GR Research Open Baffle Sub for Quad ESL??

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Prolly good to keep the low freq signals out of the Quads. You'll likely be using an active XO.

Why open baffle - to match Quad shape or sound or size or....? My best "match" to ESL sound was a Klipsch corner horn that just floated great gobs of very clean bass all over the room, like a dipole does for treble.

I think motional feedback is really great. But I haven't seen information on how they do it.

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The GSR sub uses the Rythmik coil feedback servo control. It uses a secondary coil for sensing, and the coil output is used in the negative feedback in the amplifier. I have not heard them personally, but have all reports say they work well. Like all such systems the motion feedback limits the bandwidth.

I also recommend limiting the bass in the Quads. Play close attention to phase tracking in the crossover as I believe this is most important in achieving good integration. I am using a Harsh crossover configuration between my Maggies and the sealed box subs. I am also using a Linkwitz transform on the subs to reduce group delay and extend the response. The Rythmik amps have something like a Linkwitz transform built in. Adding the subs to the Magnaplan 1.7's really improved the depth and dynamics of the system. Using a phase optimized crossover helped the overall coherence and also the cleanness of percussive attacks.
 
Custom-wound feedback coil seems a good approach. And everybody cranks up the bass to some degree of satisfaction.

But fussing about phase adds a new complexity to OPs task, popular as this topic may be at the forum everlastingly. Esp with panel speakers you get into spatial locations that make fretting about phase moot.

Ridiculous as it may sound to the wannabee-engineer, you can try reversing polarity to a sub or subs (as per the effective room acoustics strategy using some out of phase subs). Even reversing polarity, you may be hard pressed to decide which wiring is better. So if 180-degrees is hard to parse, then bits of degrees in a little narrow crossover freq band surely must be too.

B.
 
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