New MTM's in the Worx...

[JonMarsh]

Well, due to overwhelming popular demand, there's a new series of MTM's in the works here at Mission Possible. This will comprise some "conventional" MTM's and some with driver and construction to optimize for center channel duties, which isn't all that easy becuase of the typical dispersion issues that come up.

And of course, we'll have to do these designs with the current favorite bang for the buck drivers, er, uh, with a few exceptions approved or designated by yours truly... this isn't a democracy around here, you know! Well, not completely, anyway...


Being the fundamentally lazy guy that I am, as well as being way to tied up with a couple of projects that I've seriously over extended myself on, I'm going with PE cabinets again for these. Yeah, I guess that's sort of a sell-out for the hard core DIY crowd. You know who you are.

But can you make cabinets that look like this, in piano black, for example, at under $100 a pop? I can't either. It's not easy to catch in digital photography how these enclosures look and their high quality of fit and finish, but maybe this gives you an idea.





The two first projects in this series are an MTM with dual RS180's and SS D2904/6000-01 for the center channel configuration, and a stand up MTM
with dual RS180's and Seas 27TDFDC. The center channel is slated for my living room, and a friend wants a couple of these also for his living room and bedroom systems. The latter is probably also going to be used by someone else in the Bay area in a larger cabinet, something like a Thor TL design.

I don't think I'm going to have much down time this Xmax break, but on the other hand, I'll have some good projects to try out my Dewalt 621 router on.

I'll detail the info as it develops over the next two weeks, for those with similar needs or requirements.

~Jon

[TacoD]

Wow, those cabinets look great. I like building speakers, the electronical / measuring/ designing part that is. Once in a while I can enjoy building a pair of cabinets, but with the life cycle of my speakers, it is just too much hassle.

[PMazz]

I'm with you Jon for the center channel. I won't have the RS180s for another month tho, according to PE, so that should give you plenty of time to test drivers, engineer the xover and finalize the design . What happened to some time off thru the holidays? I thought I remember you kinda looking forward to that.

What sized enclosure are you using? As you know, the enclosure I'm going to use is "built-in" to the soffit. Internal dims are 12 1/2" x 26" x 14 1/2". Sealed or ported alignment?

Pete

[JonMarsh]

Dimensions as follows:

External Dimensions: 22" H x 9" W x 13.5" D
Internal Dimensions: 20.5" H x 7.5" W x 11.75" D
Internal Volume*: 1.05 cu. ft.

I've got both Cherry and Piano black sets of cabinets; my current plan is building two cherry finish CT's, and a set of MTM in Piano black, though they might get converted to Cherry before I'm done. These are intended primarily for HT applications, compact size and esthetics being a criteria where I'm using them, as well as my local friend. I've split up my gear into two seperate systems, one for HT, one for music.

Actually there's another MTM project which I mainly bought Piano black enclosures for, but it's the upper module in a three way system, and will use Peerless 850439's for the midwoofers, and Focal TC120td2 for the tweeters; the goal is a system efficiency of about 92 - 93 dB. Dual RS270 10" woofers on the bottom in custom boxes.

I know, sounds a little odd, but a few folks have been asking for a high efficiency box system at moderate cost, this sort of just fell into place as some of the Peerless 6-1/2" midwoofers were available used from an acquaintance at 1/2 the usual cost, and we're sitting on a few pairs of Tc120tdx2 focal tweeters. I've got all the bits on hand to do these...

It will use the Wilson Audio trick (as used in SLAMM, Watt/Puppy, Sophia) of a slow roll off on the bass speaker to provide most of the BSC comp on the midwoofers, so they can be run without BSC for efficiency gain. The Peerless are about 88.5 dB/watt each; before crossover insertion loss, that's 94.5 dB on the paralleled midwoofers; so figure a couple of dB BSC on the midwoofer crossover; the tweeters are 93.5 dB/watt; the 10" Dayton RS270 are 90.5 dB/watt, so two paralleled would be 96.5 dB before crossover insertion loss (DCR of inductors), after crossover losses somwhere around 94 dB? Should be quite do-able. The Dayton RS270 are very clean up to past 600 Hz, with very low energy storage signature in that range, so they're a pretty ideal combination of decent Xmax, upper end extension, good sensitivity, and relatively low cost (go look at what a nice SS 10" woofer goes for!)

