As previously described in my last post, the cabinet was designed as small as possible to match a MIDI system and big enough to allow enough bass.
The final dimensions were set as 200x200x150mm, giving us a 3.9 litres internal volume. The cabinets were provided as a mirror-image set.
This is a bass-reflex arrangement with a 2.5cm diam, 4cm long port generating a -6dB point at 70 Hz.
Because the voice coil is longer than the standard version, one can adjust the bass control on the preamplifier (sacrilege...) and extend the bass output down to 50 Hz without distortion at normal listening level in a small room - which was our target market to start with.
We will discuss in a later post how we increased the bass output with the addition of subwoofer(s)
We had been looking at various materials to build the cabinet, and apart from the obvious MDF, we wanted something that, once varnished, would show the material itself, so we settled on a very dense marine birch plywood, 15mm thick.
We shopped around for a cabinet maker and Philippe Lesage recommended we talked to Jean-Paul Guy of Guy HF in Bourbon-Lancy, a small town in the centre of France, not far from St-Etienne, famous for its manufacture of weapons, and for us audiophiles, where Focal-JMLab and Jacques Mahul operate.
Philippe had said to me that Jean-Paul was rarely taking new clients on, but that my design might interest him.
It is quite a trek from Paris to Bourbon-Lancy, but after a few phone conversations, I decided to go with a pair of prototypes made in Paris.
Jean-Paul asked many questions, listened to our product, and started suggesting a few modifications/upgrades and manufacturing tips: We were in!
At the time, Focal had started 4 years before and was beginning to be a serious client for Guy HF, so much so that it ended up taking 49% of the shares in 2003 and bought the company over when Jean-Paul decide to retire. He is still a technical consultant and the factory is making the top of the range from Focal-JMLab today, as in the Utopia and avatars.
One thing that Jean-Paul was an expert at, among other things, was the varnish: we never found anybody that could do a varnish that was not shiny, but smooth as an anodised aluminium finish. That gave our speakers an elegance that nobody else could match at any price.
We chose a Linkwitz-Riley configuration because this 12 dB per octave crossover is designed to solve the problem of centering the main lobe of the forward radiation pattern of a two-way speaker system.
This crossover is unusual in that each filter is down 6 dB at crossover (in this case around 5400hz, to avoid the resonance of the tweeter and take advantage of the full range of the bass/midrange driver) and that the two drivers are actually "in phase" at all frequencies when the drivers are wired in opposite polarity. That is, even though the filters have their own characteristic phase responses the phase difference between the two output signals is the same at all frequencies. As a result, each filter section has the same group delay.
This crossover is recommended over the 2nd order Butterworth type due to its accurate summed frequency response and forward pointing main lobe.
We also aligned the two drivers physically to adjust the time delay between the tweeter and bass/midrange driver.
This also gave the cabinet its typical look, showing the structure of the plywood on the edge of the "pyramid" supporting the main driver, a very ancient Egyptian concept. This eventually translated into the name and the logo of the company.
Here is a schematic of the filter and a photo of its implementation: