SL3-eXtreme Center channel speaker – a design and build story

Genesis:

After enjoying 12 years of the fabulous Logos center channel from Martin Logan, I was yearning for a larger and more powerful center to pair up with the big Monoliths.
The Logos is a great speaker, but is dynamically limited when pushed. Its single 6.5” mid-bass driver would bottom out at high SPL’s, requiring a fairly high 100Hz crossover and a steep 24db octave slope to remove enough low-frequency energy from it. Also, the midrange and treble was not quite as powerful as what the Monoliths can do (physics having something to do with it ;-).

Looking at Martin Logans subsequent efforts, the Theater and Theater-i, the improvement over the Logos was detectable, mostly due to dual mid-bass drivers. But overall, not worth the high cost of tradeup.
In 2005, ML announced upcoming centers based on the new Xstat technologies that would deliver greater output per square inch of panel. The new Stage was announced, and finally seems to be available now in the spring of 2006. However, at $3,500, it’s both too expensive and still not powerful enough for my needs.
For that kind of money, and given my ability to host a really large center behind my perforated screen I decided to pursue something completely different. Which is always more fun anyway

Concept and Design

Concept and Design

Given that center channels must carry the weight of the dialogue and much of the music in most movies, this speaker needs to have not only great range; it must also have the ability to play loud and evenly across its frequency spectrum.
Since I already have incredible bass capabilities in my system with an infinite Baffle subwoofer, the center will only need to cover frequencies above 85Hz.
We all know Martin Logans excel at midrange and high end. Where they, and most dipoles, are weaker is in the mid-bass, where people say they lack ‘punch’. They generally do not have good output at frequencies below 500hz. Therefore the design will need to address this critical range in the center.

I decided that I would start with a standard 4’ panel from one of Martin Logan’s existing lines. I settled on using SL-3’s, which have a 4’ panel and a decent width of 12”. This is the same panel on my Sequels and they have proven to play reasonably loud.
The plan is to take one SL-3 and chop it to essentially be just the 4’ panel and support structures. The electronics will be repackaged to fit into the ‘trimmed’ panel support.
The OEM passive crossover will be totally bypassed.

As pointed out above, the under 500Hz range is still a concern, so to address that I decided to take a page from Martin Logan’s own design book and build a line array of mid-bass cone drivers to handle the 80 to 500hz frequency range.
Line arrays allow the designer to achieve these benefits:
• Contours the vertical sound dispersion such that floor and ceiling reflections are minimized
• Maintains a wide listening area with room filling, nearly constant sound intensity
• Provides exceptional dynamic range and linear performance

All speakers produce sound in both the near field (close to the speaker) and the far field
as distance is increased. Conventional point source speakers generate a spherical
wavefront and they place the listener entirely within the far field while line arrays can locate the listener within either near or far fields. For a line array the near
field is where the radiated sound resembles a vertical cylindrical wavefront much like the one generated by the ESL panel. It is this match of polar radiation and power curves that make a line array of mid-basses the ideal companion for an ESL.

This line array will flank the electrostatic panel. The combined width of the two elements will be around 20” wide and both will be 4’ tall.

As in the Statement e2, this design delivers the smoothness of the electrostatic panel, moves the crossover low enough to be out of the critical midrange and delivers the kind of ‘punch’ only a large set of dynamic drivers can at these mid-bass frequencies.

Besides the careful positioning of the drivers in the array and the array relative to the high frequency panel, the next biggest issue is the crossovers. To handle this in as flexible a manner as possible, I’ll be using a DBX DriveRack 260 again (I already use one for the Monoliths). The key design considerations for the crossover will be types and slopes to use for the following elements:

Bass – 85Hz crossover to Infinite Baffle sub, 24db / octave LR
MidBass – 85Hz highpass, 24db / octave LR, 450 low-pass 12db / octave
Highs – 450Hz highpass, 12db /octave


Power for this system will be provided by the following:
Highs on the SL-3 panel – Sunfire Cinema grand signature 800 watts (4ohm)
Mids on line array – Sunfire Cinema grand 400 watts (4 ohm)

I call this design the SL3-eXtreme Center, in part inspired by the use of the Adire Extremis 6.8 mid-bass driver, ergo the use of ‘extreme’ in the name. But also because, it is a bit ‘out there’ in terms of uniqueness and performance goals

The short-hand naming for it will be the SL3-XC.

Here is a rendering of the concept:

Install context

It will be screwed onto the top of the Infinite Baffle (which is screwed and glued into the floor). It will be very solid. And before you ask, that IB does *not* shake. The room does
Of course, the Logos will be removed.