Sort of a mini X1 SLAMM, smaller drivers, similar efficiency and dynamics, though just down to 30 Hz or so. Yeah, a narrow band system, 30 Hz to 25 kHz.

For the CT's, I'm not really concerned about bass extension, and planned on going sealed, especially since they'll be pretty much right up against the screen wall, and the rear would have been the most logical place for the port exit, given the amount of stuff on the front panel. The little Daytons have better than typical Xmax for 7" midwoofers, and should hold their own. The stand up MTM I might port, still scrtaching my head on that one.

Thor Klone

It looks like there will be another interesting variation on this design built, as a local Bay area gentleman and his son are interested in doing a "Thor" clone; this was the dual Seas 7" plus Millenium Excel tweeter in a folded transmission line. The cabinet design will be very similar, obviosly, but the crossover based on this RS180 MTM design. Now, this is a scary thought, that we migt get 80-90 % of the performance of a kit that sells for $1600 or $2000 depending on the crossover component choices, with drivers costing about 1/4 to 1/5 as much.

~Jon

[Brian Bunge]

John,

Cool, man! I'm really excited! When you say "conventional" MTM do you mean with the tweeter centered on the baffle between the mids?

I'm really interested in seeing how this project comes together.

[JonMarsh]

Yes, Brian, that's what I mean by "conventional".

The typical trick of making a center channel speaker by laying down a "convnetional" MTM doesn't work very well, for several reasons.

First, what is normally the "vertical disperion) of an MTM, and which becomes the horizontal when laid down, is dependenton the crossover type and order- the most "popular", because of driver protection and ease of compensation for different acoustic centers from the baffle, is the Linkwitz-Riley 4th order. But in an MTM it gives a narrow lobe in the crossover region along the axis of the driver. It's rationalized that that's OK in a vertical MTM, as it reduces the floor and ceiling bouncce, but at the typical crossover frequencies of 2.5 - 3 kHz, floor bounce is rarely a problem, and not often ceiling bounce, either. It DOES affect the room power response in the crossover region quite a bit, which is why Bullock and White came up with a compromise alignment that has a bump in the axial amplitude response, and less of a suckout in the net room power response.

Now, if you look at this for a laydown MTM, obviously many of your listeners aren't directly on axis in front of the speaker, so this is a big problem.

Other conventional crossover alignments have different crossover region power response- for example, the 3rd order Butterworth (not to be confused with the Mrs Butterworths syrup I like on pancakes sometimes) has a very nice and wide vertical dispersion for an MTM- which is why I used it in the X1 Klones.

The other point rarely discussed is the driver CT-CT distance - again, I probabyl use criteria which are too "relaxed" (Center to center, 1 wavelength), but that criteria is far more stringent than I see implemented in commercial systems. So, for an 8" Ctr-Ctr distance in the center channel MTM, the crossover frequency should be no higher 1700 Hz; preferrably lower. At this time, I'm thinking a crossover pretty similar to the M8 and Arvo, in the range of 1250 to 1400 Hz.

For the vertical MTM, the driver CT spacing will be as high as 12", I'm afraid- it can't be fully optimized for laydown, but the net room power response should be optimized, so a 1200 Hz region crossover would be indicated- I'll squeak by with 1250 or so; may model it as high as 1400 with the cauer-elliptic, see how it looks vertically.

Another way to get around the MTM lay down center channel is do make it a 2.5 way system- only one midwoofer radiating in the upper range- but the crossover cost is pretty high, taking that approach. Space is a problem, too.


~Jon

[Brian Bunge]

Jon,

I'm familiar with the issues of a horizontal MTM, but for a vertical MTM I do prefer the tweeter centered on the baffle between the woofers from an aesthetic standpoint. I understand that there are issues with that as well, but I do prefer how it looks. So if you're going that route with the 7's and the 27TDFC that'd be great.

Also, since I'm wanting to go with a 3-way tower I wouldn't mind a 3-way center as well. I've heard a couple of WTMW designs with the TM vertically oriented and liked them very much. So feel free to design one of those as well!

