"Oh no. No, we think our loudspeaker is very poor"
-- Peter Walker (TAA 3/78)
The Quad ESL (Electrostatic Loudspeaker) was introduced in 1956. It was a truly amazing speaker for the time, and is still a product to be reckoned with today. Dubbed "Walkers Wonder" after Peter Walker, head of Quad and designer. Many people, myself included, believe that it is the most faithful and natural midrange transducer ever made. At the frequency extremes, it also holds up well, the treble is never spitty or tizzy. The bass is fast and clean without ever being boomy, it only goes down to about 40 Hz, so the bottom octave is missing. Production of the original ESL stopped in 1981 shortly after the introduction of the newer (and different although not necessarily better) ESL63. Over the 25 year production run, approximately 60,000 speakers were made.
But the Quads also have some downsides. First, they are fragile. They need about 15 Watts of the best amplification you can get your hands on (see the Heathkit UA-2 EL84 (6BQ5) or Fisher SA-16 amps). If you use a larger amplifier or try to overdrive the speakers, you risk arcing. Arcing occurs when the potential across the stator panels is sufficient to ionize the air and spark across the gap and through the membrane. Arcing will ruin the panels, if it occurs enough times or is severe. The Quads will play up to about 100 dB, which is much louder than I play music even when playing loudly. The second downside is that they are a difficult load for the amplifier to drive. They are highly reactive loads (primarily capacitive). Their impedance drops as the frequency increases. This makes the Quad ESL a bad choice for single ended tube (SET) amplifiers.
The Quads have a very directional radiation pattern, so you have to position them carefully for best performance. Lastly, they, like all old British things, are quirky and temperamental. Living with a set of Quads is like living with an MG. You will have to do work on them from time to time. Like an MG, when working well, they are a true joy to use. Unlike an MG, after a rebuild, Quad ESL's will work well for decades, if not abused.