CSS EL70 LX521 FAST (or something like that) - diyAudio
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Old 5th January 2013, 08:26 AM   #1
slomatt is offline slomatt  United States
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Default CSS EL70 LX521 FAST (or something like that)

There's probably some creative meta-acronym name for this project, but for now let's just call it the "CSS EL70 LX521 FAST".

At last year's (2012) Burning Amp event in San Francisco I had the opportunity to listen to the Linkwitz LX521 dipole speakers and was very impressed with them. Imaging was excellent, they had the magic "weightless" sound that can come from dipoles, and as expected the bass was very tight and musical but also surprisingly powerful. The design intrigued me and I wanted to be able to listen to it more, but at the time the plans were not available for sale and the $2k+ parts price was prohibitive.

Fast forward a few months... I had recently purchased a pair of CSS EL70 4" full range drivers and was trying to figure out what to do with them. My initial plan was to build a FrugalHorn MK3, but I hadn't decided on the aesthetics of the enclosures. Around this time I discovered that Home Depot was selling decent (not great) 3/4" cabinet grade plywood for a bit over $30 and figured that a sheet of the plywood combined with the EL70s and 4 8" woofers in a FAST arrangement would be inexpensive and fun to try out. As an added bonus I could try building a dipole setup roughly based on the Linkwitz LX521.

The last time I build dipole speakers was somewhere around 2001, and they were 8' tall line array. I liked the sound of the dipole radiation pattern, but was never really happy with these speakers. (see Dipole Line Source Arrays (DLSA) - Blank White Page).

So, the plan was to build some poor man's LX521 clones that could later be turned into the real deal by swapping woofers and building new midrange units.

DISCLAIMER: I did not pay for the LX521 plans because they were not available at the time. Instead I reverse engineered the dimensions of the W-frame dipoles based on what I saw in person and notes on the Linkwitz Lab's website. My enclosures should be quite close in dimensions. This W-frame design is 100% the property of Linkwitz Labs and I will not provide the dimensions I used out of respect for Siegfried's work.
LX521 Description


The 8" woofers are no-name units I received from a friend who worked at a major subwoofer company. I've had them sitting around for over 10 years and was happy to finally use them in a project. Their rough t/s parameters are below. The Qts is not optimal for an open-baffle, but since I'll be using a DSP that can be compensated for.

fs 34.5 Hz
Qms 1.907
Qes 0.357
Qts 0.300
Fts 114.8
Mms 38.58 g
Cms 0.551 mm/N
Rms 4.386 kg/s
Vas 35.84 litres
Bl 11.576 Tm
Eta 0.40 %
Lp (1W/1m) 88.22 dB
Dd 16.51 cm
Sd 214.1 cm^2


The full range drivers are CSS EL70s which are well documented on this website.

Ok, enough introduction. Here are some photos of the build process and the speakers themselves.

I didn't take pictures while building the W-frames, but they are cut out of 3/4" plywood and held together with screws. No glue is used at this point so that I can take out the baffles if needed. The parts fit tight enough that air leakage is not a concern. Each woofer hole is chamfered on one side to smooth the air flow. The woofer baffles are mounted at 90 degrees relative to eachother and meet the top and bottom at a 45 degree angle. They are placed so that the acoustic center of the drivers are in the middle of the W-frame. All straight cuts were done using a hand held circular saw and a straight edge, the hole were cut using a router and a circle jig.
Click the image to open in full size.

The LX521 midrange baffles have a somewhat strange shape that is designed for optimal polar response, they also include two midrange drivers and two tweeters (one rear firing). Since my plan was to use a full-range driver I built a much more simple baffle with the driver centered in the middle so that the response to each side is the same, though delay will vary based on frequency. I'm no where near an expert, but this should be good enough for now. The ends of the baffles are rounded for aesthetic reasons. I have not measured or calculated the baffle step response yet (more on this later).
Click the image to open in full size.

The EL70s are recessed to prevent edge diffraction, and the baffles are mounted to simple bases built using scraps from the rest of the project.
Click the image to open in full size.

The rear of the openings are chamfered to smooth airflow. The quarter circle vertical supports were cut from the scraps removed from the woofer cutouts.
Click the image to open in full size.

I built some quick binding post holders out of scrap MDF. Unfortunately the plywood de-laminated in two places because of forces applied by the screws. Possibly the pre-drilled hole was not deep enough.
Click the image to open in full size.

Click the image to open in full size.

For a month or two I didn't have an active crossover so I ran the two woofers in parallel with the midrange on each side. This sounded surprisingly good and gave me hope for the final project. For Christmas I received a balanced MiniDSP which I'm now using as a crossover (more later). The MiniDSP is stacked with a MiniDIGI to add support for digital input, and I'm using unbalanced connections at this point.
Click the image to open in full size.

