Morel MDT 33
A soft-dome tweeter with a sealed metal back cup, a 500 Hz resonance, and a reputation for speed that kept it in DIY designs for over a decade.
Overview
The Morel MDT 33 is a 28 mm soft-dome tweeter with a double-magnet structure and Morel's Hexatech voice-coil winding - aluminum wire wound in a hexagonal cross-section instead of the usual round. Morel's own advertising billed the MDT 33 as a new design in the Duet two-way, and the driver was also sold separately to DIY builders through distributors like Madisound. Published sensitivity is 92.5 dB at 1W/1m, with an 8 Ω nominal load. Resonance sits at 500 Hz, well below the 4 kHz crossover point most builders used, so the dome can take a considerable signal input well below its crossover without complaint. Morel calls it an "extremely fast" tweeter, with a rise time of 10 µs.
A rise-time figure like that sounds like marketing until you see what it implies. A dome that settles in 10 microseconds tracks transient information well past the top of the audible band. In a first-order crossover system the tweeter has to carry substantial energy an octave or more below the crossover point, and there the speed matters more than raw sensitivity. The MDT 33's frequency response curves show it essentially flat from 1 kHz to 15 kHz at 45° off-axis, and flat to 20 kHz on axis. The 45° number is the one that separates a tweeter you can live with from one you can't. A dome that stays flat that far off-axis holds its tonal character as you move around the room instead of collapsing into a beam.
The mechanical design is where the MDT 33 earns its keep in real builds. The rear of the driver is totally enclosed by a metal cup sealed to the body, so the tweeter is acoustically isolated from whatever else shares its enclosure. That is not a trivial detail. In a three-way system where a midrange driver sits in the same box with an open back, the tweeter's rear radiation would otherwise be modulated by the midrange's back wave. In a shared mid/high enclosure, the sealed cup means the box volume is determined only by the midrange. One builder working from published data alone noted that this isolation made the driver convenient for exactly that kind of shared-enclosure arrangement, and that the MDT 33's enclosure volume is simply the cup itself, so there is no choice to make.
The published performance was strong enough that at least one designer selected the driver purely on paper, having never seen or heard one before committing it to a design. That designer paired the MDT 33 with a JBL 2235H woofer crossing at 400 Hz and a Focal 5K013-L midrange crossing at 4 kHz, with first-order networks throughout. The logic was that the MDT 33's flat response and phase behavior could support the substantial overlap a first-order crossover demands. The tweeter would be handling real signal well below 4 kHz, and the 500 Hz resonance gave it the headroom to do so without distress. Boxmodel, the enclosure-modeling software of the day, could not calculate the acoustic performance of the MDT 33, so the design relied on the published curves.
Morel MDT 33 tweeters also turned up in the builder's own reference system, crossed at 4000 Hz alongside a Morel/MSD 56 midrange, an Audax 2025 TSN lower midrange, and an Isobarik subwoofer. That is the pattern with this driver. It was a component for people who designed their own systems, not a finished product with a model number on a cabinet. It appeared in the Duet two-way as the MDT 33 tweeter, and it was sold loose through Madisound, which was an official Morel distributor and offered technical help to builders. The Audio-Express (November 2001, p.22) listing described the flat off-axis response and the sealed rear cup in the terms a DIY builder would care about - the cup as a construction convenience, the off-axis response as a crossover-design constraint.
The MDT 33 has no documented production run, no original list price, and no body of owner reports about failure modes. The driver shows up in period advertising and in DIY project write-ups, but those sources describe it as a component to be specified, not a product to be reviewed on its own. HiFi-Stereo (February 1988, p.158) listed the MDT 33 as the tweeter in the Morel Acoustics Duet, a two-way double-magnet speaker rated 28-18,500 Hz ±3 dB. That is the Duet's spec sheet, not the driver's. The driver's own numbers come from Morel's published data and from the builders who used them.
Anyone hunting one today should know the MDT 33 is a component, and components get separated from their systems. A pair pulled from a parted-out Duet or a dismantled DIY project is the usual way one surfaces. Builders who used the MDT 33 tended to use it well, with first-order crossovers, careful driver spacing and beveled baffles to control diffraction. The surviving examples were often installed in competent designs rather than thrown into whatever box was handy.
The MDT 33 sits in the Morel line as a reference-grade soft dome from the era when Morel was primarily a supplier to the DIY market through Madisound and similar distributors. It was not a budget part, and it was not a mass-market one. It was the tweeter you specified when you wanted a dome that could keep up with a first-order crossover and a midrange that didn't fight it.
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