Copyright 2009 Troels Gravesen
Ole L. came by with these cabs including a pair of D2904/9800 alu domes and 18W/8531-G00 midbass drivers. He'd bought the cabs for 125 EUR (the piano lacquer alone had been four times as much).
With a tilted and chamfered upper front it was time to revise the old SP98 crossover and who knows what has happened since then in terms of crossover design and overall voicing.

The total volume of the cabs are close to 43 liter making the 18W produce an F3 of almost 30 Hz! Hearing the 18W in a too large cabinet, I couldn't help thinking of the unfortunate 2.5 clone. It goes immensely deep and as long as we do not crank up volume, everything goes well. Playing a bit louder the bass becomes boomy and un-precise. Should someone still be thinking of building the 2.5 clone, take a look at this construction and build something significantly better. Yes, it's a bit more expensive, but I think it's time to bury the clone and get on with our lives. This speaker is at least twice as efficient, goes just as deep and has better midrange and treble. Overall it's a significant improvement.
Fortunately Ole also brought two 10 liter sand bags to be placed at the bottom of the cabs bringing the effective volume down to 33 liter - that's more like it.

Cabinet dimensions

Click images to view large. Cab dimensions left and cab volume calc right.

Cabinet damping: Use 10 mm felt or polyester material on all internal walls. Next you need two rolls of MDM3 or similar. Half the roll is folded and placed at the bottom of the cabinet. The other half is folded and placed behind the 18W driver and towards the top.

Download drawing on bracing here.
Download drawing on damping

The Crossover

Basically an 18/18 dB filter with a notch filter smoothing the ever so important upper midrange.

Crossover Components

Crossover kit excl. drivers available from Jantzen Audio: contact@jantzen-audio.com
Damping materials also available (felt/bitumen pads and wool/acoustilux/dacron)


Left: Left and right speaker. Quite a matched pair! System sensitivity is around 86 dB/2.8 volts.
Right: Impedance of driver without crossover attached.

Left: Impedance of final system. Right. Reversing tweeter polarity creates a dip around point of crossover, here being 3 kHz.

The best alu dome I know of!