# THX spec DIY LCR's?



## eyekode (Jan 19, 2008)

I am not a big fan of THX certification, but I think they picked a good crossover . Any recommended LCR's with 2nd order 80hz roll-off?

I guess this would just be a typical sealed alignment but I have been modeling drivers and it looks like I need to go down to a 5" driver for 80hz rolloff. The 6-7"'s tend to roll off lower.

I am leaning toward a MTM configuration for power handling.

In the absence of an existing design I was thinking of:

Neo3PDR tweeter
2x Dayton RS125 or Tang Band W5-1685

I have not seen any DIY designs with the W5-1685, but the specs/price look nice.

I am leaning toward the Neo3PDR + Dayton RS because I built a pair of LouC's Cryolites and really like them.

As for the center channel I plan on rearranging my entertainment center to avoid a horizontal center channel. The seating arrangement is really wide and relatively close so off axis response is very important. And I really want my LCR's to be matching but don't want the complexity of 3 way systems. Seems like a waste considering I want such an early roll-off.

I also have a really odd requirement... any suggested tweeter must be difficult to damage with little fingers. So no exposed silk/metal 1" tweeters .


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## looneybomber (Sep 20, 2006)

eyekode said:


> I also have a really odd requirement... any suggested tweeter must be difficult to damage with little fingers. So no exposed silk/metal 1" tweeters .


Compression driver maybe? JK:neener:

As you mentioned, some ribbons have a screen over them which could help. But, speaker grille's will pretty much be a must. I have a young one running around and after 21 months, my Cerwin Vega towers (in the main room) are still in fine shape thanks to the grilles. My expensive gear I keep out of his reach in the office though, just in case.


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## eyekode (Jan 19, 2008)

I have 4 little ones. The oldest just turned 5 . I have a pair of speakers in a "public" room that have an exposed RS180 (metal cone woofer) and a Neo3PDR. And (knock on wood) they have never dented the RS180's. But a silk/metal dome tweeter is just too fragile.

And I cannot bear to put a grill in front of RS series Daytons . If they were ScanSpeak then maybe!


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## 1Michael (Nov 2, 2006)

Check these out. http://www.htguide.com/forum/forumdisplay.php4?f=39
You can add a grill.


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## eyekode (Jan 19, 2008)

Yeah, I frequent htguide. They have a great DIY speaker community. But none of the basic designs fit what I am looking for (maybe a Modula MT sealed, but that rolls off too late). They are more concerned with "serious" 2 channel listening.


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## looneybomber (Sep 20, 2006)

Put a 12db/oct HP filter @ 80hz on an existing design?


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## mayhem13 (Feb 2, 2008)

Does the freq response of the B&C 8NDL51 look good for you.
http://www.bcspeakers.com/sez/popup_graph.php?id_prodotto=53&id_foto_prod=76&tipo_descrizione=40


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## eyekode (Jan 19, 2008)

Wow, that is a serious driver. 200w thermal handling? 92+db sensitivity? Definitely does not have the Xmax to take 200w at 65hz, but still impressive.

But the price and diameter takes it out of the running for a "cheap" 2 way system.

I have been modeling some speakers in WinISD and the RS150 looks like it would work and two of them would have sufficient power handling. There are a couple designs using the Seas 27 and the RS150 but the crossovers are a little more complicated then I would like. A second order electrical at ~2k with parallel and serial traps looks pretty good. Maybe I will buy the drivers and tryout my own crossover. I am a noop with SpeakerWorkshop so I definitely could have done something wrong. If my crossover fails I will go with CJD's well respected crossover.

I have attached the crossover and the system frequency response. A couple things to note about the freq response graph:
1) my FRD data is wonky on the Seas tweater > 15k or so. That is where the roll-off is coming from.
2) the bass response has ~4db of baffle step.
3) the amplitude scale is very zoomed in.


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## eyekode (Jan 19, 2008)

Whoops, lost my attachments...


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## mayhem13 (Feb 2, 2008)

wooops-sorry-didn't know these were on the cheap. I bought 4 of em from a damaged line array system and i'm dying to make some 2 ways or even a pair of mtms but still learning so poking around here and there. 2 of these ported in 18L gives an f3 of 71hz at a whoping 117db ! Now thats some MTM.


