Ekta-TL

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ScanSpeak Ekta-TL
Copyright 2024 © Troels Gravesen

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DRIVERS CROSSOVER CABINET WORKSHOP PICS MEASUREMENTS SPEAKER-KIT CROSSOVER LAYOUT

Every time I set up a transmission line I hear something I don't hear from conventional ported boxes, a quality of bass reproduction that is unique in its lightness and ease of reproduction of bass notes. By lightness I don't mean low or vague, but as the bass driver has been relieved of some acoustic resistance. I haven't made a lot of TLs, because it takes space and for a 3-way it's not always that easy to incorporate the midrange driver in the physical design. However, making the TL here is no more complicated than making a conventional vented box with braces.

Basically a TL is a quarter wave tube with the bass driver placed around 1/3 down the tube and the initial square area around 1.5 x the membrane area and ending around 0.5 x the membrane area. It's not that critical as can been seen from examples on the web. Learn more about transmission line speakers on Wikipedia. Technically a TL's roll-off is the same as a vented box, 4th order.

I wanted to try out the concept once more with basically the ScanSpeak Ekta-mkII setup to further improve bass response. In addition to this I wanted to use the new and magnificent 3/4" dome D3004/712000 tweeter. This tweeter has been on my shelf too long.

Now, to make a slim floorstander with a conventional arrangement of midrange driver we run into the problem of the driver needing its own cabinet, thus blocking the transmission line. For the SBA-7MD this wasn't a problem as the midrange dome comes with its own small rear chamber that didn't obstruct the motion of air. The 12MU needs more volume than that, and one way to solve the problem is placing it on the top. This will reduce the length of the TL and to compensate for this we skip the base support of the SBA-7MD and get some extra centimeters there.

Basics:
3-driver speaker.
Dimensions: 24 x 30 x 105 cm, WxDxH.
System sensitivity: 87.5 dB/2.8V/1 meter.
Impedance: 4-8 Ohms.
Power requirement: 30+ watts/channel.

Useful links (Please follow all links before e-mailing!):
http://www.troelsgravesen.dk/tips.htm
http://www.troelsgravesen.dk/tips.htm#CONSTRUCTION_OF_CROSSOVERS
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/LCR-RC.htm
http://www.troelsgravesen.dk/Inverted-Polarity.htm
http://www.troelsgravesen.dk/choices.htm

DRIVERS
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Download specs here: D2104/712000 12MU/8731T00 18WU/4741T00

CROSSOVER
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CABINET
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Cabinets were made from ~21 mm Baltic birch with 25 mm black MDF (HDF/Valchromat) for front panel lined with oak fillets as seen on drawing. Midrange needs an additional 12 mm panel to be time-aligned with tweeter when listening on mid height.
You can make front panel from 21-22 mm MDF, but the mid panel must be 11-13 mm.
Chamfering of the front panel is optional. Remember to chamfer driver holes for bass and mid.
Damping materials basically added the same way as the SBA-7MD cabinet.

Damping of midrange cabinet: Fold a piece of 20 cm wide felt and add as seen on drawing. Fold a piece of 20 x 20 cm acoustilux (blue) and place as seen on drawing. Further add a piece of acoustilux, 10 x 50 cm. Sides are covered with felt. See images below.

Workshop pics
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As always, the first thing to do is to make a test baffle and see/measure if this idea has any future.
The midrange cabinet was made pretty close to the drawing as this was the driver I was the most anxious about - would I get a good frequency response without the need to elaborate corrections in the crossover?

I certainly wasn't disappointed, on the contrary, this is probably the best I've seen from the 12MU - ever.
You can never tell what a new baffle will do to the frequency response.

D2104/712000 on test baffle compared to my measurement on an "infinite" baffle, actually 80 x 100 cm.
No trouble here either.

18WU/4741T00 on test baffle, green, vs. response from the Ekta-mkII. No problems here either - and from these measurements an easy crossover seems possible.

Going for the real cabs:

All parts ready for gluing.

Start drawing all internal panels on left side panel.

Left: All refelectors. Right: Start gluing upper parts and make sure top panel is 100% perpendicular to the side panel.

Left: Glue bottom and make sure it's perpendicular to side panel.
Right: Fasten internal panels as seen on photo.

Some more pics of internal panels.

Left: Slot to the midrange enclosure, 10 x ~200 mm.

