Dual Voice Coil as Eddy Current Brake - diyAudio
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Old 9th April 2016, 12:55 AM   #1
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Default Dual Voice Coil as Eddy Current Brake

Hello All,

I've been tinkering (and trying to learn along the way) with a dual voice coil driver (Boss CH10DVC). I'd like to understand the effects of shorting one of the voice coils. This, I presume creates and Eddy Current Brake like a shorting ring but let's expand on what that means for the driver in real-world use... Because I would love to know

I've included two free air measurements:

Green - Both coils wired in series 8 Ohm
Magenta - Single 4 Ohm VC in use, 2nd VC shorted

Who can help me interpret these results?

Cheers!
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Old 9th April 2016, 03:33 AM   #2
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Very interesting. Old timers might remember when meters arrived with their terminals shorted to keep the moving coil from swinging around (even though nominally balanced).

The shorted curve looks a whole lot better, eh. That's because you are using motional feedback or at least passive degenerative feedback (pretty much the same thing).

Like true active motional feedback, effectiveness depends on the absence of correlation of the two coils and on the inherent linearity of the magnet system (which ought to be pretty linear). It does correct for the inept way Rice-Kellogg drivers are suspended with a weird piece of rubber and some phenolic cloth (as opposed to some intelligent kind of sensible linear Boyle's Law mechanism).

Little of this will make much sense to those who think everything you need to know about drivers is enclosed in the little world of Olsen/Thiele.

Ben
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Old 9th April 2016, 04:09 AM   #3
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You simply changed the driver Q. You can fix a potentiometer to the non-powered coil and adjust this effect. I found a document that explains this from a couple of decades (from Adire). See:
http://www.diy-audio.narod.ru/litr/RDOOperation.pdf
I also found this doc, that might be helpful:
Dual-VC
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Last edited by CharlieLaub; 9th April 2016 at 04:11 AM.
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Old 9th April 2016, 04:58 AM   #4
JMFahey is offline JMFahey  Argentina
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Yes, you damped the speaker a lot with an eddy current brake.
The shorted coil also strongly attenuated the residual inductance which rises impedance at higher frequencies.
I bet the effect at lower frequencies is even stronger than a plain shorting ring, because the latter is static and only attenuates by magnetic coupling while the shorted coil not only shorts the magnetic field but also acts as a generator driving a shorted load (the shorting ring does not do that) so at least conceptually the braking effect must be stronger.

FWIW after hating aluminum former coils for ages , preferring non conductive formers, now I'm reconciling myself with them, seeing that they require a different approach but can do their own thing.

Personally I don't lightly despise Olsen/Beranek/Thiele/Davis and other old timers, every time I read them I find something "new" and useful, go figure.

Kapton/Nomex/Kevlar/fiberglass/carbon fiber/epoxy/neodymium/etc. are new *materials* they didn't even dream of, but the Physics Laws remain the same
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Old 9th April 2016, 05:26 AM   #5
specd is offline specd  United States
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Quote:
Originally Posted by bentoronto View Post
... least passive degenerative feedback
Hate it when that happens, but there have been times I deserved it.
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Old 9th April 2016, 05:33 AM   #6
Ron E is offline Ron E  United States
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Basically the shorted coil increases the effective mechanical damping. The reduced Qms results in a really small impedance peak. The shorted coil acting like copper cladding or shorting rings in the magnetic circuit is an added bonus, but I haven't tried to predict that part
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Old 9th April 2016, 11:07 AM   #7
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We all agree. Wonderful.

And while it (or shorting rings or much else) really isn't in the Olson model, nobody doubts it can be portrayed in the electrical-analogy model (perhaps as simply as doing something imaginative to Qes).

But now the real question: does it improve the driver's performance? Seems to me, any reasonably linear negative feedback has to help. The charts show the expected reduction in resonance (or Q perhaps) and the improvement in phase error. Does this help the sound?

BTW, would it be true to say the sim advocates have no way to evaluate the question and particularly matters of transient performance?