An acoustically transparent screen goes between the Monoliths and will hide this beauty. Sort of a shame to not be able to see it every so often. But it’s all about the sound. Lot’s of it

Construction

Construction

As noted above, I’ll need an SL3. So luckily, I found a suitable pair on Audiogon a few weeks ago and purchased them for a good price.
A reason for the good price is that one of the units had been dropped and the box slightly damaged and the panel had a ‘dimple’ in it. They played ok, but had seen better days.
I ordered and received the ESL panel updates from the factory this week, so I’ll be updating the SL-3 panel with a brand new one (and my Monoliths as well, since I was ordering panels).
This ‘donor’ SL-3 will have the Woofer section cut out, and the Crossover board totally re-arranged to both fit under the modified structure, but to also remove the passive elements out of the X-over. Bypassing is pretty simple, just feed the step-up transformer through a 1-Ohm resistance set (reusing three of the existing 3 Ohm resistors that are already wired in parallel).

The selection of the mid-bass driver is the most critical element in the process and the clear-cut choice here is the Adire Extremis 6.8. This driver is widely regarded for its smooth and wide range, extending cleanly past 1Khz. Its excellent low-frequency abilities are also desired for a smooth transition to the sub.
Adire’s guidelines are for roughly 9.5L per driver in a sealed alignment, therefore the line array box is a simple tower that measures 48”x10”x12.5” to yield an internal volume of roughly 60 Liters in a sealed alignment.
The inside will be 100% stuffed with synthetic speaker stuffing.
Using two internal braces for reinforcement, the box is pretty simple to construct. All sides are 3/4” 16 ply Birch plywood. As the speaker is never really visible, the finish will be flat black paint (since the perforated screen does let some light through, we don’t want anything reflective back there).

Internal dimensions for 60L box:

48-1.5 = 46.5”
10-1.5 = 8.5”
12.5-1.5 = 11”

Drawing of the box is attached.

Now, I’m off to the lumber yard for the imported Baltic Birch 16-ply wood sheets for the build. The cutting of the cabinet starts Tuesday.

Before then, I’ll be re-arranging the X-Over components onto a new board that will fit behind / under the ‘trimmed’ SL3.

I’ll keep up this thread with Pics and posts of the progress. Including measurements with ETF, comparing the Logos to the new SL3-XC.

Should be a lot of fun.

JonFo said:

Construction

This ‘donor’ SL-3 will have the Woofer section cut out, and the Crossover board totally re-arranged to both fit under the modified structure, but to also remove the passive elements out of the X-over. Bypassing is pretty simple, just feed the step-up transformer through a 1-Ohm resistance set (reusing three of the existing 3 Ohm resistors that are already wired in parallel).

Click to expand...

Hi,
On all the ML's I've checked, the 'crossover' is more than a simple crossover, it also includes equalizing network(s) of various R, L and C's in different models that act to flatten the frequency response. You should either retain the equalization portion of the 'crossover', or else be prepared to use an external, inline equalizer to do the job.

Also, double check those resistors, my experience has shown those are usually used to pad down the woofer volume (as it typically has higher efficiency than the ESL panel).

HTH,
Peter

Peter Hogan said:

Hi,
On all the ML's I've checked, the 'crossover' is more than a simple crossover, it also includes equalizing network(s) of various R, L and C's in different models that act to flatten the frequency response. You should either retain the equalization portion of the 'crossover', or else be prepared to use an external, inline equalizer to do the job.

Also, double check those resistors, my experience has shown those are usually used to pad down the woofer volume (as it typically has higher efficiency than the ESL panel).

HTH,
Peter

Click to expand...

Hi Peter,

The woofer is no longer there, so no need for it’s passive elements. If you inspect the schematic for an SL3, it really is a high and low section x-over design.

As for the EQ and phase for the ESL, no problem, that’s what the DriveRack speaker processor is for. I can pick any number of styles (Linkwitz-riley, butterworth, and Bessel) four different slopes, full phase angle adjustment in 1 degree increments. I can have low-pass be one slope /style and combine it with a different HP slope and style. I can also add delay at either inputs or outputs (each individually). Very, very flexible.
Using ETF measurement software and my calibrated mic, I’ll be adjusting sub, mid-bass and ESL integration for delay / phase as well as EQ. These are things passive x-overs only approximate.
Remember, I already use one DR 260 for my Monoliths. Works waaaay better than the factory passive.

BTW- the SL3 factory crossover is a 2nd order at 250. I am going to use a 2nd order at 450.