[JonMarsh]

I've done some 3 way WMTW designs for folks, such as with MB dome mids, etc; work pretty well, just a lot of crossover components. Sigh...

May do one some time with LPG dome mid and tweeter, and Daytons- if this one doesn't work out. But I think it will- not the the component cost will be much cheaper, (considering the tweeter) but it will be much easier to package. (three way crossovers being fairly bulky...)

~Jon

[BrianJ]

Jon,

Wow....looking forward to this one! In fact, I just joined this forum because of this project. I think this will be an incredible value, since I've heard both the RS series and the 27TDFC. Any plans to make a matching center with these (not the SS)?

Also, I was really impressed with the black PE cabinets. Last time I saw a set was at Chicago DIY and they are really striking.

Thanks, Brian

[JonMarsh]

The only difficulty with regards to the whole center channel issue is optimizing the driver spacing and crossover frequency. For good lateral dispersion, that would dictate as low a crossover frequency as possible- say, dropping it to ~1200 Hz with the cauer elliptic- main drawback being somewhat more expensive, bulkier components. It would be about the same crossover as for the CT with the SS6000. I don't have plans to build one myself that way but I'll do a crossover design if you're interested.

~Jon

[Brian Bunge]

Jon,

Since we're talking about the MTM's, what do you like for a sealed enclosure with the dual RS 10's? Unibox shows a 100L enclosure heavily stuffed gives a Qtc around .6, which I think would be perfect for me. If I go with an enclosure that's 50"H x 12"W x 18"D, assuming 1" MDF, that gives me around 126L. I figure with bracing and all I could get 90-100L for the RS 10's and use 20L or so for the dual RS 7's. One thing's for sure; they'd be mighty heavy enclosures.

[Spearmint]

Jon,

Those speakers look fantastic!!!

BTW fantastic information in this thread, but I also have a question.

I pick up my mains & centre tomorrow, and am a little concerned about your comments re the centre channel. How would I go about checking my centre channel to ensure that it has been modified i.e. that it is not the vertical just laid on its side. Is there a good music cd/dvd or can I check it with some software like ETF5, or TrueRTA? I have no doubts it should be okay, but your post has me a little worried.

BTW here is a picture of the speakers enclosures getting lacquered.

Image not available

[David Meek]

I don't think I'm going to have much down time this Xmax break, but on the other hand, I'll have some good projects to try out my Dewalt 621 router on.

"Xmax" break? A freudian slip there, my friend? :B

[JonMarsh]

Jon,

Those speakers look fantastic!!!

BTW fantastic information in this thread, but I also have a question.

I pick up my mains & centre tomorrow, and am a little concerned about your comments re the centre channel. How would I go about checking my centre channel to ensure that it has been modified i.e. that it is not the vertical just laid on its side. Is there a good music cd/dvd or can I check it with some software like ETF5, or TrueRTA? I have no doubts it should be okay, but your post has me a little worried.

BTW here is a picture of the speakers enclosures getting lacquered.

Image not available

Very nice looking cabinets, Richard.

To check out your center channel MTMs, I'd use ETF5 or TrueRTA; the latter might be easiest to get going if you haven't worked with either.

Set them up with a pink noise test signal, then first check the tweeter axis response at 1 meter. This should look fairly flat, with a slight downward tilt in the highs (because you're looking at direct and reflected energy in the room - no gating in TrueRTA). Move the mic laterally so that an angle from the front panel centered at the tweeter is in 10 degreee increments- check the response at 10 degrees off axis, 20 degrees, and 30 degrees. The last will be toughest, and might be a bit lumpy in the crossover region. This is where my cauer ellitpic crossover works well, because it allows a low crossover frequency, which makes the drivers look like they're close together relative to the wavelength, and it has a nice phase transistion through the crossover- as a result, even an 8" two way has good ressponse at +/- 30 degrees along the driver axis (usually vertical in most speakers), and excellent uniformity in the more conventional direction- due to not running the midwoofer up to where it's beaming.

Jon,

Since we're talking about the MTM's, what do you like for a sealed enclosure with the dual RS 10's? Unibox shows a 100L enclosure heavily stuffed gives a Qtc around .6, which I think would be perfect for me. If I go with an enclosure that's 50"H x 12"W x 18"D, assuming 1" MDF, that gives me around 126L. I figure with bracing and all I could get 90-100L for the RS 10's and use 20L or so for the dual RS 7's. One thing's for sure; they'd be mighty heavy enclosures.

