Toccata Grande, Part 1

By Lars Mytting
20 Jan 1998

A fistful of drivers

But I guess the previous doctrine would make it strange of me to choose a single 4.5" for this project. The heart of the Toccata speaker is a 10" driver from Seas, whose specifications more or less dictated the rest of the system. Since the goal is to achieve something more than just a slight improvement in sensitivity, I decided to try using two of these woofers.

The star of this show is called Seas CA25FEY/DD, and the drivers have (after regular standards) a giant magnet system. The paper cone is only 26 g, very light for a 10". This gives a average sensitivity of 95 dB for 1 watt input, and the lightweight cone promises good for the lower midrange quality. It is possible to cross the driver in the region of 300-500 Hz without muddling the midrange too much.

But: The big magnet and the light cone gives the driver a low Qts of 0.27, and the other T/S parameters makes it difficult to get deep bass response in both closed and vented boxes. In a standard regular bass reflex (QB3), the -3 dB point is around 50 Hz. This is not too satisfying, since the resonance frequency of the driver is a fair 31 Hz.

The solution I wanted to try was a transmission line enclosure. The low end response of a t-line depends on the resonance frequency of the driver, and the parameters that causes the high rolloff in bass reflex cabinets (Vas and Qts) are less important in a t-line [3]. T-lines also have very flat impedance in the bass area, making a less demanding load for the amplifier -- another nice match with our goal. Neither had I built a t-line before, adding both risk and scientific value to the project.

Seas was so kind to supply me with drivers for the project, and the other drivers are also from their line. For midrange, I chose Seas MT14RCY (H804). This is a 5" midrange driver with TPX cone (a clear plastic material). Its average sensitivity is 89 dB. TPX drivers have a very "free, open and airy" sound quality, if you accept such a unscientific description of low energy storage. A very similar TPX driver is also used with great success in the famous ProAc Response 1S speaker.

For tweeter; I went for the Seas T25. This little sweetheart is full of goodies. Double magnet, silver wire in the voice coil, dome of Seas' best fabric, heavy frame with 6 mm frontplate. Earlier tests have proven it to sound very relaxed and calm while yielding great detail -- again something that makes it a good player on the Toccata Team.

As I should use two midrange drivers, I decided to place them in a MTM-configuration (the tweeter between the midranges). This is sometimes called a d'Appolito configuration, but to achieve a "true d'Appolito" requires a very low crossover frequency. Choosing MTM was a slightly random decision. With a "normal" crossover frequency, you can get a somewhat improved vertical dispersion, but you can also end up with a overall poor radiation pattern. MTM is very popular nowadays, but remember that it is not necessary to use MTM just because you have two midranges! You may be just as good or better served with the mids together. (I guess this would be called MMT or TMM). Many great speakers, like B&W 800, have the drivers placed in MMT.

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