RL-p18 with a Linkwitz Transform... good, bad, or ugly?

**Owen Bartley** · **04-01-07, 11:53 AM**

I, like many of you, am lusting after one of these monsters, and after thinking about it a bit, I am leaning away from LLT and more towards LT. One of the reasons is because I've already done a ported sub (which I'm very happy with), one of the reasons is that I've never really experienced a GOOD, powerful, sealed sub, and I think the kicker was thxgoon's review of his monstrous LLT TC1000's. I want to see what a sealed sub can do for me, and see what I'm not hearing from my EBS Tempest.

Now, having read up some on LT's over the years of lurking and posting, I still don't have any practical experience with them. So are there parameters of the driver that make them either inherently good or bad for this application? I would think that an RL-p18 in a very sturdy box with the right LT and enough power has the potential to be a phenomenal sub... but what do I know? Thoughts? Ideas?

**Exocer** · **04-01-07, 11:58 AM**

Three important factors are Xmax,Xmech and power handling. You'll want a lot of all three

Also, if box size is of importance aim for a driver with a relatively low Qts (close to .3).

The Rl-p 18 has very healthy amount of everything necessary for LT.

My understanding is its smarter to go with the largest box you can fit at first to begin with a low natural Qtc and design an LT around that already low Qtc alignment.

**Owen Bartley** · **04-01-07, 01:53 PM**

Beautiful, thanks Exocer. According to the website, the ideal sealed volume is about 7 cubes, would this be the best place to start? Or would bigger be better? That is about the size of my Tempest, and I could easily live with a box that size.

**Exocer** · **04-01-07, 02:37 PM**

Can you go any larger? If not, then i'd say you'll be ok.

Qtc with about 200 liters (aka 7cubes) is above a tad bit above .707 You are not entering huge enclosure territory and its starting to make more sense to go ported...unless of course you can swing two rl-p 15's which would LOVE a 7-8cube box.

**Owen Bartley** · **04-01-07, 04:23 PM**

Well, this isn't for a REAL specific project yet, more just musing about the new 18. If I were to go this route at some point, I think I could most likely use a larger box, especially if the performance gain would be significant. Now that I know it COULD work, I'll have to go back and read up on the how's and why's of LT and maybe even do some calcs. Thanks for getting me started!

**Exocer** · **04-01-07, 11:51 PM**

sounds like a plan.
No problem!

**terry j** · **04-02-07, 04:29 AM**

hi guys

hopefully someone can provide a little education for me. What are the essential characteristics of an LT that would make it preferable to plain old EQ?

For me, it is easy to do couple of measurements in REW and throw a few parametrics at the response and end up flat, with appropriate cuts and boosts as required. I see no reason that the boost would be different in either case, ie if the driver can handle the boost supplied by an LT it can handle the boost supplied by a parametric.

So, what are the characteristics that would make one approach preferable to the other??

Hope the question made sense

**Owen Bartley** · **04-02-07, 06:00 AM**

Terry, I know I've read the reasons somewhere, so if I come across them again in my reading I'll post a link. I can't remember exactly what they were, but I do remember they made sense to me when I read them.

**BoomieMCT** · **04-02-07, 02:32 PM**

Quote:

Owen Bartley wrote:

Terry, I know I've read the reasons somewhere, so if I come across them again in my reading I'll post a link. I can't remember exactly what they were, but I do remember they made sense to me when I read them.

The Sticky thread "LLT Explained" at the top of the DIY subwoofer forum gives some reasons why you'd want to go LT. Is that what you were looking for?

**Sonnie** · **04-02-07, 03:54 PM**

Quote:

BoomieMCT wrote:

The Sticky thread "LLT Explained" at the top of the DIY subwoofer forum gives some reasons why you'd want to go LT. Is that what you were looking for?

Actually an LLT is a Large and Low Tuned sub, while an LT is Linkwitz Transform circuit.

Here's a pretty good explanation of how it works.

Here's a design program for it... some of those names may ring a bell.

And here are some circuits via Rythmik Audio with some more good info.

**BoomieMCT** · **04-02-07, 04:34 PM**

Quote:

Sonnie wrote:

Actually an LLT is a Large and Low Tuned sub, while an LT is Linkwitz Transform circuit.

Doh! I dropped an 'L'.

**Owen Bartley** · **04-02-07, 07:19 PM**

Thanks Sonnie, always on the ball.

Yeah Boomie, thanks for the link anyway, that is a good read for anyone who hasn't been through it. Basically they're two pretty distinctly different approaches to bass, one favoring a massive enclosure and porting to get down to those golden low notes, the other using a much smaller sealed enclosure and lots of (specific) equalization and amplification.

**Brian Bunge** · **04-02-07, 10:22 PM**

I had an original Tumult in roughly 2ft^3 (15" internal cube). The natural Q was around .7 before adding the LT. I used the LT circuit to lower the Qtc to .6 with an Fb around 20Hz. That along with a little EQ via a BFD gave me flat response to 10Hz in room in this enclosure:

**terry j** · **04-02-07, 10:44 PM**

oh dear, I don't think you helped me at all ha ha!! I assume the pictured sub is sealed, and if so my limited understanding at the moment says that the only variable that affects the Q then is enclosure volume. Now you said you changed the Q by other means...think my brain is about to explode..help!