Brian, we're thinking VERY similarly; my target was a Q of 0.566 (Bessel) to 0.6. I haven't drawn a concept yet, but since I can get ripped BB ply at various sizes including ~12", I had in mind a 14" wide front panel. I'm going to steal from my M8ta construction concepts a little, planning to use a combination of 3/4" BB ply overlaid with 1/4" HDF on the exterior (ease of finishing or veneering), with additional 1/4" pieces on the interior, but with clear channels between acting as guides for the braces. This pic shows the concept pretty well, as used in the M8ta.



Between that and judicious use of bracing, I'm hoping (perhaps not realisitically) to keep the weight reasonable, and the stiffness high.

The last "bass bins" I did for the X1 clones had 1-1/2" MDF walls, and were hideously heavy... just ask ThomasW. Ask my poor back, too. That's why I'm taking a carefully considered approach to this system; smaller diameter drivers, but almost 2X the Xmax, and low energy storage in the woofers up to 600 Hz; and a lighter, stiffer cabinet.

With the right room placement controlling where boundary gain kicks in, LF extension to pretty low frequencies should be possible- I'll do a MathCAD work up of the "optimum" placment during the break.


And speaking of Xmax, David, what can I say? I've caught myself several times doing that this year, but obviously a few are slipping through. Maybe I just have more of the Xmax spirit this year?

~Jon

[David Meek]

Internal Bracing

The photo showing the internal bracing brings up a newbie question. Is there some estoteric need to calculate the total volume of the holes in each of the pieces of bracing or are they just "drilled out" as a weight-saving measure? :scratchhead:

[GrahamT]

David,

Not esoteric, all that volume adds up. For my bookshelf speakers I had to subtract like 6 liters in total. You need to add up the volume of all the braces and subtract from the gross volume. You also need to subtract the port, drivers, and crossover etc.

[JonMarsh]

As Graham points out, you do have to take into acocunt the volume in the enclosure used by bracing, drivers, port, crossover, etc. Braces 1" thick don't really take up much volume each, but with three or four, it starts to add up. 4" out of an internal height of 36" is 10%. For a reflex enclosure this is more critical than for a sealed box. The holes are necessary so that the acoustic volume of the entire box is "shared" - I wasn't trying to seperate them into subenclosures. For some designs, say, a cone based three way, you might actually do that - seperate sections for each driver. Than, you'd probably want to make the holes a little smaller....

[Brian Bunge]

Jon,

I really need to see if I can get 3/4" BB ply locally. I know one supplier that sells 1/2" but they don't carry 3/4" for some reason. I'd imagine that somewhere in a city the size of Atlanta that someone sells it.

I think 14" might be a little wide for me but I could probably go a bit wider than 12". Not much though. I'd still like them to "appear" somewhat tall and thin.

BTW, I got that job! So in a few months I should be able to start scraping together parts for the towers! While it's still a good ways from my old salary at BellSouth, I'll be starting $10K higher than my current position so the pay checks will be noticeably heavier.

[JonMarsh]

Congratulations! That's a good way to lead off the holiday break!

Well, there's a Rockler store here:

6235 Roswell Road - Suite B, Sandy Springs, GA 30328

Which is part of metro Atlanta, and every Rockler store I've been into so far carries at least 2-1/2' wide by 5' long and 1' wide by 5' 3/4" BB ply, so hopefully we've got you covered! And in a city the size of Atlanta, there should be even more options- probably a nice little workout for Google!

If you prefer narrower, probably no reason not to; I was just thinking in terms of my cabinet construction, and a 12" inside rear and front panel; 14" is two inches narrower than my old X1 bass bins. I'm a little concerned about the tip over factor, especially living in the land of shaking hills and mountains; for you, NBD.

Anyway, congrats again on the job, and have a great holiday!

~Jon

[Brian Bunge]

Jon,

Yeah, that Rockler opened up earlier this year. I haven't had a chance to go by there yet as it's not really close by. About an hour's drive from here, but not to far from where I will be working. Also, in those size pieces, I can get them in my little Civic with the back seat folded down.

