I’ve started a new build project called “Prestigious Two – Monitor DXT” and currently I’m in the fine tuning, listening test and final measurement project stage. This time I’ve used less expensive driver units from the “Seas Prestige line”. The mid-woofer used is the U16RCY/P, which has a woven polypropylene cone and phase plug.
The mid-woofer has a smooth extended frequency response with a controlled roll-off, without nasty cone break-ups. For its size it has a large radiating surface (99cm²) and the bass performance is very good.
A variant of this mid-woofer is used in the Sonus Faber Liuto loudspeaker series as well as in the Abrahamsen FS401 loudspeaker.
The tweeter used is the well-known and among DIY:ers popular 27TBCD/GB-DXT tweeter, with its unique DXT lens and great off-axis frequency dispersion.
For further details and measurements on these driver units see:
SEAS H1499 27TBCD/GB-DXT
SEAS H1520 U16RCY/P
I’ve used both these drivers in active as well as passive loudspeaker designs before, but I haven’t been fully satisfied with the end result and I have always thought they could perform even better and now I think I’ve succeeded!
The DXT tweeter isn’t the easiest tweeter to work with. Don’t misunderstand me, it’s not that hard to shape it in to a working cross-over that on paper looks good, but in my opinion this tweeter has a tendency to become dull, un-engaging and less musical in certain configurations.
I found this tweeter to work and sound the best in a second-order LR filter topology (acoustically). This means that in order to use a simple electrically first-order filter it needs to be crossed-over a bit higher up in frequency to ensure that it operates within its “comfort zone”. In this case I’ve targeted a 3kHz cross-over point for the design.
On the other hand, this requires a mid-woofer capable of a 3kHz cross-over point without sacrificing any off-axis dispersion (beaming) and to match the DXT tweeters excellent and controlled off-axis frequency characteristics. The Seas U16RCY/P fulfills that together with a very smooth and controlled frequency slope roll-off, without the need to do corrections for nasty cone break-ups.
To sum up, this all ended up in a very simple cross-over design, which was one of the goals of this loudspeaker design.
The mid-woofer cross-over filter section is an electrical first-order and is very simple and consists only of a large coil (L1) that shapes the cross-over slopes to a LR2 roll-off with a targeted 3kHz cross-over point. The inductor (L1) also tunes the “Baffle Step Compensation” (BSC).
The tweeter cross-over filter section consists of a single tweeter padding resistor (R1) and a first-order electrical filter (C1) that shapes the cross-over slope to a LR2 roll-off with a targeted 3kHz cross-over point. The value of (R1) can be changed to tailor the tweeter level to personal preferences.
The tweeter is connected with reverse polarity and (C2+R2+R3) shape the tweeters frequency response and flatten the response at higher frequencies.
Simulated 15deg off-axis frequency response with a targeted second-order LR topology (acoustical). Cross-over point at 3kHz. Note the tweeters sharp peak at the hard dome break-up frequency!
More info coming soon….
Regards
/Göran