CNO mkII (cost
Copyright 2009 © Troels Gravesen
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.
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
Cabs should be tilted 5 degrees to render the flattest frequency response.
Left: SPL response from
drivers without crossover from test cabinet.
Left: Summed response of
drivers merged at 350 Hz with bass nearfield response.
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: -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.