What's interesting is that in checking the Rockler website the list the largest dimension of the BB ply to be 30". They only list 12x30 and 24x30 pieces in 3/4" material. Maybe they sell larger pieces in store and only list these smaller pieces as what they are willing to ship. I'll see if I can get by there during the holiday.

Tipping might be an issue for me too, as I have two dogs, one ~80 lbs. and one ~100 lbs. So a little wider might not be a bad idea. I know my old dual subs were 25.5"H x 15"W x 22"D and when spiked they never moved no matter how hard they were "bumped". They were constructed of 3/4" MDF with a 1.5" thick baffle and extensive internal bracing. I'm guessing with the drivers installed they each probably weighed 100 lbs.

[JonMarsh]

Those subs sound a bit like the X1 cabinets I built...




but I weighed these, with Audax Pro drivers mounted, they were about 180 lb. each. Just the bottoms....

I'm hoping for a little less this time....

[Spearmint]

To check out your center channel MTMs, I'd use ETF5 or TrueRTA; the latter might be easiest to get going if you haven't worked with either.

Set them up with a pink noise test signal, then first check the tweeter axis response at 1 meter. This should look fairly flat, with a slight downward tilt in the highs (because you're looking at direct and reflected energy in the room - no gating in TrueRTA). Move the mic laterally so that an angle from the front panel centered at the tweeter is in 10 degreee increments- check the response at 10 degrees off axis, 20 degrees, and 30 degrees. The last will be toughest, and might be a bit lumpy in the crossover region. This is where my cauer ellitpic crossover works well, because it allows a low crossover frequency, which makes the drivers look like they're close together relative to the wavelength, and it has a nice phase transistion through the crossover- as a result, even an 8" two way has good ressponse at +/- 30 degrees along the driver axis (usually vertical in most speakers), and excellent uniformity in the more conventional direction- due to not running the midwoofer up to where it's beaming.

Thanks Jon,

I really appreciate your help.

The speakers are now up and running, so over the weekend I will do some tests.

BTW here is a photo of the centre...

Image not available

Brian congratulations on your new job.... must admit it is probably a better Christmas present than my new speakers.

Take care, and BTW you may recognize these cabinets match my subs...

For anyone reading this Brian was a GREAT help to me when I was deciding on my new subs.

Richard

[JonMarsh]

Richard,

Another very nice looking cabinet, Richard. And I think the Millenium Excel's are the sexiest lookig dome tweeter on the planet. Haven't decided where the pair I recently bought is going to end up for sure


Brian,

BTW, when you get to the point of ordering drivers for the high efficiency system (I'm getting the impression you've almost decided to go ahead with that?), if you have any trouble finding the Focal TC120Dx2 in the channel, ThomasW has several sets, and we can probably turn you a pair at a decent price. PM me to discuss, when you're ready next year. Focal is pulling out of the DIY channel, unfortunately, so I look at our stock of Tc120dx2's as something pretty lucky.

It would also be possible to juggle the configuration and kill some efficiency in the LF cabinet (wire RS270 drivers in series) and use the Dayton RS180 midwoofers and Seas 276DFC tweeter, if saving money were a big issue. But you wouldn't save a bunch of money (maybe $90 per speaker, maybe less considering crossover changes), so I'd recommend going with the higher efficiency design. It would be like doubling your amplifier power, not a small thing...


~Jon

[Brian Bunge]

Jon,

Are you speaking to me or BrianJ? I've never considered using the Focal tweeters you mentioned so I don't think you're speaking to me, but your description of the rest of the drivers sounds like maybe you are.

Anyway, if you are, let me know about the details on the Focals. I do just happen to have a template for those funky Focal faceplates.

EDIT: After re-reading your post in the other thread concerning the Focal tweeters in the X1's along with the RS180's I'm thinking the higher efficiency design might be a cool way to go. At the moment I'm still just using a receiver for power so more efficiency might be a good thing. It is a Sherwood Newcastle receiver, one of the better manufacturers IMHO, so it's not like I'm throwing some Best Buy Pioneer or Sony POS receiver at them. I've run several Newcastle receivers with various 4 ohm speakers in the past and had zero problems. So I think we'll need to talk when the time comes.

