Ekta Check out big brother Ekta Grande The sliced paper driver 18W/8531-G00 is one of the most potent 6-7" midbass I have ever experienced. It did great in the SP95 set-up. It did even better in the SP38 construction and here's the best I have ever heard from this driver and probably due to the cabinet made from curved and laminated side panels producing the most rigid enclosure I have tested. Un-coated paper cones will always leave a fingerprint on the reproduced sound and the sliced paper drivers are no exception from this rule. Having a 6-7" un-treated paper cone handling a very wide frequency range in a two-way construction is tough. It helped a lot taking the first point of crossover down to 1600 Hz by the D3806/8200 tweeter and here we have the option 2 again from my thoughts about various ways of designing 2- and 3-way speakers with regard to the chosen points of crossover: http://www.troelsgravesen.dk/Design_criteria.htm After doing the 3½-way Zahra: http://www.troelsgravesen.dk/8543.htm, I was happy repeating this approach and in particular with drivers I knew from other constructions. I've made a crossover for a construction including the coated version of the 12M middriver (not published - and shall not) and it did very well despite having to mate two dedicated 8" bass drivers at around 450 Hz. Not my favourite set-up, but this was how the cards were played from the constructor.
The construction shown here is the result of another DIY-service project and again the owner has allowed the publication of his construction. Thanks to Jesper. This time I was presented with the finished cabinets and only had to do the crossover. However, a "12M" project was definitely one of the things I would like to do and my own thinking about this small mid-driver was either a single 18W/8531-G00 bass driver + 12M + tweeter or a big 3-3½-way from two 18W/8531-G00 drivers. More on that one later. Getting the 7000 tweeter mating a 15M mid-driver wasn't successful in the wide baffle set-up, but you can never tell how a driver will perform in another set-up. Maybe the 12M would be the ideal partner for the much too expensive 7000 tweeter. Well, the other drivers are ripoff too, but not much we can do about it unless we form a Sliced Paper Club and buy 500 pcs each.
The cabinets are made around the SP38 front baffle layout. The front panel size, tilt and placement of drivers are almost identical to the SP38 and you can use this cabinet by adding a 1.5 litre mid-driver enclosure. Cabinets made from multiple layers of plywood. What Jesper has produced here is probably the most rigid cabinet I have ever seen - and felt. The knockletest is like hitting a solid log of hardwood. The side panels are made from 4 layers of plywood with an overall thickness of 30 mm. The front panel has a subpanel, thus reaching a thickness of 45 mm. This takes some serious chamfering of driver holes to make the drivers breathe. Don't report back it sounds crap if you have stuck the 12M into a tube! The cabinet volume is 32 litres minus mid-driver enclosure and bitumen pads (4 mm). The vent tuning is 33 Hz (65 (ID) x 150 mm vent).
Cabinet details:
The Crossover: Again the drivers were linearised by passive components and a 3-way electronic crossover was attached and adjusted to produce a reasonably linear frequency response. The point of crossover between bass and mid was varied from 350 to 1000 Hz and from 2000-5000 Hz between mid and tweeter. This was done while listening to various kinds of music and vocals to possibly render the best points of crossover. Taking the 12M down to 400 Hz made the speaker sound rather thinnish with lack of volume to particularly vocals and somewhere between 600 and 1000 Hz appeared to make a good balance between bass and mid. The point of crossover between mid and tweeter was somewhat more difficult as the differences weren't so clear. I have a CD with a particular sibilant female vocal and this recording suggested a crossover in the 3.5-4 kHz region to work the best. Next thing to do was creating the
frequency/phase and impedance data files from all drivers
and start LspCAD simulations to see whether a passive
crossover could be constructed with the chosen points of
crossover in mind - and still rendering a reasonably flat
FR profile. Preferably with the simplest possible