When all internal panels are in place, put on the right panel and mark internal panels with a marker to help placing the felt before final gluing. See below.

Add felt before gluing the final side panel.

Right side panel before gluing.

Routing for drivers:

Left: I always make test routings before hitting the final panels to make sure diameter and depth is correct.
Right: Routing the outer midrange panel.

Left: Routing for bass and tweeter.
Right: Due to the 25 mm thickness of the basic front panel we need to chamfer deep for the midrange driver to eliminate any acoustic resistance

Left: Upper panel in place. Right: Basic front panel seen from rear.

Testing all driver rebates.

Cut a piece of 25 x 50 cm acoustilux and fasten on top of center brace and pull into/down towards crossover hatch. Fasten at top with staples and leave the rest loose.

Cut a piece of 25 x 100 cm acoustilux and place as seen on images above. Fasten with staples.

Left: The crossover board.
Right: May be time for some lacquer - and beeswax.

Place a piece of 10 x 50 cm acoustilux in midrange cabinet.

The rear panel and - almost - finished speaker with prototype crossover - next to the Revelator-851.

First time fully set-up with test crossovers.
Bass as expected, articulate and deep.
Midrange and treble - maybe the best I've heard from the 12MU - and seamless integration with the Revelator tweeter. This works really, really well. Time to make the final crossovers.

Regardless of what I threw at the Ekta-TL, they did well. Actually made one of my tiny 1948 niche workshop windows rattle, so I had to add some Superfix putty round the edges...

Connecting the driver wires to the crossover board.

Job done! Speaker finished.
I always want adjustable feet on floorstanders, here a 12 mm oak frame for my preferred - and very cheap - feet.

Ekta-TLs ready to rock!
Here bi-amped with Hypex FA501 for bass and EAR-861 for mid-tweeter.
Kuzma StabiR turntable with 4Point9 tonearm and Ortofon Windfeld Ti cartridge. I love it!

A friend of mine lend me this 1962 Quincy Jones recording and it's a really harsh mix, lots of midrange, lots of treble and very little bass. Yet, and this must be the litmus test, the Ekta-TL did it without ever sounding more harsh or shrill than it should. This from my Ortofon Windfeld Ti cartridge.

MEASUREMENTS
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A few comments on MEASUREMENTS before you start interpreting the readings below.
First of all, if we think measurements will tell us how a speaker sounds, we're wrong. The perception of sound is way too subjective to be reflected in any measurements we can perform. A loudspeaker system is meant to give us a satisfying idea of an acoustic event and for some people a pair of 5 USD ear-plugs are enough, others spend 200 kUSD on a truly full-range pair of speakers - and the latter may not be happier than the former.
Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas, although dispersion characteristics play a vital role here. A two-way 7+1 and a three-way 7+4+1 may display similar horizontal dispersion, yet sound very different. Measurements may tell us about bass extension if far-field measurements are merged with near-field measurements. In addition to this, ports may contribute to bass extension. Most of we diy'ers do not have access to an anechoic room for full-range measurements from 20-20000 Hz.
What cannot be seen is what kind of bass performance we get in a given room. Bass performance is highly dependent on in-room placement of your speaker and the same speaker can be boomy in one place and lean in another. Actual SPL level at 1 meter distance and 2.8V input is useful for en estimate of system sensitivity and combined with the impedance profile may give an idea of how powerful an amplifier is needed to drive the speaker to adequate levels.
What measurements do not tell is the very sound of the speaker unless displaying serious linear distortion. The level of transparency, the ability to resolve micro-details, the "speed" of the bass, etc., cannot be derived from these data. Distortion measurements rarely tell much unless seriously bad, and most modern drivers display low distortion within their specified operating range.
Many people put way too much into these graphs and my comments here are only meant as warning against over-interpretation. There are more to good sound than what can be extracted from a few graphs. Every graph needs interpretation in terms of what it means sonically and how it impacts our choice of mating drivers, cabinet and crossover design.
What measurements certainly do not tell is the sonic signature of the speaker, because speaker cones made from polypropylene, aluminum, Kevlar, paper, glass fiber, carbon fiber, magnesium, ceramics or even diamonds all have their way of adding spices to the stew. Nor do measurements tell what impact the quality of the crossover components add to the sound, from state of the art components to the cheapest of coils and caps, they all measure the same if values are correct, yet sound very different.

Final frequency response, normalised for 2.8V/1 meter.