The Andromeda, or building a reference speaker
By Tony Gee
02 Apr 2000
A fresh start
I had moved to a new house and had sold my previous seven pairs of loudspeakers before moving. My new living room was big enough for a large loudspeaker system and I wanted to build something that was the best I had built yet; so no compromises.Fig. 1. The Andromeda
What did I want?
The bass: I believe that you can only produce "real" bass by moving a large area of air gently and not by moving a small area of air violently. This means that anything smaller than a 10-inch woofer would simply not do. I wanted to play it safe so I went for a 13-inch woofer. The midrange: This was the most difficult part, there are so many possibilities: 4-inch, 5-inch, 6-inch or 7-inch? Split it into lower-midrange and upper-midrange? Use cone or dome units? Crossover at 2kHz or anywhere up to 6kHz? In the end I decided to go for the simplest solution (is there a simplest solution?) and look at it as a 2-way system with a sub-woofer added. That meant I needed a 6 or 7-inch unit with a smooth response and a large bandwidth. The treble: I like the treble to sound clear and detailed without getting harsh, so no cheap aluminium dome tweeters. I was going to use a 6 or 7-inch unit for the mid, so the tweeter should be able to go low enough; that meant a 25 to 30mm dome. Good dispersion up into the high treble is also important for a nice clear and wide stereo-image. What did I end-up with? Fig. 2. The drivers
The tweeter.
In the end I chose the Focal TC120Tdx2, an inverse dome tweeter with a titaniumdioxide coated titanium membrane and a cone-shaped diffuser mounted on the front. This diffuser gives very wide dispersion right up to 20kHz - 30 degrees of axis at 20kHz is only down 4,5dBs. It has a double magnet to create a very strong magnetic field in the air-gap and it is also shielded. The cavity inside the magnets is filled with a mineral wool to dampen resonances.
The mid-woofer.
Nothing but the best here: Scanspeaks 18W8545. The perfect 18cm mid-woofer! It has a low resonance frequency of 28Hz, a large long throw 42x19millimeter voice-coil and a very large magnet to give a strong driving force. The cone is made of carbon fibre filled paper and heavily coated. All this should result in a nice dry, warm and detailed sound. I also considered the 18W8545K, basically the same speaker but with a kapton voice-coil former. Its bass was more detailed and seemed a little stronger, but I preferred the mid-range of the standard 18W8545 in being clearer.
The woofer.
For a long time I had been wanting to use one and now I had the financial means: A woofer from the Focal Audiom range. A 15-inch one would be a little too big as it needs about 200 to 250 litres to work in; the Audiom 13KX only needs about 90 litres! I chose the Audiom 13KX because it has a rubber surround, as apposed to the foam surround of the Audiom 13VX. Efficiency is high enough at 91dBs and fs low enough at 28,8Hz. The magnet is not what you see every day: It is made up of twelve 72mm standard magnets painted bright red mounted between two chrome-plated pole-plates with a total diameter of 232mm! The cone is made of so-called Poly-Kevlar: two layers of finely woven kevlar sandwiching a layer of polystyrene. Fig. 3. A pair of Focal Audiom 13KX
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