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## eyekode (Jan 19, 2008)

They definitely sound like interesting drivers! Ported doesn't really fit either because they will roll off with a 4rth order slope. Then again I am not sure how much I _really_ care about the 80hz 2nd order slope. I have a BFD so I could equalize the sub regardless.

And unless I buy an acupower there is no way my sub will be able to hang with 117db!!! Whew that would be 132db if you consider LFE with 15db of headroom


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## willy-be (Nov 15, 2006)

Due to off axis lobing MTM designs don't make for good center channel speakers. 

Why do you care about the roll-off? The speaker size and bass management settings in the receiver takes care of all that.


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## avaserfi (Jul 5, 2007)

willy-be said:


> Due to off axis lobing MTM designs don't make for good center channel speakers.


This is not necessarily the case. Poor design of an MTM will result in off-axis irregularity, but with proper steps taken these irregularities can be dealt with. An article on the subject can be found here.


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## willy-be (Nov 15, 2006)

That site was 'news' about 10yrs ago....

Most center channels are horizontal not vertical MTMs...


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## thxgoon (Feb 23, 2007)

willy-be said:


> Why do you care about the roll-off? The speaker size and bass management settings in the receiver takes care of all that.


Not necessarily. For example the bass management in a THX receiver is designed to mate with spkeakers that have a 12db/octave roll off for a total of 24db/octave. Then the same is applied to the sub for a smooth transition. If the speakers do not possess this roll off you would end up with something other than 24db/octave.


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## willy-be (Nov 15, 2006)

THX specs are primarily marketing driven.

There are far more important considerations involved in the design of a center channel loudspeaker than the roll off frequency of the box.


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## avaserfi (Jul 5, 2007)

willy-be said:


> That site was 'news' about 10yrs ago....


An articles age has nothing to do with its validity. 



willy-be said:


> Most center channels are horizontal not vertical MTMs...


I fail to see the issue here. Certain principles discussed in this article can be used in a horizontal MTM design as well. Generalizability, of research is a common practice and what actually makes research useful.


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## thxgoon (Feb 23, 2007)

willy-be said:


> THX specs are primarily marketing driven.


They are market driven, but were developed for quality assurance. While you don't need to buy something with THX on it to get a great piece of gear, some of the things they spec help to achieve better performance. The crossover in question in this thread was known to be great before THX adopted it.


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## eyekode (Jan 19, 2008)

I think everyone here has a point. THX certification by itself is maybe not so interesting. But they chose a very good crossover for power handling and this is why I choose to use it. And many receivers (like mine) have a THX mode where they will use a 12db per octave crossover at 80hz. Integration of satellite with the sub will be easier if I come close to the specs.

Also many movies are produced to THX specs and have very little content around the THX crossover, so 80hz looks better all the time.

As for the question about "is an MTM good for a center", I need very wide dispersion due to my seating arrangements. And as I said in my original email I will have a vertical MTM as my center channel.


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## thxgoon (Feb 23, 2007)

eyekode said:


> As for the question about "is an MTM good for a center", I need very wide dispersion due to my seating arrangements. And as I said in my original email I will have a vertical MTM as my center channel.


You will be good to go then. Another big part of the THX spec is horizontal coverage and most THX speakers (including mine) achieve this via vertical MTM. I can't wait to see what you come up with!


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## avaserfi (Jul 5, 2007)

With regard to wide, linear, horizontal dispersion a vertical MTM configuration is not needed. Use of a MTM design will allow for more dynamics (about 6dB) and lower distortion, but does not necessarily have a substantial effect on dispersion patterns. Driver choice and baffle design are far more important. 

For example: If one uses transducers with wide, linear, dispersion pattern and mounts it on a appropriately designed baffle* maximum horizontal dispersion could be achieved without regard to driver quantity.

*This baffle must be designed in such a way to minimize wave diffraction.


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## eyekode (Jan 19, 2008)

The choice of a vertical MTM center is not really about getting wider dispersion. I chose MTM for mains for dynamics, power handling and low distortion. I decided on a center to match the mains. And because it is an MTM I decided on a vertical MTM for the center for the best dispersion.


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