[Hank]

Brian, congratulations on the new job! Say hi to Kay and Merry Christmas to you and your families! Same to the Grand Poohbahs and forum members!

[JonMarsh]

OK, see post number 1 & 4. I know, too many configurations to track. Life is complicated.

Center channel MTM: Dayton RS180 plus Scanspeak D2904/6000-01. Cauer Elliptic Crossover at 1250, to optimize lateral dispersion. 1 cu ft PE cabinet

"Standup" MTM small main/satelite: Dayton RS180 plus Seas 27TDFC; Crossover in the range of 1500 Hz (could be lower; higher is lower component cost). RS180 ETC curves get wonky above 1700. If done with BSC for near wall mounting (HT), ~89-90 dB/2.83 VRS. If done with full BSC for position 1 ft or more from wall, ~ 87 dB/2.83 VRMS.

High Efficiency WWMTM: Peerless 850439 6-1/2 with Focal TD120dx2, crossover about 1800 Hz, dual RS270 in 100 liter sealed box, crossover to Mid woofers around 125-150 Hz on MTM, slow slope on RS270 to 600 Hz to provide baffle step comp. ~93 dB/2.83 VRMS.


Best wishes for the Holidays to Hank, Brian, Richard, Grahm, Taco, Thomas, Mark and all you Mission Possible types in HG Guide land!

[Davey]

Why does everyone use a tipped-over MTM for center channel duty? Two MT's (one on each side of the screen) is the obvious solution to all the problems of using a horizontal MTM.

Something else I just don't....get.

Merry Christmas everybody.

Davey.

[GrahamT]

Why does everyone use a tipped-over MTM for center channel duty? Two MT's (one on each side of the screen) is the obvious solution to all the problems of using a horizontal MTM.

People used tipped over MTMs because they fit well on top of or under a TV and they look good.

Two MT's, one on each side of the TV should still have the same (if not worse because of the spacing of the tweeters) comb filtering problems that a horizontal MTM can have. One MT aimed at the listening position is my personal preference, but SAF is low.

[JonMarsh]

What Grahm said.... :T

Just for fun, Davey, try listing to center channel only, the way you have setup; both with music and pink noise, walking from one side of your seating area to the other.

Then take your MT's, stack them with the tweeters towards each other in the center, and repeat the experiment with both program material and pink noise.

I bet it will be interesting....


I don't like lay down MTM's in most cases- most of the time, their design is flawed as described above. But with care they can be made to work. A better arrangement is a WMTW, with the woofers latteral, and the MT stacked between them; I've designed several of these for folks. But they're bigger and more complex, obviously. For this project, I wanted something for a relatively small "mini" HT which is actually in what would normally be my dining room; (advantage of no WAF required), as living room is pure two channel music now. Also, some friends locally need something like this, so the effort won't just be for mine.

Have a Merry Christmas, Davey!

~Jon

[Davey]

Gents,

Yeah, I understand the theory, but on the tipped-over MTM systems I've listened to the dialog was always shifted above or below the screen (depending upon where the speaker was) which was disconcerting to me. I think the vertical image localization ability of twin MT's trumps the MTM setup.

Anyway, I only use my center channel for movie watching and I'm always on the coach and not walking around. If I'm watching an action movie or chick flick with my cheeka hangin' on tight I couldn't care less about lobing problems.

Cheers,

Davey.

[JonMarsh]

Well, if it works for you, then it works for you.

In considering the optimum design for a system with multiple listeners (not all of us go solo), then how the center disperses towards the side, say in a 30 degree arc forward, especially in the presence region where the crossover point usually is, becomes critical to intelligibility.

I mention the use of pink noise for testing because it's very revealing, more quickly identifying problem areas and shifts in balance than just music or dialog- the latter will show up the problems, but often takes listening to more material to identify the problem area.

Any solution, though, that works for you in your situation, is by definition, a good solution.

Regards,

Jon

[Dennis H]

With a normal TV, I like Davey's idea. The C-C spacing of the tweeters is far closer than a normal stereo triangle. I don't often hear hardcore stereo guys complaining about the evils of comb filtering so I think the problem is a bit overrated.

