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Old 15th December 2011, 02:22 AM   #19231
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Simon;

as I said before, when I design for some range of hi-fi speakers that are 2 or 3 way, and have dips almost down to zero, of course I would do all my best to ensure that the amp has low output resistance, and does not start wildly distorting when load resistance goes way down. And I am hundred percent sure that Stereophile would give very pleasant review to it. But it is not the single way to reproduce music, so some amp with very linear single-ended triode output would get very poor review, even if with corresponding speakers it will sound even better.

You know what happens when people enforce some artificial standards? Right, it forces the rest to pay money for "boutique" solutions that satisfy their tastes better.
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Old 15th December 2011, 02:30 AM   #19232
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Pavel,

Quote:
Originally Posted by PMA View Post
Anything else than voltage drive of the dynamic speaker is a great nonsense
If you apply a voltage to a dynamic driver/speaker a current flows.

This current flowing is what exerts a force on the diaphragm which hence is displaced from it's position.

In terms of variables outside the speaker that can change/generate the acoustic output the only variable is current, the acoustic output of the driver/speaker is dependent on the current in the voice coil and only the current in the voice and nothing but the current in the voice coil.

Now if we drive the speaker which reacts to current, only current and nothing but current with a voltage this voltage must be converted to a current somehow. This is done by the impedance of the speaker.

This impedance is composed out of the voice coil DCR, which is variable with temperature and hence will rise as we apply more power to the speaker, this is called power compression. Power compression in speakers which Sound On Sound showed as high as 5dB for some HiFi Speakers they tested) exists only with voltage drive and is cancelled using current drive.

Further the impedance of the speaker contains an inductive component. As this is created by winding a coil around a solid piece of steel (the pole piece and yoke) the current flowing through this coil with a given applied voltage will be distorted according to cubic function, that is creating odd order harmonics with the third highest and rising to the square of the applied power... This distortion mechanism is a direct function of the use of a constant voltage source to drive the speaker and it mainly shows up in middle and high frequencies where the voice coil inductance forms a large part of the drivers impedance.

In another twist, as the magnetic force is not constant for a given excursion of the driver and the Back EMF of the driver is not directly proportional to the diaphragms movement but also includes a signal variable term. As the Voltage Source driving the speaker short circuits the back-emf we have an electromechanical analog to the classic feedback loop, which, if the Amplifier used to drive the speaker includes looped feedback, extends into the actual amplifiers feedback loop. This mechanism causes unpredictable transient distortions and dominates with rising excursion.

So to me, knowing how a speaker actually works and what driving it with Voltage causes, driving a dynamic speaker with voltage is great nonsense, which does not prevent it from being the dominant mode employed. Even I do, as I need to maintain a system that is compatible with a wide range of commercial gear.

But just because everybody does it it is not necessarily right or optimal.

If I make an active speaker it will use current drive.

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Old 15th December 2011, 02:55 AM   #19233
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Hi,

Quote:
Originally Posted by vacuphile View Post
Agree for the most part, certainly for woofers it is most dangerous. At the high impedance peak(s) caused by the one (closed) or two (BR) fundamental resonance(s), voltage would sharply increase at the exact moment when excursion is already maximum and badly damped. Results like VC's bumping backplates or cones crumbling are likely.
I believe you are greatly confused. What you typed in this quote is a great fallacy. If it is not immediately obvious to you why " Results like VC's bumping backplates or cones crumbling" are not likely you may wish to peruse any of the good books on speakers that exist...

You are talking about damping. This a whole other kettle of fish. Lack of damping as such does not destroy any Speaker...

If you do not deal with the behaviour of the Mass/Spring resonant system of the driver and apply damping to it by other means (apperiodic damping was used with German Studio Speakers derived from the Eckmiller well into the 1980's, experimentally we used seamless voice coil formers made from Alu but they overdamped the drivers even for current drive).

When applying current drive Qe approaches infinity and the Drivers Qt becomes essentially Qm. Seas used to have some drivers with a very low (< 2) and linear Qm in their lineup, but last time I looked I could find them. But they seemed ideal for exploring current drive.

In a pinch you can even just equalisation to correct the frequency response (and yes, doing so MUST correct the impulse response too) as has been demonstrated in a number of publications...

At any extent, of course you cannot just take a generic off the shelf 3-way bass reflex box and drive it with a current source. Well, you can, but the results generally won't be pretty, though not woofer destructive (it should be obvious why).

