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Definitive Technology - Center Channel

The Parts Express buyout of Definitive Technology drivers proved to be too much of a tempation for me to resist.  I initially purchased a pair of the cheapest woofers and a pair of tweeters just to mess with.  Then Parts Express went and put the 299-706 drivers on sale, and the Ariel Clones were born.  I then decided that I might as well go ahead and build a complete home theater system, as I was already over half-way there.  I was able to secure two pair of 299-708 shielded 5-1/4" woofers, and a pair of 279-306 shielded aluminum dome tweeters.  Three of the woofers and one of the tweeters will be used in the front center channel, and the other woofer and tweeter will be used in the rear center channel.  More on that later...

The idea for this project centers around the center channel used in Vance Dickason's Audax home theater system.  It is a 3-way system designed to provide better off axis dispersion.  The mid and tweeter are arranged vertically along the center of the baffle, with the woofers mirrored on either side.  This arrangement reduces the lobing problems associated with the typical "MTM on it's side" center channels, that often have poor off axis response.

As with the Ariel Clones, I used the published frequency and impedence responses supplied by Parts Express.  I traced the plots, created baffle correction curves (see Figure 1), combined them and designed the crossovers in Speaker Workshop.


Figure 1 - Baffle layout simulation

The baffle layout and enclosure dimensions are shown in Figure 2.  The mid and tweeter are isolated from the woofers in a small sealed portion of the cabinet.  Not only does this reduce the interference between the mid and woofer backwaves, it also adds some structural rigidity to the cabinet.  The mid/tweeter cavity is filled with pillow stuffing to prevent midrange reflections from being transmitted back through the cone.  The cabinet uses a 1-1/2" thick front baffle, made by laminating two layers of 3/4" MDF.  The rest of the cabinet is just 3/4" MDF.  All drivers are flush mounted, using the same template created for the Ariel Clones.  Each woofer is also in it's own enclosure, further reducing interference between drivers.  I used Parts Express's 260-402 1-1/2" flared port tube, trimmed to about 3.5".  The woofers each see about .5 cuft of volume, and the ports tune the enclosure to about 45 Hz.  This gives an f3 in the low 40's - more than sufficient for a center channel.

The crossover for this speaker uses all 2nd order electrical slopes, which provides all 4th order acoustic slopes.  This is my first attempt at a 3-way system, which was a little intimidating.  I chose to spread the crossover points as far as possible to minimize the overlap between the woofers and tweeter, so the mid wouldn't have to deal with much bandpass gain.  I was able to get a better frequency response using different components, however, the phase relationships weren't as good, which resulted in very shallow reverse phase nulls.  I opted for the better phase tracking.  Figure 2 shows the crossover layout, with the Parts Express part numbers for each component.  The 7mH and 4mH inductors are both iron core inductors.  They were chosen to keep the cost down, and to provide lower DCR.  The other inductors are 18ga (.9mH) and 20ga (.8mH) aircore.  The large 50uF and 33uF capacitors are electrolytic because using poly would have been extremely expensive.  I chose to only use poly caps on the tweeter.  The metal dome tweeters tend to sound a little harsh on their own, and I figured electrolytic caps there wouldn't help anything.  The caps in the high pass section were relatively small values anyway, so it didn't add much cost.


Figure 2 - Crossover schematic

Figure 3 shows the system frequency response with the drivers connected as shown in the schematic (mid and tweeter with normal polarity, and the woofers in reversed polarity).  I should also note that the woofers are connected in series.  Note the slight dip at the woofer/mid cross point, followed by a slight hump.  There were a few crossover combinations I tried that smoothed that out considerably, but screwed everything else up.


Figure 3 - System frequency response

Figure 4 shows the system with the mid polarity reversed.  The mid/tweeter has a nice deep null.  The woofer/mid dip is not as deep, but is nice and symetrical.  The overall system response is flat +/-2dB from 45Hz-14kHz, then there's the dip/bump above that.  In actual listening, I don't think that anomaly is actually noticeable (it's above the range of hearing for most people anyway).


Figure 4 - Reverse phase nulls

Figures 5, 6 and 7 show the low pass, band pass, and high pass responses plots, respectively.  Each is plotted agains a target response that was used for optimization.  After the optimization, the individual component values were adjusted to get a better frequency response, impedence, or phase response.


Figure 5 - Low pass response


Figure 6 - Band pass response


Figure 7 - High pass response

Figure 8 shows the system impedence response.  There are a few spots where the impedence phase jumps a bit, but they are at reasonably high impedences, so it shouldn't present a problem to an amplifier.  The mid is a 4ohm unit, thus the 5ohm minimum around 700Hz.  This is still a reasonable impedence, and the speaker should easily be considered a 6ohm nominal load.


Figure 8 - System impedence

Figure 9 shows the cabinet layout.  The enclosure is a little bit larger than "standard".  In my case, I have a rather large opening above my TV in the entertainment center, so I chose to use all the space I had.  The cabinet could be made narrower with the ports located on the rear if space is a concern.  Make sure you adjust the depth to maintain the correct internal volume.  The cabinet is constructed of 3/4" MDF all around, except the front which is 1-1/2" thick.  As mentioned before, the mid and tweeter are in a separate sealed portion of the cabinet.  The sealed partition also isolates each of the woofers, as well as provides additional internal bracing.


Figure 9 - Cabinet drawing

Impressions:  Coming soon...
 
Qty Description $ ea $ Total
3 Definitive Technology 5-1/4" shielded woofer $15.88 $47.64
1 Definitive Technology 1" shielded aluminum dome tweeter $12.56 $12.56
1 7.0mH Fe core inductor $9.08 $9.08
1 4.0mH Fe core inductor $7.58 $7.58
1 .9mH aircore inductor $4.75 $4.75
1 .8mH aircore inductor $3.74 $3.74
1 50uF electrolytic capacitor $1.00 $1.00
1 33uF electrolytic capacitor $0.85 $.85
1 10uF electrolytic capacitor $0.55 $.55
1 5.6uF electrolytic capacitor $0.50 $.50
1 1.0uF Dayton poly capacitor $1.15 $1.15
1 .68uF Dayton poly capacitor $0.85 $.85
1 16 ohm non-inductive resistor $1.50 $1.50
1 1 ohm 10W resistor $.39 $.39
Total
$92.14