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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 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