Congratulations! Bet it sounds great

BTW I tried a DBX Driverack PA in my Monolith III system. All I can say is that, if anything, you understated how much better Monolith III's would sound by getting rid of the stock crossover. I was bowled over. I was getting bass (for me in my system) I only dreamed of previously. I have a Driverack on order.

Did you replace your 12" drivers with Peerless units? How did that work out? No need to answer I'm sure it's great.

Happy listening!

ted betley said:

BTW I tried a DBX Driverack PA in my Monolith III system. All I can say is that, if anything, you understated how much better Monolith III's would sound by getting rid of the stock crossover. I was bowled over. I was getting bass (for me in my system) I only dreamed of previously. I have a Driverack on order.

Did you replace your 12" drivers with Peerless units? How did that work out? No need to answer I'm sure it's great.

Happy listening!

Click to expand...

Hi Ted,

Glad to hear the active x-over and bi-amping is delivering improved sound. It’s amazing isn’t it?

A DR 260 is much more flexible than the PA. I especially like and leverage the fact that the 260 has RS-232 control and a great software UI for managing it.

Updating the Monolith woofers is part of the agenda this week. As you’ll see in a picture below, I also got new panels for the Monoliths and a set of peerless drivers.

So I’ll be removing the rails, re-staining them with aniline dye (dark purple, to match the décor in the HT), updating the panels, and testing the new Peerless drivers.
I also bought some Deflex panels to put in front of the electronics module inside the Monolith woofer cabinet as well as plan to take the stuffing percentage up to 50% (factory is 30%). Small tweaks to decrease mid-bass resonance in there.

Good thing I have the next six days off….

Lot's of parts

OK, here a shot of all the stuff (or most of it anyway) going into the project.

I only pulled out one of the Adire Extremis; stacking six was more work than I wanted to do just for a shot

The SL3 x-over is out so I could examine it and plan the mods. Anyone need some spare x-over parts?

Monoliths are Big

Just in case you wondered, a Monolith is indeed a big boy.

Keeping up with two of those will surely be a challenge, even for this new center. Hopefully, we have enough juice and radiating surfaces here.

Here is the 'donor' SL3 next to the Right Monolith.

Screen is up in this pic.

Before State - Logos Frequency response

Just to be complete, I took some measurements of the current rig in the ‘before’ state and marked the exact locations / levels used. This is so I can do an ‘after’ set.

Here is the Martin Logan Logos center speaker Frequency Response (FR). Remember that it is behind an acoustically perforated screen, so some of the resonances, reflections are from the screen. But in general, the FR is not too smooth.

Monolith and Logos Frequency response

It’s interesting to compare how a Logos FR (blue line) approximates a Monolith (green line). The only similarity is in the rapid high-end drop-off.

The dip at 1.2K is likely a crossover anomaly in the Logos. I hope to avoid this type of stuff by using actives and by crossing over much lower, since I have a big panel to work with (from the SL3).

Of note is that this is the Monolith panel before the swap to the new ones. Also, it's only the panel, so anything over 250 is comparable. Another thread will pop up describing that, along with more measurements.
So this is two 10+ year old speakers being compared.

JonFo said:

Of note is that this is the Monolith panel before the swap to the new ones. Also, it's only the panel, so anything over 250 is comparable. Another thread will pop up describing that, along with more measurements.
So this is two 10+ year old speakers being compared.

Click to expand...

Hi Jon,

I would be very grateful if you could post about your Monolith 'rebuild' if you have the time and energy. Our Monoliths are a decade old and we can see ourselves needing to do something similar in the next couple of years. In particular I would be interested in your rationale for the choice of woofer and any practical tips in fitting new woofers.

Kevin

Waterfall plots

Here is a waterfall spectrum graph for the Logos, not so hot in my book. There are way too many resonances and issues even early (<3ms) in the graph. Some of it is due to the reflections from the back of the screen, but we’ll have to re-measure once the screen comes down and compare.




For comparisons sake, here is the waterfall plot for the Monolith (Still the old panel).



Much better, only resonances are in the mid-bass.

OEM Crossover bypass

Part of the concept is to go full active for crossovers, so I need to modify the factory crossover to retain only the ESL drive elements and remove the passive elements from the signal path.

After due diligence and research, including help from knowledgeable people, the following schematic is what I’m putting together.

It depicts the high-frequency signal path for the ESL panel (the low pass signals for the mid-bass come straight from the DriveRack, see post further below).