With a big FP screen, I kinda like one MT below the screen and an inverted one above. That moves the dialog up to center screen where it belongs. You don't have any serious combing problems unless you stand up and even then it's not all that bad. Considering the angles, it's about like leaning a foot off center with your stereo system. And not many people do serious listening standing up anyway.

[Brian Bunge]

Jon,

As long as I can use the Dayton RS 180's for mids I'd be interested in the high efficiency version. If not, I'll stick with the Dayton's and the 27TDFC. BTW, not to throw another wrinkle into things, but do you think the Vifa XT25 would be better than the 27TDFC?

Hank,

Thanks and Merry Christmas to you and your family, too. BTW, tomorrow is also Kay's (and her identical twin, Kim's) Birthday.

Merry Christmas Everyone!

[Brian Bunge]

Jon,

I'm bumping this up since I think you missed my last post.


Happy New Year Everyone!

[JonMarsh]

Happy new year, Brian!

Re the high efficiency version, can probably use the RS180's too; it will complicate the tweeter crossover a bit, because the Focal requires a higher crossover point, whereas the RS180's like a lower point. From ETC curves, I'd rather not run the RS180's above 1400-1600, certainly not at 2K at all; whereas the Peerless is pretty nice up to 1800, and even at 2K the "anaomolies" are down about 30-35 dB. So, it would be a bit happier choice with the Focal Tc120, which I wouldn't take below 1500-1600.

Re the 27TDFC vs. XT25, the 27TDFC has less distortion down low, and subjectively has a clearer, smoother, more detailed presence region, much like the difference of dropping in an SS9800 in place of the XT25. Considering the 27TDFC costs less, and also has somewhat better off axis dipsersion in the top octave, it's rather a bargain.

The XT25 has a slightly "sweeter" sounding top octave, and might be more forgiving of inexpensive electronics, (due to the recessed presence response and "sweet" top octave) but for me it wouldn't be the preferred choice. I suspect this latter point is why it's popular wtih some designers like Polk and Krell, maybe even AV123, which may normally pair their speakers with "agressive" sounding electronics. That's just a surmise on my part; who knows?

Been making some sawdust, and have some front panels routed and cut, working on the porting for the standard MTM set (8" long 2" flared Precision port, rear firing, tunes box to about 32 Hz, relatively low Q alignment because of small box size, but extends the -6dB point from 54 Hz sealed to 37 Hz ported). Should be doing measurements sometime this weekend.

~Jon

[Brian Bunge]

Well, I think I'd be better off sticking with the 27TDFC vs. the Focal because I really want to use the RS180's and 270's for this project. I guess you'd call it form over function? Also, I've heard both the TDFC and XT25, albeit in totally different designs, and I really like the smoothness of the TDFC over what seemed to be a more aggressive sound of the XT25. I guess you'd say that the TDFC seemed to be more "hi-fi" sounding. Also, the design using the XT25 was obviously tipped up a dB or 2 in the bass and the treble so it sounded more aggressive in general.

[JonMarsh]

Well, there may be a slight problem there, for the high efficiency design with woofer cabinet, the 27TDFC may not be sensitive enough, at 91 dB/watt. I'll have to see how it all looks; the other possiblity is living with a slight downward tilt in the treble, or tweaking the crossovers to reduce midrange efficiency.

I'd hoped to realize a system efficiency around 93 dB with this driver set, but that's why our "stock" of NOS Focal Tc120's come into play. The Focal has a sensitivty of 93.5 dB/watt. We've got about 8 of those, and could reserve a set for you if you're interested.

[Frank L.]

John, I have been lurking for quite a while and had decided on building the M8a design, however I have been following this thread quite closely. My question is how closely the center w/SS 6000 tweter will be able to be voiced in comparison with the HT speakers using the Seas tweeters. Would this be effective, or will the response be different enough to cause a problem. I understand the need and advantages of the SS for the center, but the cost of using it for all 5 channels would be a bit steep for me. My plan was to build the center, 2 fronts, and later build the 2 rear channels.
On an unrelated note, the first of my Shiva subs (thanks Thomas W.) was finished last night. (88.5 l. sealed w/500 watt PE amp) I will post some pictures soon with a more detailed report. I am still dialing everything in, but so far I am extremely impressed with the sound quality--tight and quite musical.

