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.

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

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

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

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

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