Quote:
Originally Posted by vacuphile View Post
Hence my earlier question to Thorsten how many woofers he had destroyed in the time when he says he used current drive for all his drivers.
Your question was based on not actually thinking about the real systems involved and how they behave, but simply what you could use as Bon Mot to shoot down an idea you failed to understand but disliked...

Quote:
Originally Posted by vacuphile View Post
There are some other additional good reasons not to use current drive with woofers, such as the effect it has on Qes.
So what effect does it have on Qe (besides the obvious one)?

Quote:
Originally Posted by vacuphile View Post
Hawksford in his paper identifies some interesting properties of current drive though, such as flattening out Bl irregularities.
Yes, doesn't it just.

Quote:
Originally Posted by vacuphile View Post
Please note that in practice all mid and high drivers in passively filtered more way system have current drive to some extent, since they are usually attenuated in order to line up with the sensitivity of the woofer.
There is such an extensive number of misconceptions underpinning the above statement that I do not know where to start, so I won't. However, attenuating a driver does NOT create current drive (neither does putting a 1 Ohm resistor in series - which is what Bob Carver does in some of his Amp's for the "tube" output), it merely raises driving impedance to the driver somewhat. Current needs a source impedance that approaches infinity and not one that is a few ohm.

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Old 15th December 2011, 03:13 AM   #19234
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Quote:
Originally Posted by hitsware View Post
> Sorta kinda like listening to an open baffle from the side ... and rotated to appear a table ......
Hey Mike!

Yeah, something like that. Had some photos of it but can't seem to find them. I'll see if my friend still has them.

se
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Old 15th December 2011, 05:03 AM   #19235
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Quote:
Originally Posted by ThorstenL View Post
This is done by the impedance of the speaker.

This impedance is composed out of the voice coil DCR, ....

Further the impedance of the speaker contains an inductive component....

In another twist, as the magnetic force is not constant for a given excursion of the driver and the Back EMF of the driver ....

So to me, knowing how a speaker actually works ...
Thorsten, your view of speaker impedance is oversimplified.
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Old 15th December 2011, 05:11 AM   #19236
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http://www.klippel.de/fileadmin/klip...ity_Poster.pdf
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Old 15th December 2011, 05:34 AM   #19237
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Default World's Largest Woofer

Well, I almost didn't admit to this; but part-way down on the page:

Worlds loudest subwoofers - crank up the bass

you will see the Concept Design woofer. I was part of the engineering team that came up with this 60" "woofer." The first time we drove it, it was sitting face down on the lab floor, and it started hopping around, and we scrambled to cut power to it. It was scary seeing a 200+ pound woofer bouncing around out of control. Once mounted in the panel truck and driven at 1/4 max excursion, it proceeded to blow the windshield and side windows out of the cab, destroyed the latch assemblies and blast the doors open, balloon the sheet metal of the truck all over, and we just plain couldn't keep the rear doors shut. It created so much pressure that using a B&K mic and a scope (outside the van) you could see the asymmetry in the resulting pressure field due to the non-linearity of the air: you can compress air to many ATMs, but only decompress it to -1 ATM before it creates a vacuum. Crazy stuff, I hated being around it when it was on; it gave an instant headache and you felt weak and queasy for a while afterwards.

Was it hifi or musical? Hell no. Was it designed to make max SPL playing by contest rules, yup.

Another crazy tale from the trenches of car audio...

Howie

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Old 15th December 2011, 05:36 AM   #19238
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Joachim, that's correct, some of the elements of the schematics I posted are non-linear. Anyway, the speaker impedance may not be simplified to voice coil resistance, voice coil inductance and some mysterious "back EMF". This "back EMF" is just mechanical impedance and radiation impedance recalculated to electrical side of speaker schematics.
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Old 15th December 2011, 06:14 AM   #19239
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Pavel,

Quote:
Originally Posted by PMA View Post
Thorsten, your view of speaker impedance is oversimplified.
I merely presented three major points. You should not conclude that they represent all or even most of what I know or that I intend for them to represent a comprehensive picture. That would require several volumes without the references section and I have no time or will to write them.

The simplified model of a speaker you posted however is grossly oversimplified to reductio ad absurdum level and hence useless as it excludes all non-linearities...

Ciao T
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Old 15th December 2011, 06:37 AM   #19240
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Actually, there is a lot more to SUCCESSFUL speaker design that what can be commented on here. It is interesting that everybody has their opinions, but it is fairly amateur for the most part.
Of course, there is Beranek's law of speakers which goes: "If you design a loudspeaker and believe in the tradeoffs that YOU made, it is the BEST loudspeaker" Or something like that. '-)
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