Power to the MiniDSP stack comes from a ~16v wall wart running through a quick-n-dirty 12v regulator setup. The digital audio signal comes from lossless files on my Mac, fed over USB to a UCA222, and the over fiber to the MiniDSP. Supposedly the UCA222 is bit-perfect in this configuration, but I don't have 100% proof of that. Alternatively MiniDSP sells a streamer that converts USB audio to I2S for input to the MiniDIGI.
Click the image to open in full size.

Here is the final setup in all it's unfinished glory. The midranges are driven by a MiniAleph, and the woofers are driven by a LM3886 based chip amp.
Click the image to open in full size.

Click the image to open in full size.

At some point I may build a "bridge" to physically lift the midrange baffle over the W-frame, but for now they are just clamped together with the driver's acoustic centers roughly in line.
Click the image to open in full size.


... more to come ...
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Old 5th January 2013, 08:38 AM   #2
slomatt is offline slomatt  United States
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At this point I've only eyeballed a rough crossover setup on the MiniDSP. The midrange and woofers are crossed over using LR4 slopes at 150 hz, and I've put a small lift on the bass response below 80 Hz to compensate for loss below the open-baffle's re-enforcement frequency. At some point I'll get around to measuring the actual frequency response and setting up a better DSP configuration, but for now these are very enjoyable to listen to. There is a definite bump in the mid-bass that will need to be eq'd out at some point.

Overall I'm quite impressed with the sound from the EL70s. The midrange is smooth and they image very well, my only small complaint is that they lack some top end and the "shimmer" is missing from cymbals. This is to be expected from any full range speaker, and I might add dedicated front and back super-tweeters at some point. There is much more bass than I was expecting and the woofers are barely moving so there is a lot of room to eq in a lower frequency response in the future. Bass guitar sounds much more musical than most sealed/ported enclosures.

For now I'm going to keep listening to the speakers in their current configuration. Currently I have no real volume control and instead have to use the input levels in the MiniDSP configuration, in the near future I need to wire up a potentiometer for digital volume control. With the current gain structure I'm using almost full volume anyway, so I should not loose much data resolution when lowering the volume.


So, that's my "quick" update on this project. I'm very interested in any input, positive criticism, or questions. The next major step is to get my measurement setup working and to start tweaking the DSP settings.

- Matt
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Old 5th January 2013, 01:35 PM   #3
xrk971 is offline xrk971  United States
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Fantastic job - well done! Looks great and I bet it sounds even better. The dsp and biamping is definitely the way to go. I noticed that the Linkwitz woofer cabinet has a vertical panel at the top. I wonder if that is needed for acoustical reasons?
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Old 5th January 2013, 04:30 PM   #4
slomatt is offline slomatt  United States
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Quote:
Originally Posted by xrk971 View Post
Fantastic job - well done! Looks great and I bet it sounds even better. The dsp and biamping is definitely the way to go. I noticed that the Linkwitz woofer cabinet has a vertical panel at the top. I wonder if that is needed for acoustical reasons?
Thanks. I considered adding a vertical panel similar to the Linkwitz cabinet, but I couldn't think of a strong reason to do so. One reason for the panel might be simply to partially block your view of the upper woofer so you can't see the wires. Or, it could be acoustically relevant and used to tune the response of the w-frame, though if that were the case I would expect he would have placed a similar panel on the rear to make things symmetrical. My guess is it is purely there for aesthetic reasons.

- Matt
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Old 6th January 2013, 10:13 AM   #5
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by slomatt View Post
This W-frame design is 100% the property of Linkwitz Labs and I will not provide the dimensions I used out of respect for Siegfried's work.
Such W-frames have been around for many years, and SL is using them in a straight forward way. So I don't see a real need to keep the dimensions secret.
Usually you make them as wide as the driver dimensions demand or as high as your concept requires. For ease of building most everybody puts the driver baffles at 90, but that is not mandatory. In most cases you end up with the frame depth equal to the width, but you can vary that too.
When you increase the frame depth, the W-frame will deliver more bass, but the quarter-wave resonance of the frame will get lower and become more prominent.
Quote:
Originally Posted by xrk971 View Post
I noticed that the Linkwitz woofer cabinet has a vertical panel at the top. I wonder if that is needed for acoustical reasons?
I don't see any severe acoustical reason. What I would do is connect the side walls in the middle of the front opening by a horizontal brace. That's were the side walls could resonate most.
M_Baffle1.JPG
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Old 7th January 2013, 06:59 AM   #6
slomatt is offline slomatt  United States
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Rudolf,

Thank you for the reply. My intention was not to claim that Linkwitz invented the w-frame dipole design, only to note that since I based part of my project on his specific implementation I am respecting his intellectual property.

- Matt
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Old 5th March 2014, 12:09 AM   #7
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hi
do you have the dimensions woofers box? i have 10" woofers
thanks
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Old 4th April 2014, 02:08 PM   #8
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Well, I miss that little EL70 driver. Amazing bass in the proper enclosure. Is there anything out there that comes close now?
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