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Magnepan 20.7 and Subs

I've been reading about supplementing my Magnepan 20.7s with a sub, the general preference being REL.  However in room I'm getting fairly deep bass with the 20.7's, so my sense is that I would put the crossover on the REL pretty low, probably at 30Hz, leaving only sub 30Hz material porting "also" to the sub to augment the natural deep bass of the Magnepans.  I'm just wondering since there's precious little info in the 15hz to 25hz range, do I really need a PAIR of RELs, or could I get by with just one?  And shouldn't that one be capable of going REALLY low, like the G1, or what's the point?  Any advice would be appreciated.  Thanks.
pwhinson
bdp24's avatar
bdp24
9,816 posts
 

Although Elizabeth doesn’t like it when I say this, 3’ away from the wall behind them is insufficient for any planar loudspeaker. 5’ is about the minimum, to create at least a 10ms delay between the front wave and back wave reaching the ear. As Liz accurately said, it is actually not the distance from that wall that matters, but the time differential between front and rear waves reaching the ear. As long as an at least 10ms delay is created (the more the better), it doesn’t matter how that is achieved.

Subs with a phase control allowing continuously-variable adjustment---as do the better Rythmiks (0 to 180 degrees)---make possible time alignment between loudspeakers and subs. I would not buy or own a sub without that capability.

I’ve long understood the opposite of dipole to be monopole, but that may just be semantics. I’ve never heard a bipolar loudspeaker I’ve liked, including the highly-regarded (by others) Mirage. "Line source" and "dipole" are not synonymous.

mijostyn's avatar
mijostyn
8,570 posts
 
No they are not and I agree. I have never heard a full range bipole speaker that I have liked. Although I think unipole line sources are a step in the right direction all the ones I have heard have suffered next to dipole ribbons and ESLs. Variable phase in a sub is also a step in the right direction but very difficult to set up by ear and it does not correct the time issue. Assuming the Subs are behind the satellites as is usual. Even though the phasing can be correct the timing can not be. Using digital delay on the satellites works perfectly especially when you have a computer making the adjustment. We use to move the subs back and forth to get them in phase but as soon as you separate them from the wall you lose power, a no win situation. As for keeping dipoles away from the front wall it depends on where you want to amplify the bass. 5 feet will give you a peak at around 100 Hz which will give bass more impact but below that things will roll off quickly. You also interfere with the room more. Again the best solution is to move to a subwoofer array at 125 hz which gives you much flatter bass and allows you to keep the dipoles closer to the wall. I always dampen the wall directly behind the dipole with acoustic foam tiles which kills the primary reflection above 250 Hz tightening up the image. There is no significant radiation in any other direction except straight forward. Perfect!
bdp24's avatar
bdp24
9,816 posts
 

The time/phase issue between speakers and subs has to take into account the nature of the x/o between the two. 1st, 2nd, 3rd, and 4th order filters all create different phase relationships, both between the drivers within the loudspeaker itself, and between it and any sub. Brian Ding has a detailed discussion of this issue in the technical section on his Rythmik Audio website. His phase control provides delay of the woofer output in milliseconds---which is the same as moving the sub further behind the speaker. He reminds readers that the opposite is not possible: moving the output ahead in time ;-).

But of course, if you delay the sub enough (16ms on the phase control), it will be in opposite polarity (180 degrees)---equivalent to moving the sub in front of the speakers. Danny Richie of GR Research also sells the Rythmik Subs, and his "baby" is the OB/Dipole model, as well as the "G" version of the Rythmik F12G (the G is from GR Research, and that sub has a paper cone woofer. The standard F12 has Brian Ding's aluminum cone woofer).

Danny is a crossover design expert and OB speaker enthusiast/specialist, and is happy to provide advice on phasing (and any other matter) between the subs and any speaker you may be interested in pairing with the OB/Dipole or F12G subs. His main business is DIY speaker kits and subs; you make the enclosure (or buy it from Parts Express, Meniscus, etc.) and assemble the x/o (parts provided), then install the kit's drivers, the x/o board, and the internal stuffing into it.

Rythmik offers some of it's subs as kits, and it is those kit-version Rythmik subs that Jim Salk installs in his incredible real wood veneer enclosures. Take a look at the bracing design he came up with! It's a honeycomb-type construction, the best bracing I have ever seen in a sub. Danny Richie's F12G design is great too---double-walled, the space between the walls filled with sand. Reminds me of those big speakers from the 1950's that did the same. Was it Bozak? Wharfdale? 

mijostyn's avatar
mijostyn
8,570 posts
 
Wharfdale did the sand. Bozak's claim to fame was infinite baffle speakers. There are lots of sub kits out there now and if you have some woodworking experience and some basic tools by all means. Dayton makes a very advanced plate amp with room control.

It is extremely difficult to control resonance in box speakers. The kit manufacturers because of weight limitations and cost to not dig very far into the problem. If you are capable,  making your own enclosure is the way to go. Calculating volume is easy once you know your driver's parameters. It is just plug in math. Tricks to control resonance are first use very thick walls. In MDF 2 inches minimum. Keep the sides, top and bottom as narrow as possible but make the top and bottom wider than the sides are tall so that they resonate a different frequencies. You want to make things so stiff that the cabinet resonates at a frequency as far above the woofers cut off point as possible. The heavier the better. You do not want the whole cabinet shaking. Always three spiked feet and right into the floor. No pads. Ideally with the woofer pumping out it's lowest frequency you should feel nothing with your hand on the enclosure. Any movement or vibration that you feel is distortion. You will never feel a commercially made sub that quiet with maybe the exception of the big Magico subs. Get a piece of granite cut to size and silicone it to the top of your enclosure. The added weight always helps and you can get granite cut offs for cheap. I prefer external amps and cross overs. JL Audio makes a nice external crossover.  
pwhinson OP
158 posts
 
As the OP, I have to say first of all that I listen to pretty much only classical music. OK, a bit of jazz here and there. I find the bass on the 20.7’s not only adequate but PERFECT really. I think my original idea to supplement them with subs comes from the fact that I CAN (financially anyway), and also from the fact of my never ending need to "improve" things. So I’ve pretty much decided not to invest in a pair of subs because I'm rather convinced that I would simply be creating more "problems" to solve.  There are folks in this thread who really know more about what they’re talking than other folks but its up to you dear reader to DISCERN that. Just remember, more experience = better advice. @jafant The only other development is that ODDLY, I have gone back to the Aesthetix Atlas as my amp of choice with the Maggies at the moment. I’ve gone back and forth between the Atlas and the Pass X150.8 (also thinking that perhaps I should move up in the pass line to a more powerful amp). But the Pass product is SIGNIFICANTLY warmer in the midbass. The Atlas is incredibly airy, perhaps a very slight TINY smidgen "clinical" but still very very good. I did think about attenuating the midrange panel using a resistor on the 20.7s with the Pass to see what that would do and I may still experiment with that. There’s absolutely no doubt that the Pass did WONDERFUL things for my Thiel 2.4s but I don’t like it anywhere near as much with the Magnepan 20.7s, MUCH preferring the Aesthetix Atlas as my amp at the moment.
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