MOB3W - my open baffle three way

 

Prototype

This system is inspired by Siegfried Linkwitz. After his discovery of the benefits of a dipole tweeter setup I just had to try this too. I can fully confirm all positive findings as reported by Linkwitz. Especially his page on Room optimized stereophonic sound reproduction is highly recommended reading material.

This prototype has been based on the dipole bass system I developed for my ultimate monitor. I tried several driver combinations and found that symmetrical radiation is very important. This is difficult to achieve with two dome tweeters back-to-back, the back tweeter acoustical centre cannot be aligned with the midwoofer other than with strange baffle design not pleasing the eye. Therefore I changed the dome tweeters for a true dipole tweeter, the B&G NEO3PDR. The midwoofer is the Visaton AL130, this driver performs even better than the Peerless HDS152excl. of my ultimate monitor.

I have had the opportunity to compare this prototype 1-on-1 with the Orion from Linkwitz. The owner was very friendly and invited me and my system to his home. He let me listen for hours to both systems. The Orion appeared the most beautiful sounding loudspeaker I ever heard!

So where did I stand with my prototype system? Well, it had even some more precision in the 3D-picture, purer and clearer highs while not being sharper. The big BUT of my system was less warmth, natural warmth. This is difficult to describe, you have to hear the Orion to know how natural a loudspeaker can sound. With this auditory reference in my head I started the MOB3W project.

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Drivers

Peerless SLS 12" Subwoofer Type Number: 830669

Visaton High End Midwoofer AL130

Mundorf Air Motion Transformer AMT2340

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Baffle design and building

The base for the woofers is build out of 12mm thick multiplex panels. They are 35.2cm wide and 58.9cm high (excluding the angled part).

Between each pair of multiplex layers there is a layer of 3.2mm density fiber board. Since there are 6 multiplex panels, there are 5 density board layers. The total depth of this base is 6x12mm+5x3.2mm=88mm=8.8cm. The most upfront 12mm multiplex panel has 28cm diameter openings for the woofers. The centre of the lower woofer is 16.7cm from the bottom, the centre of the upper woofer is 48.9cm from the bottom. With each layer of multiplex and hardboard, the diameter of the woofer openings increases with 3mm. So the last multiplex panel on the backside got woofer openings of 29.5cm diameter.

The woofers base is fixed to a bottom panel of 35.2cmx33.8cm. It is 15mm thick.

Here the base panel of 8mm thick for the AL130 and AMT2340 is shown. It is 35.2cm wide and 97.9cm high (not 98.9cm because it will be covered with a piece of 1cm).

This part is really tricky. Material has been cut out so that the AMT2340 fits exactly. Behind the AMT2340, the MDF is only 1mm thick!

The back of the woofers base is covered with 6mm MDF.

The top sides are also covered with 6mm MDF.

The upper part of this 6mm MDF cover is 12mm in depth. Therefore it is sticking out 4mm at the front.

The front side of the cabinet is covered with 3.2mm density fiber board.

The bottom panel is covered with 6mm MDF.

The top is finished with a piece of MDF. Heigth is 10mm, depth is 12mm.

In this picture, the top is finished.

The crossover cabinet is made out of 15mm multiplex. The pieces are 4.2cm high.

Front and back of the crossover cabinet is covered with 6mm MDF.

The sides of the woofers base are covered with 6mm MDF. This gives the total width of 36.4cm to the speaker cabinet. The crossover cabinet is at its final stage and is 7.1cm high. This leaves room for a cover plate of 4mm or 6mm MDF, depending on the thickness of felt between the cover plate and the cabinet (4mm and 2mm respectively). A cover plate of 4mm thickness gives room to crossover parts that are 4.5cm thick or high.

The front cover panel is made out of 8mm MDF. The cut outs are 11cm for the AL130 and 28cm for the woofers. The distance of the woofer cut outs is 4.2cm to the sides and between each other. So the centre of the lower woofer is at a heigth of 18.2cm and the centre of the upper woofer is at a heigth of 50.4cm.

The back of this panel is routed to fit the drivers.

The cover plate and an 8 pole NEUTRIK speakON chassis connector.

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Crossover

Passive crossover for midwoofer and tweeter

Several values for the L-pad before the tweeter. I myself use the loudest option, i.e. only a serial resistor of 3,3 ohms. I provide only options for weaker highs with respect to my own situation. It is my experience that other listening rooms support the higher frequencies more than mine.

Active filter to cross between mid/highs and the two woofers. Normally the woofers will be wired parallel and connected to a power amplifier able to deliver a minimum of 120Watts to 4ohms. The woofers have to be connected in reversed phase, because there is an inverting stage in the active crossover. The amplifiers for the woofers and mid/highs must be identical (identical gain). The level of the woofers can be set to your own taste (+1.6dB/-2.5dB). Furthermore, by means of the other potentiometer, the crossover frequency can be adapted from 28.4Hz (maximum value) to 72.2Hz (minimum value). This gives you the opportunity to adapt the bass level below 72.2Hz to your listening room.

Combined curves of passive and active filter

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Measurements

Response of midwoofer and tweeter with only the passive crossover in place. The tweeter is playing at the loudest option.

Response of the whole system, note how the active crossover equalizes the response of the midwoofer and tweeter. Below 400Hz the system plays louder, this is the result of having the loudspeaker on the floor and is necessary to get the optimal tonal balance. The system sounds very very realistic and all what I searched for, having auditioned the Orion, I have found with this system.

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Nikos from Greece made his OB3W from plexiglas

I provided the passive crossover and a custom made active crossover with balanced inputs and outputs for Nikos

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