Friday, March 09, 2007

The Loudspeaker part 4

This is the classic electrostatic loud speaker. the design is imperfect, but is revisited even today by audiophiles looking for more accuracy. Nakamichi just unveiled a version that costs about $9,000 per pair. A healthy descendant of patent No. 1,983,377.

One of the big down sides is that they use very high voltages to operate. The 5000 volt DC bias is usually supplied by a power supply running off 120 volt AC electrical circuits which is dangerous enough I'd not recommend it be anywhere children can reach. This nut makes them.

They use a thin flat conductive diaphragm. It's usually a plastic sheet impregnated graphite. This diaphragm is sandwiched between two electrically conductive grids, with a small air gap between the diaphragm and grids. For low distortion operation, the diaphragm must operate with a constant charge on its surface, rather than with a constant voltage. hence the need for the juice and the tube amp and the transformer and the dedicated circuit in the basement. [OK, I'm exaggerating but you're starting to get the idea what nobody uses these.]

The reason they exist is that they are amazingly precise. Which is why the audio nerds at Stereophile Magazine get all dreamy-eyed when some manufacturer ships them a pair for testing. The diaphragm described above is driven by two grids. Using grids on both sides cancels out non-linearity. The result is almost the total elimination of harmonic distortion. it sounds crisp to say the least. The downside to this design other than voltage and cost is that unlike your cheapo speakers it projects this perfect sound over a very narrow area. It's sonic field is very narrow, especially for those of us that are getting spoiled by surround sound these days.

In 1957 Quad ESL marketed as the first real electrostatic loudspeaker, referred to as the Quad 55 or the Quad 57. Those bad boys were designed by Peter Walker (pictured) and David Williamson. But they were based on a patent owned by Edward W. Kellogg's from 1934. that patent was based on a research paper he did way before that in 1925 while still working at General Electric. Kellog worked in tandem with a gentleman named Chester Rice. They published a paper on amplifier design that was important in boosting the power transmitted to loudspeakers. In 1926, RCA used this design in the Radiola line of a.c. powered radios.

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