VAST majority of drivers is the DC
RESISTANCE OF THE VOICE COIL.

Ignore this point, and, Peter, you WILL always come to the wrong
conclusion.

To go back to your motor analogy, it's NOT the difference between
the motor coil being open circuit or dead short, it's the difference
between open circuit and a fairly hefty series DC resistance.

All and at the same time, DF is only one (1) single factor in
how amplifiers interact with speakers.


And for the VAST majority of amplifiers and driver combinations,
it is among the LEAST significant of the bunch.

If you are so fixated on damping factor and you abjectly refuse
to consider it in it's correct context, then at least calculate
the right damping factor. The right damping factor, i.e., the
one that actually describes how the system is damped, is NOT
the ratio between the amplifier's output resistance and the
nominal impedance of the speaker, it is the ratio between
the voice coil's DC resistance, all divided by the SUM of the
amplifier's output resistance plus the voice coil's DC resistance.

Calculate it ANY other way, and you get a completely wrong answer
for damping.

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.


Please, what does overshoot and sagging have to do with one
another (especially as you have used "sagging" without a clear
definition of what you mean)?

And "overhang" is simply a function of the total Q of the system at
resonance. And the total Q of the speaker at resonance is a function
of the electrical and mechanical Q, to wit:

Qt = (Qm * Qe) / (Qm + Qe)

And the electrical Qe is a function of:

Qe = 2 pi Fs * (Mms * Re) / (B^2 l^2)

where Fs is the resonant frequency, Mms is the total effective moving
mass, Re is the DC resistance of the voice coil, B is the flux
density on the active portion of the voice coil gap and l is the
length of the voice coil wire in the active portion of the gap.

Now, adding the resistance provided by the amplifier changes that
electrical Qe:

Qe' = Qe * (Re + Rg) / (Re)

where Rg is the output resistance of the amplifier.

Clearly, this last equation shows that unless the amplifier output
resistance is significant in relation to the voice coil resistance,
it is the voice coil resistance that completely dominates the total
damping of the system.

THese are not my equations: go back and look at Thiele from the
the early 1960s, go back and look at Small from the early and
mid 1970s. If you wish to dispute these relations and the whole
issue of damping and damping factor, you'll need to argue it with
them with the same mathematical rigor that they formulated them
to begin with.

A 15" Zenith speaker driven by a single-ended 6F6 is
an entirely different animal than a 12" Long-throw woofer from an AR3a.


No, they most assuredly ARE NOT, not from the viewpoint of how
the physics of each work and how the mathematics describes those
physics very accurately, thank you.

Dick Pierce
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On 9/11/2019 6:50 am, Peter Wieck wrote:
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.


**And again: It is the output impedance that is important. The so-called
'damping factor' of an amplifier has (almost) nothing to do with damping
a speaker. The reason why speakers sound different on high output
impedance amplifiers (like most valve amps) is due to the frequency
response variations, caused by the interaction of the output impedance
of the amplifier and the impedance variations over the audible range of
the speaker system. See my previous submissions from the Stereophile
graphs.

Locate a speaker that exhibits an almost resistive load and check for
yourself. Maggies are a pretty good start. As is almost anything that
uses ribbon HF drivers (obviously, LF variations will depend on what
kind of bass driver/s is used). Maggies exhibit a highly resistive load
from top to bottom:

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

Such a speaker can be expected to perform very well with any valve (or
SS) amplifier, since frequency response variations due to a poor source
impedance (extant in most valve amps), will be minimal.

Even this one will be fine, provided the amp can cope with a slightly
tougher load:

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

At anything below 2kHz, the KEF R107 is a good one too. Note the
resistive nature of the impedance below that figu

https://www.stereophile.com/content/...1-measurements

Such a speaker will perform quite well with relatively high output
impedance amplifiers (like most valve amps).




--
Trevor Wilson
www.rageaudio.com.au
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Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
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Default Motional feedback in speakers

On 9/11/2019 12:25 am, wrote:
On Thursday, November 7, 2019 at 4:32:20 PM UTC-5, Trevor Wilson wrote:

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 first system I saw with that arrangement was the Infinity RS1. It
introduced as many problems as it solved. Amplifiers with 'floating'
output stages encountered some problems. Bridged amplifiers too. That
said, the bass extension available from a rather modestly sized, sealed
enclosure was impressive.


May well be the case, but it wasn't because of feedback. If there
was anything done electronically, it was EQ which, itself, is a
completely legitimate way of getting bandwidth, if done properly*.

* Which, of course, is subject to Dick Pierce's First Law
of Acoustics: Anny idiot can design a loudspeaker and,
unfortunately, many do.



**True enough. In fact, I've found somewhat excellent results using a
Behringer DCX2496 in place of the original Infinity crossover for these
speakers. The original crossover can be a PITA to integrate with some
systems, due a number of design limitations. The Behringer, OTOH, has
some useful features that make it an excellent, economical substitute.

--
Trevor Wilson
www.rageaudio.com.au


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[email protected] dpierce.cartchunk.org@gmail.com is offline
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Default Motional feedback in speakers

On Sunday, November 10, 2019 at 9:55:11 AM UTC-5, Trevor Wilson wrote:
On 9/11/2019 6:50 am, Peter Wieck wrote:
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.


**And again: It is the output impedance that is important. The so-called
'damping factor' of an amplifier has (almost) nothing to do with damping
a speaker.


No, it is not the output impedance that's important it is
the total loop resistance of amplifier-speaker system that
determines damping. And to attempt to drive the point home,
let me repeat these two very crucial points in an attempt to
emphasize their importance:

1. It is THE TOTAL LOOP RESISTANCE in the amplifier-speaker
SYSTEM that determines damping. And that, at the very least
includes the DC resistance of the voice coil, the speaker
leads and the amplifier.

2. And it is the RESISTIVE components of each of these that
are the mechanism that determines damping. You and Peter keep
saying "impedance", when, to be technically accurate, it is
NOT the impedance, but ONLY the restive component of the
impedance that is [part of the total loop resistance that
determines damping (in the electrical domain). Reactive
components to the impedance MAY change the frequency response,
as you suggest below, but they DO NOT change damping.

The reason why speakers sound different on high output
impedance amplifiers (like most valve amps) is due to the frequency
response variations, caused by the interaction of the output impedance
of the amplifier and the impedance variations over the audible range of
the speaker system.


I'd not necessarily go so far as saying this is THE reason
they sound different, but it certainly is one of the significant
contributors to real differences in acoustic output.

Locate a speaker that exhibits an almost resistive load and check for
yourself. Maggies are a pretty good start. As is almost anything that
uses ribbon HF drivers (obviously, LF variations will depend on what
kind of bass driver/s is used). Maggies exhibit a highly resistive load
from top to bottom:


As do many KEF systems that incorporate complex conjugate
networks do compensate for reactive impedance variations,
resulting in impedances that are almost uniformly 4 ohms
across the audio band (you give one example).

Dick Pierce

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