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The Andromeda, or building a reference speaker
By Tony Gee
02 Apr 2000
Mixing it all together
When speakers get this big I feel they blend into the interior better if they have a real wood finish like a piece of furniture. I chose to cover both cabinets with cherry veneer and 6 layers of clear durable floor varnish, sanded between each layer. Fig. 9. The top enclosure sanded
Time alignment.
Time alignment is one of those things that until recently was rather under-estimated. But it is all so logical (as Spock would say).
If two drivers placed at different positions vertically are producing the same tone (what happens around the crossover point) but one is further away from the listener than the other, they will interfere. The tone from one speaker will reach your ear before the other one does. In a standard flat baffle the horizontal offset between a dome tweeter and say a 7-inch woofer can be 2 to 3 centimetres. Of course it is all-dependent on how high the units are placed in comparison to the listening height, but the principle stays the same. In this case the baffle of the top cabinet slants backwards at 5 degrees to time align the tweeter with the mid-woofer. The whole top cabinet is set back 106mm to time-align the mid-woofer with the woofer, this seems a lot but the depth of the Focal 13KX cone is 66mm. Fig. 10. The aligned cabinets
The crossover network.
Once you have got these very nice drivers in a very nice box, how do you blend them together electrically? The standard way would be to design a parallel-network and then tweak it until it sounded the most satisfactory, but a parallel filter has one very big problem: Most of the time the signal going in is not evenly divided across the drivers. There will be electrical overlaps or "underlaps" between drivers that will cause unwanted phase shifts even if the acoustic amplitude is flat. The only way to get around this problem is to use a series-filter. The electrical signal sent in is evenly divided across the drivers because the drivers and filter are wired together in series as one total system. The pass- function of the components is an exact mirror image of the stop-function. The total voltage measured across the drivers is identical to the voltage measured across the amplifier outputs (minus the losses in cabling etc of course). For example if you take a basic first-order series-filter consisting of a capacitor, an inductor, a woofer and a tweeter and you shift the cross-over point of the tweeter upwards by choosing a smaller value capacitor the cross-over point of the woofer will go up with it as the capacitor runs parallel to the woofer.
As I designed my system as a 2-way system with a sub-woofer added, I combined both types of filter: The Scanspeak 18W8545 and the Focal 120Tdx2 are wired together using a second-order series-filter and the Focal Audiom 13KX "sub-woofer" is connected parallel using a first-order network. The Focal Audiom 13KX has two extra parallel compensation networks consisting of a LC-network to cut out the cone break-up peak at 2kHz and a RC-network to compensate for the rising impedance due to the voice-coil inductance. The Scanspeak 18W8545 also has a RC-network to compensate for the rising impedance. Fig. 11. The filter under construction
My personal findings of the series-filter is that in most cases a series-filter sounds more homogeneous, more open and more natural than a parallel-filter.
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