CROSSOVER     CABINET     MEASUREMENTS     T29 TWEETERS         

During crossover construction for CNO-T25 (T25C003 tweeter), I decided to take a fresh look at the CNO crossover, thus an mkII version, providing an overall more balanced presentation compared to the "up-front" tuning of the first version. From the feedback on first version, some people like this tuning, despite its elevated upper mid presentation, which may be very engaging, but leaves a few recordings on the shelf. The version shown here will make a flat midrange presentation and both T29CF001 and T29CF002 (Crescendo) tweeters can be used with minor changes to tweeter attenuation.

The T29CF002 Crescendo tweeter is one of the most expensive dome tweeters available. Hence the name, "cost-no-object", for this modest 24 liter floorstander.

These new SEAS drivers have classic paper cones, coated on the rear and applied a rough Nextel coating on the front. Further specifications at the SEAS website http://www.seas.no/. The finish of the drivers is second to none. SEAS is also a true follower of fashion and the magnets are covered with soft rubber boots.

Having a pair of suitable test cabs I only needed to make some new front panels for the drivers. My test cabs have a max. volume of 31 litres and this made some deep bass from the W18, although not too well defined. View cab simulation below. Closing off the lower sections of the cabs, I have some 24 litres left, and this is better. Pure math dictates 17 litres, QB3 alignment, but the W18NX001s can easily handle 24 litres without loosing control and we get better low-end extension. The cab width is a modest 200 mm. View details below.

Before going into details, a little on the sound. To my ears the sound from these nextel coated drivers fall between the SEAS Prestige CA and EXCEL magnesium drivers. This may suggest sound quality inferior to the magnesium cones. It isn't. It's as good as the magnesium cones, only different due to improved responsiveness. I don't think they beat the magnesium cones in terms of neutrality, but these Nextel coated paper cones provide a very neutral, yet vivid sound. The best of both worlds? Well, only your ears can tell.
The Nextel drivers were not only made to look good, but also provide a sound clearly distinguishable from the average. High-end drivers built on classic virtues. As always we apply to the law of diminishing returns when we buy expensive stuff. Small improvements may cost small fortunes. I think these nextel units are the best sounding drivers SEAS has ever built.

The crossover
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A lot of simulation work went ahead of actual set-up and fine-tuning, and both parallel and series crossovers were tried. The crossover shown below is a series crossover and was finally chosen for presentation. Due to drivers' intrinsic performance and baffle geometry, we have a bump in the 700-1000 Hz range and an LCR circuit gently equalises the response here.
For newcomers to series crossovers: These cannot be bi-wired. Read about series crossovers here: http://sound.westhost.com/parallel-series.htm#3.1

CNO series crossover.
For T29CF001: R1061 = 5R6-6R8 and R1051 = 5.6 ohms.
R1061: Try it out before deciding!

Suggested crossover layout for T29CF002 tweeter.
Note: The orientation of components in the LCR circuit doesn't matter.
It can be LCR, CRL, RCL, etc. Doesn't matter (I have this question all the time).

Crossover Components

Crossover components from Jantzen Audio.
Kit will contain resitors for both options. Specify tweeter on order.
e-mail: contact@jantzen-audio.com

The Box
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SEAS TS data

LspCAD box simulation. Port has to be shorter than calculated here, multiply by 0.7

Bass performance from 24 litres vented cab, port tuning 40 Hz.
PORTS for Fb = 42 Hz (just to pick something..):
55 mm (ID) x 90 mm (L)
60 mm (ID) x 110 mm (L)
65 mm (ID) x 130 mm (L)
70 mm (ID) x 155 mm (L)
75 mm (ID) x 180 mm (L)

Cabs should be tilted 5 degrees to render the flattest frequency response.

Cabinet construction

Same cabinet as CNO-T25. Click image to download large drawing.

Test cabs with painted front panel for the benefit of the photo shoot.

Measurements
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Left: SPL response from drivers without crossover from test cabinet.
Right: Impedance of drivers mounted in cabinet.

Left: Summed response of drivers merged at 350 Hz with bass nearfield response.
Right: Point of crossover is 3100Hz. Blue = reverse polarity of tweeter. Reading taken at 1 meter distance, tweeter height.

Left: Impedance of final system. Minimum impedance = 6 ohms.
Right: This speaker has excellent vertical dispersion, here measured at
1 meter distance with microphone from just above tweeter to just below woofer.

Left: CSD, 20 dB. Right: CSD, 40 dB

SEAS EXCEL T29-tweeters
T29CF001 & T29CF002 Crescendo
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SEAS T29CF002 and -001 tweeters have a different magnet systems. Read data files at SEAS website. The -001 has a round neodymium magnet and slightly higher, un-symmetrical impedance profile, probably due to the damping system used. The -002 looks better, but that's about it from my point of view.

Left: T29CF002 to the left and T29CF001 to the right, showing differences in magnet construction. The Crescendo - 002 - has a small damping pad under the dome on top of the foam plug placed in the centre polepiece hole. Have we seen this before? The -001 hasn't, thus some irregularities at 18 kHz.
Right: The Crescendo magnet system made from 6 neodymium magnets providing excellent ventilation for the voice coil. This -001 has not, rather a round neo magnet making a non-vented cavity behind the voice coil. So, you are getting something for the extra money. But this is about it.

Right: -001 added a felt pad similar to the -002. The self-adhesive felt pad is 5 x 15 mm and sticks to the foam plug leaving just ½ mm opening to the foam. I rounded the felt pad slightly as can be seen from photo. So, does it make a difference? Frequency response graphs below. Red = + felt pad. Blue = as-is.

Builders' response to CNO