crossover. The version 1 crossover below may actually be the 5th version of the first crossover, but I'll spare you all the details. Playing around with the LspCAD I also tried a 1st order filter, something I mostly stay far away from. The huge overlap between drivers creates - to my ears - a lot of intermodulation distortion unless played a very low volume, but in this case both FR and phase turned out so nice that I put together a crossover to make an AB test with the version 1 crossover seen below. Making a 1st order network really takes quite a few components, almost as many as the 2nd order filter. The drivers' impedances has to be linearised, e.g. the tweeter resonance peak at 500 Hz has to be removed as the bass and tweeter cross only 15 dB down around 1800 Hz. The mid has to attenuated considerably and is 4 dB below summed response due to the huge overlap of drivers. Anyway, despite having a similar amplitude profile the speakers sounded very different and what made the biggest difference was the sibilance from the 1st order filter. Some more modelling and crossover tweaking was not enough to remove this phenomenon and the 1st order filter was discarded. I have with interest read the recent review of the Audio Physic Caldera loudspeaker in Stereophile Dec. 2005. The 7000 tweeter is used here although in a very unusual fashion as the faceplate is removed and the tweeter is hung in three wires in front of the main mid-driver to resemble a coax driver. Michael Fremer had major trouble with this speaker and from John Atkinson's measurements it seems clear that the 7000 tweeter has an average 3-4 dB higher amplitude compared to the midband. Due to a narrow dip above 2 kHz there's a 10 dB jump from 2.3 kHz to 4 kHz! It sounded so bad that a new pair of speakers were required and this time the tweeter level appeared to have been attenuated further, although only approx. 1 dB. The frequency response is nice and flat from 100 to 2000 Hz and the rest is still some 3-4 dB above average. I really wonder what the 7000 tweeter sounds like from this set-up... The crossover
Crossover components
Crossover layout
Measurements:
Left: Phase tracking between mid and
tweeter. Blue = minimum phase. As can be seen there's an
almost 180 deg. phaseshift at 3800 Hz when mid and
tweeter has opposite polarity.
Left: Preliminary frequency response of
"final" crossover. Note the is a bump at 7.5
kHz from the 7000 tweeter. Initially I thought this was
no big deal, but the tweeter persistently kept reminding
me of its presence going trough a number of tracks.
Hmm... if you can "hear" the tweeter as such,
there's a problem.
Left: Impedance of system with a vent tuning around 32-33 Hz. The phase is -56 deg. (quite low) around 60 Hz combined with an impedance of 12 ohms. Now, a combination of negative phase and low impedance draws a lot of current from your amplifier, but this doesn't seem too serious here. There's not much we can do about it except for closing the vent and this at the expense of low-end extension - and this speaker goes deep. The best and deepest I have ever heard from a 7" bass driver. I have run the Ekta from my 50 wpc valve amp it does well, but a 100 wpc solid state amp has the grip on the drivers that my valve amp cannot match. Give it s the best of solid state amplification and the Ekta will deliver. I've recently heard a Cayin 252B integrated and I was highly impressed. Not least taking the price into consideration. Right: Final FR from crossover version 1, mkIII. As always: Disregard what below 350 Hz. My workshop doesn't allow reliable readings lower than 300-350 Hz unless I do nearfield measurements.
I'm sure there will be questions about the 7000 tweeter. Please another one! The price of the 7000 is a killer and I fully agree. The price is way too much. It's a fine tweeter, but it's not that great. To be honest, I'll bet that Vifa XT25TG at one-tenth the price of the 7000 will do as well as the 7000 tweeter as it only has to handle down to 3800 Hz. Vifa DX25TG and XT25TG versions: I have tried modelling the 4 ohms
Vifa DX25TG tweeter. Much easier compared to the
troublesome 7000 tweeter. The changes are:
Tweeter section:
Importance of acoustic surroundings:
The response graph seen above depicts the
FR from the DX tweeter "as is" (blue) on the
present baffle. Applying a piece of tape on top of the
front panel to eliminate the 45 deg. cut-off make the red
FR. So, the DX25 doesn't seem to have a bump at 6 kHz any
more, rather a minor depression around 9-12 kHz. Not
having the cut-off makes an overall smoother response. View Ekta builders here. |