Frank Looney

[JonMarsh]

Welcome to HT Guide, Frank!

I understand your concern about voicing matching- keep in mind, the main reason I'm doing these is for my HT setup also, as well as a local long time friend. I'm breaking one of my rules- he wants me to build him a set, at least for the living room system, maybe for his bedroom HT also (big widescreen Toshiba)

If Seas had an equivalent to the SS6000, I'd have used it, but they don't - and the Audax equivalent is no where near as good a driver. The 27TDFC is a very nice combination of performance and price- approaching the Excel series in many regards.

The basic crossover topology is the same, and of course the midwoofers them selves are identical- it's just that to do a good center, you really have to optimize certain factors carefully to get the required lateral dispersion. Some folks plan to use the CT design all around, but as you point out, that's a bit pricey.

My experience is that the critical things for matching are power response and dispersion pattern, as well as driver character- though the latter has a big impact on the former.

Besides the physical "fit" of the D2904/6000, a big factor is the extended low end, even below 1 kHz as well as smooth top end behavior to beyond 20 kHz. The SD1 motor structure realized with a neodymium magnet gives relatively low nonlinear distortion numbers in both the low end and high end of the range.



Now, the 27TDFC isn't identical, but with minor crossover tuning/voicing, the final response can be made very similar- and of course, it's much more affordable. It will work better with a little higher crossover, but it actually has remarkable output and linearity as low as 800 Hz.




Of course, the proof is in the pudding, so I suppose it woudl be wise not to try to count our chickens before they're hatched.

Stay tuned for developments...


~Jon

[Brian Bunge]

Jon,

And I assume there aren't many tweeters with efficiency in the 93dB range that meet your requirements, right? I guess I'll hang on and see what you come up with before I start buying any drivers. Just keep in mind that my main goal is to use the RS 180's and 270's in a 3-way design with whatever tweeter will work best with them. So I'll be willing to pony up for the Focal if you can make it work with the RS drivers.

I don't ask for much, do I?

[JonMarsh]

Nah, Brian! You're no more picky than I am!

Now, just a little trivia question, how many tweeters can you think of in the 93 dB sensitivity range? The TC120 has one heck of a magnetic system.... the Focal TC90 is more conventional, and has more conventional sensitivity.





Jon

[JonMarsh]

Front Panel Layouts

OK, let's get some real info up here- front panel layouts.


MTM with RS180 and 27TDFC




MTM Center Channel with SS 6000-01




Note, these assume the standard PE 1 cu ft front panel of 9" wide by 22" high. Seas Tweeter and RS180 midwoofers are mounted flush to the front panel, by routed recess as dimensioned; i.e., 7-1/8" for woofer.

Center channel is intended to be laid down.

[Brian Bunge]

Jon,

Is that Focal a metal dome driver?

Those front panel layouts look nice. I need to dig out that copy of AutoCAD that I have and learn how to use it some time.

[JonMarsh]

Yup, that's a TC120dx2. It's bigger than it looks....

I'm still using AutoCAD 2000; it does pretty much everything I need to do in 3D. I'd like to upgrade to a newer version, but I can't justify the expense. Buy drivers or upgrade software? Easy choice...

This graphic is just from printing to PDF, then clipping the desired area and rescaling slightly with Photoshop to cut the image size.


BTW, I've been making sawdust, too; I'll post a few pics tomorrow re front panels... :B got holes cut for ports in the rear of the MTM mains. I'm also modding up a Woodstyle box I have on hand to do driver measurements.

Found a broken connector on my CLIO test board today and fixed that; also transplanted the CLIO test PC (Athlon 600) to a smaller Cooler Master Centurion chasis that was on sale at New Egg last November. Lighter, better ventillation, smaller, has a window with a view, and can hold more drives. All for only $69!

[Brian Bunge]

Jon,

I finally got a chance to make a quick dash into Rockler this past weekend. They did have 3/4" BB ply in 2'x5' sheets. They also had some nice 6' long cherry boards that were around 8" wide and 1.5" thick! Damn, it's tempting to do an outer baffle made of solid cherry!