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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
mijostyn's avatar
mijostyn
8,570 posts
 
don_c55, Magnepans are linear radiators down to about 150 Hz. Below that they are point source radiators just like any single sub woofer. Maggies will make bass but it is very lumpy for lack of a better term. Not only is their response not smooth but input below 50 Hz really shakes the speaker distorting everything else. I prefer 4 subs with 20 series Maggies using 12 inch drivers and crossing up at 125 Hz essentially keeping the system Line Source throughout. You also have to high pass the 20.7s to get the full benefit so you really need digital bass management to do it right along with room control. Now we are talking serious money. It is best to avoid subs unless you can do at least two. Single subs will just get lost and cause a lot of heart burn and frustration.
Setting up Maggies to get the smoothest bass is just a matter of adjusting the distance to the wall until you get what you want but it will never be the smoothest base because you can not avoid dipole interference and cancelation effects off the front wall. Wasting money on all kind of room treatments will not change or correct this. 
bdp24's avatar
bdp24
9,788 posts
 

Don, I share your amusement at you being lectured to on the subject of Magneplanars. Those who know Don are aware of the fact that he has forgotten more about that loudspeaker (having owned them since the 1970's) than most will ever know. Don also has a ee education, and it is HE who should be giving the lecture.

Don could talk about the fact that below 150Hz the Magneplanar bass panels (separate in my Tympani T-IVa) continue acting as dipole radiators, with a null to either side, therefore exciting one less room dimension mode. There is one hi-fi enthusiast (I need to find his name) who uses Martin Logan ESL's as his mains, Tympani bass panels as woofers (it reproduces the sound of my self-recorded Gretsch 26" bass drum more like it does in life than any other reproducer I've heard), and an Eminent Technology TRW-17 Thigpen Rotary Woofer as a "true" subwoofer (20Hz-down!). Now THAT is a system I would love to hear.

There is no need for "digital bass management" to high pass the 20.7; a First Watt B4---all discrete, no opamps, no ic's---works perfectly. So will a Pass or Marchand x/o. If a 1st-order filter is steep enough for your needs, you can even install a simple capacitor/resistor filter on the input jacks of the amp powering the 20.7's.

This is all old news to Maggie enthusiasts, and much more info about the speakers is available on numerous forums, including the Planar Speaker Asylum.

mijostyn's avatar
mijostyn
8,570 posts
 
bdp24, I have owned my fair share of Maggies and have installed more than all of you have ever listened too. Yes they are dipoles and do not radiate to the sides but that has absolutely nothing to do with what happens at the front wall and how that affects the speakers bass response. I am not here to give you a lesson in acoustics if you want to learn more about dipole interaction with a wall there is loads written about it. You will never get completely smooth bass response out of any dipole unless you have it out in open air or live in an alternative universe. 
Having said all this there are very few loudspeakers I would choose over a 20.7. I just wish they had decided to do an 8 foot version. That would have been the nuts.
bdp24's avatar
bdp24
9,788 posts
 

Oh yeah? Well I own Tympani T-IVa’s ;-) . And have heard the 30.7, though in a bad sounding demo in a really poor room. Wendell Diller was not pleased. But to get back to the point of yours that I am contesting, it is your statement that below 150Hz Maggies are point source radiators. That is simply not true; they are absolutely dipoles at all frequencies, including below 150Hz. Q: What magically transforms Maggies into behaving as point source radiators at 150Hz and below? A: Nothing. Your post directly above seems to find you subtly backing off your earlier statement to that effect.

The matter of their interaction with the wall behind them is a completely different subject. By the way, @don_c55 uses four Rythmik F12 subs with his Magneplanar MGIII’s, and has chosen to employ absorption on the wall behind the panels, rather than diffusion.

don_c55
1,025 posts
 
@bdp24

I do not use any subs with my MG IIIa speakers.

The MG IIIas are in my home theater and MG 3.6s are in my main room.

They are both flat to 30Hz and I do not feel the need for lower bass.

I am more than satisfied.

Bass below 30 Hz has little musical info. Low bass peaks and dips are hard to deal with in my rooms.

I also like to keep peace with my neighbors below in my condo.

I rarely playback much over 90 dB except when neighbors are gone.
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