Now, for those not au fait with electrostatic speakers, they work in a different manner to conventional speakers (which have a cone to radiate the sound, driven by a coil of wire and a magnet). The speaker diaphragm itself is a piece of metalised plastic film, which is charged up to several kilovolts. Audio is modulated onto this high voltage by a transformer, which causes the film to vibrate, and produce sound.
The ESL 57 is so-called because Quad started production in 1957. Each speaker has three panels, two bass and one smaller treble panel. All three panels run from top to bottom. Bass left and right of the central treble panel.
The power supply consists of a transformer, which steps-up the mains voltage to around ~650VAC and feeds a rectifier and voltage multiplier, which derives the ~4KV to charge the panels.
Here's one of the audio transformers...
... and here's one of the power supply units. You can see the transformer, and the multiplier unit above it, thankfully indicating this speaker was rebuilt sometime after May 1972. The other speaker was rebuilt at the same time by Quad.
The mains wiring (externally) was in poor shape, and lacked a safety earth, so was replaced, and the speakers gingerly powered up.... both fired up and produced audio straight away, although they were both quiet. (The temptation here is to whack the volume up, but these speakers are only rated at a few watts (maybe 20), to match the original Quad II amplifier (or a 405-2 fitted with voltage limiters as mine was here.) Winding the volume up causes the audio modulating voltage to rise, and cause the speaker to arc over, and can cause irreparable damage to the panel, necessitating a re-build. ) After a few minutes, the volume slowly increased as the panels charged up.
Now I had never heard a pair before, but I had heard comments that they don't produce much bass. Well, that's true to an extent. Don't expect trouser-flapping amounts of bass, but what there is is very detailed, and the treble is incredible. A pair image fantastically. You do really need to be sat in the right place in front of them to get the best effect, but they are something else. The music seems to be in your head.
I was hooked. I then wasted away a whole evening just listening to music.
Sadly Mrs Doz is not quite so hooked. They're big (although slim), and not exactly matching Mrs Doz' impeccable interior decorating standards... so they must sadly go.