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Join Date: Feb 2010
Location: Monument CO
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Magnepan uses low-order crossovers and panels with lots of overlap in the frequency range. That provides a more seamless transition among drivers than many conventional speakers, leading to better imaging with no funny "wiggles" in the response through the critical voice range (~300 Hz to 3 kHz). Too many speakers have little discontinuities in the vocal band as the sound moves between (among) drivers to my ears. It's hard to describe but easy to hear; images "snap" into position and there is no change in the sound as voices and instruments slide up and down the (musical) scale. I have since changed to Revel speakers and find they share similar characteristics.
In the interest of maintaining that smooth sound passing through the various drivers and crossovers, it is helpful to have a subwoofer that can handle frequencies well above the crossover. Remember the crossover is not a brick wall dropping to zero on either side; a typical 12 dB/octave AVR crossover means the sub is about half as loud as the mains nearly an octave above the crossover, and the mains are about half as loud an octave below the crossover. Having a sub with a higher frequency response like the F8 helps provide that seamless transition and sense of "air" Brian described. In my case, decades ago I discovered servo subs by inventing my own design, interestingly similar to Brian's. The servo helps provide the equivalent of a much more power amplifier with much lower output impedance to better control the sub's driver. That in combination with a higher-order crossover design enabled me to produce a subwoofer that mated well with my Maggies. Thus, nearly 30 years later, Rythmik was an obvious choice, and one I have been very happy with. As I've said before, I was vexed to discover that subs I felt worthy of my Maggies were in the $3k to $5k range until trying Rythmik. The rest is history.
Oh, the Revels take the opposite approach -- multiple drivers, high-order crossovers, very professionally (scientifically) tweaked to provide a smooth transition from driver-to-driver, not only in frequency response but also in radiation pattern (much more critical in conventional speakers).
Finally, keeping the sub low enough that the energy is pretty much inaudible before reaching that critical voice band is the usual approach, and leads to the 80 Hz crossover often cited. Below the frequency most of us cannot localize the speaker, meaning the sound does not mess up the mains, and allow us to place them for smoothest in-room frequency response without compromising the stereo (or multichannel) image. That means a sub that plays cleanly with low distortion above and below the crossover point. At 80 Hz, a high second harmonic distortion term is at 160 Hz, readily heard and localized. You don't want that, or at least I don't, and another reason I like servo subs (a crossover doesn't matter if the sub's distortion is the source of the higher-frequency sound).
ESLs are tricky -- the panels are essentially big capacitors and so drop very low in impedance at high frequency. However, the vast majority use a transformer to step up the amplifier's voltage to drive the panels, so there's a big inductor in the circuit. Their HF impedance is low because of the big capacitor that is the panel, but looking into the speaker it is not necessarily capacitive. But the wide frequency range and corresponding need for a very clean sub is the same as for planar magnetics (Magnepan) or ribbons (Apogee and speakers with ribbons mid/tweeter drivers).
Sorry for all the babble, HTH - Don
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley