The Linkwitz Transform Circuit

[Andrew Pratt]

I've seen a lot of talk on using LT's to boost the low end of smaller sealed sub's lately and thought that it might be a good topic of discussion here in the DIY area. I understand from this site what they do and have a basic understanding on what's required to build one but I'd like to get into a little more specifics if possitble using a real world example...like the tumult driver in a 2.5 cubic foot box which is fairly common and what I'll very likely build this weekend.

Also while we're on the subject does anyone know how good the LT's are that are built into the new Adire plate amps?

[ThomasW]

The catch-22 is that they really need to be custom designed for each driver and it's corresponding box. Not many parts but it is an active circuit.

There are 'generic' ones the Marchand Bassis is an example and it's expensive.

Here's how they're calculated

http://www.trueaudio.com/downloads/linkxfrm.xls

Oh and I should add that it takes AMPLIFIER POWER to use a LT circuit, no wimpy amps need apply.

[Dennis H]

Most of the better digital EQ boxes, e.g. the Behringer DEQ2496 or DCX2496 can do it with ease.

The LT can be replicated as the sum of 3 filters - two band (bell shaped) filters and one 12dB/octave shelving filter. The first band filter changes the Q of the box/driver to 0.7 at it's Fb. The shelving filter changes the response to Q=0.7 at the new desired Fb. The second band filter sets the Q to whatever you want it to be, e.g. Q=0.5, at the new Fb.

In practice, you may not even need the band filters. Just use a shelving filter to boost the lows. If the final response curve looks the way you want it to, it doesn't matter how you get there or what kind of fancy name you give it. It's all just EQing the bass response to be the way you want it.

[JonMarsh]

Oh and I should add that it takes AMPLIFIER POWER to use a LT circuit, no wimpy amps need apply.

Boy, isn't this the truth! Take a long throw driver like the HE-15 that thrives on hundreds of watts even in a large ported enclosure, and the requirements really climb for a compact sealed LT setup. Instead of 500W or so, even just 6 dB of LT boost will kick it up to 2 kW at the affected frequencies (low). So, LT systems tend to be thermally limited, while ported and IB systems usually run of out VC travel before things get too toasty. OTOH, the LT systems do sound pretty nice, usually having critically damped transient response. One of these days I'm going to get around to slapping my HE-15 into a setup like that. Hopefully I'll be able to tickle it enough with a lowly Aragon Palladium....

Weather's holding here in Singapore; a few sprinkles yesterday evening, but I see shunshine this AM. 8) Heading back tomorrow. Lots of strange info picked up regarding management going's on. 8O

~Jon

[ThomasW]

Jon who.............? :wink:

At the RMAF I spent a fair amount of of time auditioning the Epiphany Audio subwoofers. These are 25" sealed cubes each containing a pair of Adire 18" Maelstroms, driven by a custom plate amp with a custom LT circuit. According to 'Mr Epiphany', Ken Bates, these subs have a "Q" of 0.5 and the group delay is 9ms! They weigh in at a dainty 202 lbs each....... :B

A pair of these subs were pretty impressive even to a person who's twisted idea of subwoofers is a 12 driver IB.

[Brandon B]

OK now you have me thinking about modifying my dual HE 15 sub again. How small a sealed box do you think an HE15 could do this in?

Cause I have this for power:

[ThomasW]

How small a sealed box do you think an HE15 could do this in?

Sorry but I don't have LspCAD loaded in this PC.

What size is the box containing the pair of HE-15's? Ported or sealed?

Given the size and weight of the driver I doubt that anything much smaller than the original 22" Stryke cube would be practical.

the Behringer DEQ2496 or DCX2496 can do it with ease.

Yea I know, but some how I don't think either of those are in Andrew's budget.

[Andrew Pratt]

Any thoughts on the new plate amps with them integrated?

[Brian Bunge]

Andrew,

An LT circuit is an LT circuit. Nothing magical about the ones in Adire's new amps vs. sourcing the parts and building it yourself. If you want a built-in amp for the Tumult I'd say the new Adire amps w/LT would be a great way to go.

[Andrew Pratt]

Brian I was wondering about the plate LT's since they seem pretty specific to your sub and box etc. Since these are adjustable I wasn't sure if the quality was the same when all factors were considered.

[ThomasW]

Well as I said before the LT circuit isn't something that's generic. It's specific to the driver and the box.
According to the Adire website.

And there's more! The preamp includes a full Linkwitz Transform circuit that also functions as a dipole equalization circuit. These circuits are laid out and ready to roll - simply choose your resistor and cap values (small 1/4W and 50V units), stuff them into the board, and open up the jumper that normally bypasses these circuits.

That's sounds like a good deal

But the most impressive feature of the preamp section is the EQ provided. Two full parametric EQ bands devoted strictly to the subwoofer range. Our research indicates that 95%+ of all bass EQ problems can be properly addressed with just a couple of EQ bands - rarely are more bands needed!

That doesn't jive with my experience...

For the Tumult you certainly want the ADA1200 amp not the smaller ones

[Bent]

Would I be correct in assuming that one would build the box, install the driver, install the amp, break in the driver, derive some parameters of the box/driver/amp assy to determine what freq. response you are starting with and then design the LT cct to allow for calculated the changes/adjustments?

I have all the test equipment (scopes, true RMS DMM's, freq generator, etc) except for a microphone - would an analog RS SPL meter suffice?

[Jerry Pease]

following this link > http://hometheatertalk.com/httalk/vi...=9010&start=60

about 5 post down Dan provides additional info.

[Jack Gilvey]

Brian I was wondering about the plate LT's since they seem pretty specific to your sub and box etc. Since these are adjustable I wasn't sure if the quality was the same when all factors were considered.

The LT in the Adire amp isn't "adjustable", as such. You are required to change resistors and caps just as if you built the circuit yourself. As such, it's as "good" as any other LT.

[Andrew Pratt]

Right I should have read the specs on their site before posting such a silly question ops:

[Jack Gilvey]

Would I be correct in assuming that one would build the box, install the driver, install the amp, break in the driver, derive some parameters of the box/driver/amp assy to determine what freq. response you are starting with and then design the LT cct to allow for calculated the changes/adjustments?

The best way to go is to measure your final assembly after break-in to determine your "starting" Fc and Qtc. This will allow you to more precisely calculate the components required for your desired final Qtc and Fc. In practice, though, I think you'd be just as well-off using Unibox predictions since the room will have such a huge effect on the FR and Q.

Right I should have read the specs on their site before posting such a silly question

Not a silly question at all...there are adjustable versions of "LT" circuits (at least as far as function) such as the Marchand Bassis equalizer.

I think the combination of that Adire LT plate amp and one of the 18" drivers discussed here would be amazing.

http://web.archive.org/web/20071005223804/http://sound.westhost.com/linkwitz-transform.htm

http://www.trueaudio.com/st_lkxfm.htm

http://web.archive.org/web/20090622151013/http://www.pvconsultants.com/audio/eq/linktran.htm

[Brandon B]

What size is the box containing the pair of HE-15's? Ported or sealed?

Given the size and weight of the driver I doubt that anything much smaller than the original 22" Stryke cube would be practical.

My present enclosure is slightly more volume (10%, about 8.3 cu' total)than the dual stryke enclosure (I added some 'debris' to correct it), and I can pretty effectively and easily seal up the PR openings without compromising the cabinet's integrity. Anyone wanna buy 6 big heavy Lambda PRs?

Does this make the sub less efficient? Being thermally limited, will its max output be substantially less than in the normal extension limited mode? The way my house is constructed (concrete slab and very open floorplan), I actually need the full capability of the dual HE15 to hit reference in my setup. I'd be fine giving up some amount to get better transient response out of the thing, but don't want to completely cripple it for HT. Or is this not actually an issue, output is not drastically changed?

BB

[ThomasW]

Anyone wanna buy 6 big heavy Lambda PRs

Ahh no thanks, but thanks for the offer. We have a pile of Stryke/Lambda PR's too. We tried them with the HE-15's and couldn't get them to sound right. That's why the AS-15 design was created.

Does this make the sub less efficient?

In general the smaller the box the lower the efficiency. Sealed is minimally less efficient than vented, given it's lack of boost at the tuning frequency.

If your current sub is at it's limits, I wouldn't recommend going with a smaller sealed LT based box. That is unless you plan on getting another pair of HE15's. I imagine some can be found. The other option is sell your sub and pick up a couple of the new XBL^2 18"s when they hit the market.

[Jack Gilvey]

I actually need the full capability of the dual HE15 to hit reference in my setup. I'd be fine giving up some amount to get better transient response out of the thing, but don't want to completely cripple it for HT. Or is this not actually an issue, output is not drastically changed?

As you'd be going from the present displacement of six 15" PR (correct?)down to relying purely on the Vd of a pair of 15" drivers for output down low...yes, output potential near the current Fb would be substantially reduced in a sealed version. In a box that big, I believe you'd be excursion- rather than thermally limited.
You probably wouldn't need a full-blown LT, either, as you wouldn't be correcting a high Qtc (as I recall...don't have the HE15 specs)...a simple shelving filter should be fine.

[ThomasW]

HE15 T/S Parameters

Fs: 26 Hz
Qms: 6.0
Qes: .345
Qts: .326
Vas: 106.7 L
Re: 6 ohm
BL: 27.3 Tm
no: .529%
1W SPL: 89 dB
Cms: .145 mm/N
Mms: 257 grams
Rms: 7.0 kg/s
Xmax: 22.85 mm
Sd: 720 cm^2
Vd: 6.5L total
PEmax: ~ 2000W
3.5" p-p excursion
dual 3ohm VC

[Brandon B]

Sounds like I'm best off just sticking with what I have for now. Thanks for the input.

BB

[Jack Gilvey]

Thanks, T. Looks like a pair would net you a Qtc of 0.43 and an F3 of ~35Hz...lots of eq and power needed sealed. Thermal limitations would indeed be a factor. That ain't a sealed driver.

[Dotay]

Does anyone want to describe what the different T/S parameters mean in dummy newb terms? I understand Fs and Xmax, but what does the other stuff mean and how can you tell if it would work better in a sealed/ported/IB/PR enclosure?

[taz13]

Dotay
Try Here

[Dotay]

Thanks taz that clears some stuff up, although I think it's going to take a couple hours for my eyes to readjust to normal pages after reading that. Holy cow was that site hard on my eyes.

On another note, what is considered an optimal Qts or maybe a better question is is what are the different characteristics of a low and high Qts?

[taz13]

The link to the site this comes from follows below, hope it helps.

Q Parameters Qms, Qes, and Qts are measurements related to the control of a transducer's suspension when it reaches the resonant frequency (Fs). The suspension must prevent any lateral motion that might allow the voice coil and pole to touch (this would destroy the loudspeaker). The suspension must also act like a shock absorber. Qms is a measurement of the control coming from the speaker's mechanical suspension system (the surround and spider). View these components like springs. Qes is a measurement of the control coming from the speaker's electrical suspension system (the voice coil and magnet). Opposing forces from the mechanical and electrical suspensions act to absorb shock. Qts is called the 'Total Q' of the driver and is derived from an equation where Qes is multiplied by Qms and the result is divided by the sum of the same.

As a general guideline, Qts of 0.4 or below indicates a transducer well suited to a vented enclosure. Qts between 0.4 and 0.7 indicates suitability for a sealed enclosure. Qts of 0.7 or above indicates suitability for free-air or infinite baffle applications. However, there are exceptions! The Eminence Kilomax 18 has a Qts of 0.56. This suggests a sealed enclosure, but in reality it works extremely well in a ported enclosure. Please consider all the parameters when selecting loudspeakers. If you are in any doubt, contact your Eminence representative for technical assistance.

Again Here

[ThomasW]

This site is a little easier on the eyes.

http://www.the12volt.com/caraudio/theile.asp

[Hank]

Lots of strange info picked up regarding management going's on.

It might finally be time to consider a change, Jon

[JonMarsh]

It might finally be time to consider a change, Jon

You know, you're probably right, Hank, at least if I wanted to keep my work life from getting overly complicated and stressful.

Problem is some "golden handcuffs", includling 19 years+ credit in a pension program which isn't available anymore to new employees, and which is beaucomp better than the current one.

Then, there's the old "devil you know", to the devil you don't at another company. Sigh....


BTW, my RS225 Daytons are here (as I am, just a few minutes ago!- been up about 24 hours, now. The Dayton's look pretty nice, feel pretty heavy, and a remarkable thing is, they cone is quite "inert" if you tap it.

They say "made in Taiwan". The frame looks like a HiVi, but that could be sourced from a common frame vendor. I still think these look like a series I saw on the Swanspeaker site earlier this year, which were later taken off. Black aluminum cones, with black phase plugs.

And they fit perfectly in the M8ta cabinets....


.... more this weekend.

~Jon