A little off topic perhaps, but on this question of FR by either eq or LT, and now Q as well, say I had two different sealed subs, with exactly the same measured FR at the listening position, achieved either by eq or LT (to be answered more fully from above). Now, the only difference between the two happens to be the Q. Make one 0.5, and the other say 0.707. Back in the old days before our ridiculously easy job of getting a flat response, one method of achieving different FR's was of course choosing different Q values, eg one went lower but different roll offf etc etc.

Now, both have exactly the same FR, will they sound different?? Even tho the FR is the same, ie the quantity of the sound, does the Q still affect the quality??

**Brian Bunge** · **04-02-07, 10:58 PM**

Yes, the sub is sealed. An LT circuit can only be used with a sealed enclosure (better known as an acoustic suspension enclosure). I changed both the Q and the Fb with the LT circuit. I could have built a larger enclosure to achieve the Q I wanted naturally and only adjusted Fb, but then I wouldn't have been able to fit the sub in the desired location.

There's no way a sub with a Q of .5 and a sub with a Q of .707 would have the same FR. The lower Q sub would have a more gradual/shallower rolloff than the sub with the higher Q.

**terry j** · **04-03-07, 12:39 AM**

thanks Brian

regarding the FR, assuming the drivers are up to it, can't I boost and cut as I would like and therefore make two different sealed boxes have the same FR? If that is possible, then I'm wondering what the qualitative difference in sound would be.

Regarding your answer above, aND WRAPPING MY POOH BEAR BRAIN AROUND IT, WOULD IT BE TRUE TO SAY THAT IN THE FINAL ANALYSIS THE shape OF THE FREQUENCY RESPONSE IS THE ULTIMATE ARBITER OF THE q??Sorry about that, caps lock and all but I don't want to go and retype it. Is that how you were able to change the Q w/out altering the volume.

**Owen Bartley** · **04-03-07, 08:36 AM**

Just been fiddling with some calculations, and keep in mind I'm still new to the technical stuff regarding LTs, but it SEEMS fairly straightforward so far. I'll attach the file I was working with, maybe I'm way way off on something.

Using the RL-p18 (dual 4 ohm) parameters off the website, I got a response that is down about 3.88dB at 10Hz in a 210L enclosure, using a Q of 8.0 which is a bit high I think, but it gave a significant bump in the low end extension. Also, I know there's a significant amount of boost being applied, should I try to stay closer to 10dB? What other programs should I be looking at modeling this in?

*edit: See my post below for the zipped Excel file.

**Sonnie** · **04-03-07, 10:30 AM**

It tells me the file is temporarily unavailable.

How big is the file?

**Owen Bartley** · **04-03-07, 12:04 PM**

Weird, it works for me about half the time. The file is about 120k.

Duh... I just zipped it and added it here.

Edit: I uploaded my original file, but I've been tweaking a few things to show less boost, and if I slightly raise the F3 to 13.5Hz and increase the enclosure size to about 300L, I get a result that's almost as good, and is down just over 5dB @ 10Hz, with the max boost kept well under +15.

**Brian Bunge** · **04-03-07, 05:47 PM**

Terry,

Sure, you can make one sub sound like another with EQ, and yes the frequency response is what determines the overall Q. Adding boost to the sub with a standard EQ (or PEQ) will increase the Q of the system. Only with an LT can you boost the low end while also adjusting the Q lower. Of course, you can also leave it the same or raise it higher if need be.

I need to recheck to see if I still have the Tumult design, but I believe I actually shot for an Fb of 18Hz with .6 as the Q. I then used a little boost from the BFD to get flatter down low. It worked great for me this way.

I would never bother with an LT for a Q of .8 unless you want to turn off the LT circuit when listening to music and have the natural Q be lower. Now, if it's completely for HT effects and you want to rattle your teeth loose then go for it. But don't try to use it for music that way as I'm pretty sure you'll be disappointed in the bloated sound.

**F1 fan** · **04-03-07, 06:44 PM**

An alternative to a fixed LT circuit is the Marchand BASSIS.http://www.marchandelec.com/wm8.html
It is basically an adjustable LT which is very versatile as it allows the amount of boost and Q to be easily changed .Therefore one can experiment with different settings and tune to taste without having to change components on a circuit board.

**Owen Bartley** · **04-04-07, 06:36 AM**

Thanks for the tip on Q Brian. It will be for probably 80% HT, but will get used for music too, although not in what I'd call "critical" listening. Maybe I'll bump it down to .7 or so to compromise a bit.

With your Fb of 18 Hz do you feel like you're missing anything down really low during movies, or are you getting pretty full range? I love my Tempest, but it seems like it is more boomy in the 40 - 60Hz region and doesn't pick up much below 20Hz. I'd like a movie sub that digs all the way down low, so I can turn the total volume down a bit and hopefully have it integrate better. In the meantime I might experiment with a steeper house curve with the Tempest.

Oh, and thanks for the link to the BASSIS, Fred. I've read a bit about them, but was curious about how they actually work. Depending on the cost of putting together a more basic LT, that might be a much better way to go. Would it eliminate the need for my BFD though, or could it be used principally as an LT circuit and I could keep the BFD in the chain for tweaking the final response?

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