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Friday 17 March 2017

Today students studying Audio Systems were treated to a visit from a guest speaker. The Extron SI 26CT ceiling speaker dropped by to show off its coaxial dual driver design and switchable transformer for line distribution applications. The 6.5 inch woofer is situated directly behind a 1 inch titanium-coated dome tweeter in an attempt to provide accurate sound reproduction over a wide angle.

The unit houses a passive crossover to protect the tweeter from higher-power low frequencies whilst allowing its light weight diaphragm to concentrate on the fast turnaround required by higher frequencies. The woofer explains: “I can’t keep up with the rapid movements of HF and simply end up distorting the sound; I’m far more comfortable taking the power of LF and generating the longer wavelengths needed for bass”.


Key to SI 26CT’s visit was the concept of 100 V line distribution systems. A specialist amplifier is used to create high voltage, but low current signals to reduce losses over long cable runs. This is useful in applications such as shopping centres and airports where numerous loudspeakers are spread over a large area resulting in enormous cable runs. Each loudspeaker has its own step-down transformer to reduce the voltage and increase the current so that classic drivers can be used. The transformer reflects a high impedance to the amplifier, hence the current is reduced, but the same power is delivered to the load.

Furthermore the SI 26CT features multiple taps on the secondary of the transformer which allows it to produce between 7.5 W and 60 W from the same amplifier. This means that the same amplifier and cable run can be used to supply louder and quieter loudspeakers depending on the throw distance and volume of the space. SI 26CT has found application in Europe at 100 V and in America where they tend to use 70 V systems.

There are alternative systems available, for example active loudspeakers could be used with a separate amplifier combined with each driver. This would allow more control over the output level from each driver but means every loudspeaker unit requires a power supply, which is expensive.

It was great to see an alternative style of audio installation and I’m sure we’ll see SI 26CT and his peers soon in classrooms and shopping centres around the local area.