Ben
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Old 9th April 2016, 02:02 PM   #8
Ron E is offline Ron E  United States
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Quote:
Originally Posted by bentoronto View Post
BTW, would it be true to say the sim advocates have no way to evaluate the question and particularly matters of transient performance?
To call an electromagnetic dissipating force "feedback" is a stretch. As far as evaluating transient response, small-signal will closely follow T/S theory, and large signal will need a much more complicated nonlinear model - or (simpler) measurement.

Will it improve sound? I doubt it. If you really need the flat impedance, it could be useful, but getting a driver that already has copper in the motor is a better solution. It throws away a coil that could be generating output so you lose 6dB of voltage sensitivity. The system damping it provides could be accomplished another way, like DSP.
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Old 9th April 2016, 03:43 PM   #9
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Interesting indeed. From what I can tell, the upside to this configuration would be a drop in Le and a smaller deviation is phase near Fs frequencies. The downside being reduced power handling and sensitivity. Considering that this is a subwoofer and the effects of Le at low frequencies are unclear to me, I may have to run some more tests and let my ears weigh in. Thanks for the information guys.
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Old 9th April 2016, 03:55 PM   #10
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Quote:
Originally Posted by bentoronto View Post
We all agree. Wonderful.
I don't want to speak for anyone but I don't think anyone has actually agreed with the premise that this shorted coil actually improves the driver.

Quote:
And while it (or shorting rings or much else) really isn't in the Olson model, nobody doubts it can be portrayed in the electrical-analogy model (perhaps as simply as doing something imaginative to Qes).

But now the real question: does it improve the driver's performance? Seems to me, any reasonably linear negative feedback has to help. The charts show the expected reduction in resonance (or Q perhaps) and the improvement in phase error. Does this help the sound?
This is exactly why your premise is wrong. Reducing the impedance peak has no practical benefit. And phase is not an error.

If you view the impedance peak and it's resulting phase as bad then everything that follows will be based on a misunderstanding of what these things are, what they mean and what effect they have.

You've spoken out in the past about the "dangers" of operating below fs, it's clear that you don't really understand how these things work.

Quote:
BTW, would it be true to say the sim advocates have no way to evaluate the question and particularly matters of transient performance?

Ben
I'd say sim advocates are in a much better position to evaluate the effects than you are.

By shorting the coil you have:
1. Decreased the sensitivity of the driver. Subwoofer drivers usually already have very low sensitivity.
2. Decreased the power handling.
3. Decreased the qts.

And you've decreased the sensitivity, qts and power handling a LOT, not just a bit.

So what effect does all that have?

First, you need a bigger box for any given goal qtc, and in fact if qts is too high even an infinite sized box might not get you into a reasonable qtc range. But if you have eq you can have any qtc you want.

Second, the decreased sensitivity AND decreased power handling. The reduced sensitivity means you have to put a LOT more power through the one operating coil to achieve the same spl. And the decreased power handling means the one operational coil can only handle some fraction of the driver's intended power handling. Combined, this means the driver will be severely limited in terms of max spl.

Because of that you will run into power compression issues very early on at low spl.

So what does the shorted coil ACTUALLY accomplish? You've squashed the impedance peak and the resulting phase. Neither of those were a problem in the first place. And you've reduced the inductive rise at high frequencies. This is basically meaningless for a subwoofer as it isn't interested in high frequencies anyways.

In exchange you've absolutely ruined the driver's performance by dramatically increasing qts (by 2x or more), dramatically decreasing sensitivity and dramatically decreasing power handling. These things are more likely to INCREASE distortion and REDUCE performance at moderate to high power levels and there's no real benefit at all.

They only real benefit the Adire paper alludes to is the ability to change the q (the sound) on the fly, with a more "musical" q and a "thicker" q. It would be a LOT better to do this with eq because if the driver is not a super low qts driver in the first place, you will never be able to achieve the low "musical" q even in an infinite size box, and the other cons to doing this (decreased sensitivity and power handling) significantly decrease the performance potential of the driver and will result in higher distortion at moderate to high power levels. Basically a lose/lose situation.
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