The positive high-frequency signal first goes through a set of 3 Ohm – 20w resistors wired in parallel. This yields a net resistance of 1 Ohm and a power rating of 60w. The reason for this resistance is that otherwise, you are hooking the amp straight up to a Transformer, and some amps do not like the reactive nature of that, nor the very low impedance at HF that it creates. Therefore a 1 Ohm load helps make this ‘friendly’ to the amp.
Unfortunately, it also lowers efficiency by a bit. No free lunch.
The good news is that some amps (Sunfires) can deal with it, so I re-purposed the ‘bass contour’ switch to a role of bypassing the 1 Ohm load. The SPST switch will bridge the resistors when engaged.

After that, it’s basically a stock ML ESL circuit topology. The Signal positive goes to the red lead on the step-up transformer; the signal negative goes to the black lead.
The transformer outputs the positive signal on the gray wire, which in turn routes straight to the rear Stator of the ESL (since the factory passive 2 nd order x-over inverts phase).
The Orange output of the transformer goes to the high-voltage board to provide a reference signal for the Diaphragm and to supply the negative Signal for the Front Stator via the Blue line on BB4.
Additionally, a reference signal ground is fed to the BB4 on the High-voltage board, and another is fed straight to the front stator via the second blue lead.

Simple, right ?

Physical Build of ESL drive board

Now that the design is done, time to build.

First challenge: the OEM board is huge, and since we are cutting the SL3 down to just 4’ of total length (size of panel only), we need to repackage the electronics.

Since I’m lazy (cough), I decided to see how much of the OEM board I could reuse. After some measuring, I decided that the ‘bottom’ 4” of the panel that contains the Input, power, and bass-contour switch could be repurposed as the new vertical ‘top face’ of the rear, so I’d need to cut 4” into the board.
To fit the rest of the stuff and have something that slid into the opening of the SL3 in a horizontal fashion, I’d need to fit the 9.5” opening and keep it to an 11” depth (and no part could protrude more than 3” vertically).

More measurements and the rest of the board was cut up to meet these criteria.

Of course, before any cutting, I had to totally remove all parts from the board.

After the cuts, I re-laid out the elements and screwed them on. Soldered leads together and basically have what you see here.

The stuff on the left are all the removed passive components. Good thing too, as they would not have fit.

Another view of new board

Here is another view of the newly re-arranged ESL drive board

JonFo said:

The Orange output of the transformer goes to the high-voltage board to provide a reference signal for the Diaphragm and to supply the negative Signal for the Front Stator via the Blue line on BB4.
Additionally, a reference signal ground is fed to the BB4 on the High-voltage board and another is fed straight to the front stator via the second blue lead.

Simple, right ?

Click to expand...

Hi,
Double check the high voltage wiring. The ML's I've seen have the transformer outputs going to both stators, and the bias high voltage between the transformer output center tap (that you haven't shown) and the diaphragm film. That way the transformer signal drives one stator negative, the other stator positive, and vice versa for the other half of the waveform, for a push-pull effect on the diaphragm.

Peter

Peter Hogan said:

Hi,
Double check the high voltage wiring. The ML's I've seen have the transformer outputs going to both stators, and the bias high voltage between the transformer output center tap (that you haven't shown) and the diaphragm film. That way the transformer signal drives one stator negative, the other stator positive, and vice versa for the other half of the waveform, for a push-pull effect on the diaphragm.

Peter

Click to expand...

Thanks Peter. You are correct that some models have the step-up wired as you describe. There are schematics floating about that depict the orange tap feeding the front stator and the gray wire feeding the rear, with a center tap feeding the high voltage board.

However, the SL3 units I have are wired as I described. I didn't even disconnect the stock wires from the High-Voltage board, xformer nor the connector.
On my xformer, all other taps are wrapped down and unused as you can see in the pic. So no center output tap was ever used.

This had me scratching my head for a while, but given that it’s the stock wiring (and I have pics of ‘before’ to double-check, plus the other SL3 unit, I feel comfortable this is correct.
It looks like ML had a design change in the HV board at some point as my boards are from Feb of ’98.
So anyone reading this and planning a mod to their ESL drives should take this into consideration, as there are various modes of wiring these things depending on which rev of the HV board you have.

Box prep

Here are some pictures of the build of the Line Array Box.

As described above, it’s a 4’ box made from ¾" 12-ply Baltic Birch plywood. It features two internal braces plus cutouts in the rear for two binding post cups and six cutouts in the front for the Extremis.
The box uses biscuit-joins to keep alignment, and the sides and tops are all glued and screwed together. All edges are routed with 1/8" round-over.

Also seen in this shot is the cut-down version of the SL3. The masking tape on the rail fronts protects the Velcro and foam strip from dust and paint.
The SL3 is cut to just the length of the panel, or 4’.