The Sub Woofer


Front and back side view of the finished bass cabinet. Notice the Amplifier and control panel in the right picture.


The lower frequencies are reproduced by an active sub-woofer that is build with four 25 cm Kevlar woofers, each of which has its own amplifier. 
Each of the speakers has an acceleration measurement device that provides motional feedback correction. 

The complete amplifier - feedback assembly together with the speakers units was provided by Backes & Müller

Detail showing the amplifier module. The double 300 VA transformers are followed by two boards totaling
80.000uF capacitors and also containing the switch on/off circuitry. The first board contains an extra
transformer (the blue square in the front above the flat cable connector) for the standby modus. 
Then follows the  board containing the filters. Control of the filters and extra phase circuitry are on a separate panel.
The last two boards each contain 2 complete amplifiers capable of delivering 180 Watt RMS/4Ohm.
Also including the motional feedback circuitry.


On the left you see the woofer, Each bass cabinet contains four of these.
On the right you can see, once the dust cab is removed, the induction accelerator sensor in the center of the voice coil.
It is etched on a piece of fiberboard and surrounded by its own set of magnets.

The Woofers are placed opposite to each other in order to minimize forces exercised by the cone movements. 
Since all woofers move out or in together the forces they exercise on the cabinet are annihilated. 

The woofer cabinets are made of 13 mm (HPL high pressure laminate) phenolic resin (the same material as used in the Manger top cabinet) glued directly to 22 mm MDF. 
This was done under high pressure using a panel press capable of delivering 60 tons of pressure. 
The complete panels where first assembled to a sandwich.

Then they where cut on a numerically controlled panel saw.  

The cabinet was reinforced by multiple braces and then extra damping was applied to the panels using Hawaphon panels. 
These are filled with small steal balls which absorb the vibration energy. 
Such panels are also used in the Sculpture and the Elance from the Aachner firm Audiodata.
or Danish Physics. These sandwich is then further damped by a layer of needle felt. 

Here you can see the stack of panels which for the woofer cabinets. 
You can clearly see the sandwich construction.

And here are the cabinets ready to be painted.

Here you have the near field response and distorion from the Backes & Müller in its cabinet.
It was measured in the living room



This is schematic of the Backes & Müller cabinet.