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Series vs Parallel XO & Maggie Physics.

Posted by DrChaos (A) on July 17, 2010 at 00:48:14

I don't yet have a complete physical understanding of what's happening but I'd like to spark some fruitful science-oriented (not accusatory) discussions. This obviously has some relevance to PG's crossover modifications.

First, let's start with some basics which I believe are accurate to the limits of their assumptions.

http://www.audioholics.com/education/loudspeaker-basics/series-vs-parallel-networks-first-order-comparison

http://sound.westhost.com/parallel-series.htm

(I think the first of the two articles is more complete but the 2nd has clearer graphics)

Upshot is that if one models speaker drivers as resistive non-interacting units then there is no difference between parallel & series.

However, speakers drivers are not purely resistive. As the two articles show if one modifies woofer impedance (e.g. as a reactive load) the summed response of the series network "self-compensates" better than the parallel.

However, there is one important issue which is critical to 2-way Maggie physics which is not discussed in the articles. The articles assume usual woofer+tweeter and briefly mentions how "back EMF" from woofer couples into tweeter and then assumes it's bad.

The physical difference is that unlike conventional speakers there is a direct physical coupling between motions on the woofer part of the panel to the tweeter part---coupling via the mylar of course.

Where does "back EMF" come from? It's because the diaphragm has inertia & momentum (and the air that it squeezes between the mylar and pole piece is compressible and bouncy) and thus there is some 'desire' to overshoot. The back EMF is the result of the coils connected to the diaphragm moving through the magnetic fields. This is what the "damping factor" of the amplifier tries to stop and dissipate/control as a result of a low output impedance. (laws of Newtonian physics are 2nd order differential equations not first, engineering would be so much easier without inertia)

Now, consider a maggie driver: the inertial motion of the woofer forward pulls forward the tweeter too in the same direction. Both will generate EMF (and if drivers are in phase) the voltage will sum in a series configuration.

Now look at the right hand side of the figure 1.0 in http://sound.westhost.com/parallel-series.htm, showing the series network & drivers. Think about EMF voltage adding. This back EMF voltage is touching the output of the power amplifier directly!! No caps/inductors in the way.

So, my interpretation is in terms of damping overall driver inertia, in 2-way maggies with series XO it is just like an amplifier connected to a single conventional driver, e.g. with active cross-overs and multiamping, one benefit of which is supposed to be a much better damping factor. This is a result of the confluence between the unusual maggie physics and a series XO like the one in the diagram.

Also consider the situation for frequencies in the crossover region.
Either the two parts of the mylar are be asked to move in the same direction, or they are being asked to move in the opposite direction. The one where they are in the same direction is energetically favorable since you do not have to stretch the Mylar as much. (More physically more of the force can be used to push the driver against the air instead of stretching the driver within itself).

As far as I know, stock 1.6's have tweet & woof out of phase. If the PG mod puts them in phase with the series XO then that could be one of the origins of the claimed increased sensitivity.

Presumably Magnepan did what they did to alleviate an otherwise bothersome frequency response anomaly (too big hump somewhere)? My guess is that the PG frame mod changes the physical properties and the good 'overall' back-EMF taming of the combined driver panel with a series XO the issue is addresed with a different solution.

I'm still trying to think of any more effects (equivalent circuit?) using the fact that the woof/tweet have a physical coupling quite different from nearly all other speakers.


Why didn't Magnepan use a series XO today? My guess is it is a marketing issue. You can't easily/cheaply jumper a series XO do bi-wiring or bi-amping and that would be unacceptable to the general market these days.
Only on an entry-level was that considered acceptable so that is why a series XO only happened on the SMGa.

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