The idea for this speaker was birthed at the 2012 Midwest Audiofest from a conversation
with Russell Kauffman, then Technical Director at Morel Acoustics. He was showing us
some new Morel drivers, using titanium voice coil formers, and I became very interested
in the design of one of the woofers and made the comment that I would be interested in
trying that one in a speaker design sometime. Russell thus challenged me to do so, and
the Solstice is that design.
The Solstice is a small tower design implementing two woofers and a dome tweeter.
I chose an MMT configuration in order to place the tweeter closer to ear height and
decided to just let the dual woofers work together in their operating range. To improve
overall bass response the tower is a Mass-Loaded Transmission Line (MLTL) design with
a slot port at the bottom of the back of the cabinet.
The woofer that had intrigued me is the Morel TiCW 638Nd. This is a beautiful 6" polymer
cone woofer with a “hybrid” Ferrite/Neo magnet inside a 3" voice coil on a titanium former
and a full copper sleeve in the motor to reduce inductance. This is truly a state-of-the-art
motor design with very low distortion. I used a pair of these woofers operating in parallel to
increase sensitivity and bass SPL.
The tweeter I selected is the Morel CAT308 silk dome tweeter, from the Classic line of
Morel drivers. Although Morel offers tweeters in their Elite and Supreme lines as well,
measurements of the CAT308 revealed it to be a tweeter with very linear response and low
distortion, and since I have used it before and was pleased with it, I decided to stick with it
in the Solstice.
The cabinet design is as special as the drivers are. I wanted a small tower where the tweeter could be set at ear level with the woofers just
below the tweeter. This would determine the height (38") and the woofer diameter would determine the cabinet width (8.75"), being a bit larger
than the woofer frame itself. The depth (10.5") was then determined based on achieving the correct internal volume for the woofers. This
gave me nearly optimum dimensions for an MLTL (Mass-Loaded Transmission Line) design. I drew up some sketches and contacted Erich
at Denovo Audio. He had some novel ideas and designed an excellent cabinet kit. The flat-pack enclosure is precut and assembles easily
into a very well-made and very strong cabinet, designed to accomplish my MLTL woofer loading. The bass loading was fined-tuned using a
combination of impedance and near-field port and cone measurements to dial-in the optimum port dimensions.
The Solstice is a basic two-way design, with both woofers working together in parallel over
the same frequency range. To avoid issues, the crossover point to the tweeter is at a low
1.7 kHz. The crossover is a symmetrical 4th Order Linkwitz Riley acoustical design with all
drivers in phase at the crossover point and with excellent phase tracking over a wide band.
This crossover is achieved with an electrical third order network on the woofers implementing
the necessary response shaping and baffle step compensation, and a second order
network on the tweeter with resistor padding.
One of the things that surprised me during
the design stage was that due to the large
voice coil and the shallow cone on the
woofer, the Z-axis offset between the
woofer and tweeter was very close to
0 mm, meaning that these drivers are
naturally time aligned on the flat baffle
when recessed flush. Time aligned
systems are usually difficult to design,
but Morel’s driver geometries make this
easy to achieve. Because of this the
Solstice is a time aligned loudspeaker.
Measured Acoustic Crossover
A Parts Express / Morel Exclusive
by Jeff Bagby