I finally made some good progress on the speakers to the point of being able to listen to them. I initially cut some MDF with the intention of building an enclosure, but then ended up using some enclosures I had been given a while ago. They were originally a set of ex R&D Duntech Ambassador XLs, but they were not really in useable condition, otherwise I would never have modified them :-) The bass drivers' foam was rotted, the tweeters were both in pretty average condition, and the twin 6" midbass drivers had varying amounts of stuff stuck to the front of them as part of something Duntech was trying many years ago. The crossovers were different in each speaker. Also, the cabinets needed a little attention as the joints were not 100%. A far cry from their production models!
I ended up keeping the low down side firing 8" woofer arrangement, although with replacement woofers. The overall height was cut down by about 120mm to make them less imposing. I wanted to try the closed box arrangement with the electrostatic panels (a la Janszen), but if it didn't sound any good, I would have cut out the sides and back and covered the whole lot with a grille cloth sock.
I cut and fit some new front panels to suit the 505s, and lined the 25mm enclosure with 10mm felt offcuts which I had. The enclosure was then about 80% filled with the Dunlop Hypersoft HS1730 foam which Duntech used in their enclosures. Luckily this one came stuffed full of it.
The bi wire speaker terminals were wired up to the woofer and panel respectively, with the panel wiring being run up through the enclosure to a compartment at the top, to make it accessible. I still need to make a cover for the transformer board under the speaker's top cap to keep the high voltage away from curious fingers. A 12v input socket was fitted to the terminal plate and the 12v power cabling run up to the top compartment. A 12v power LED was also fitted to the terminal plate.
So far I have wired but not used the 8" woofers, so it is just the panels running down to maybe 200 Hz, with the 12" Rythmik subwoofers running up to their upper limit of 200 Hz. Rythmik does not recommend running this high but I thought I'd try it anyway. 2x22uF capacitors were paralleled and used in line with the panel, plus MiniDSP on top of that, with 3rd order XO at the 200 Hz point, aiming for something effectively around a 4th order XO.
So far, they sound absolutely stunning! I really didn't expect enclosed electrostatic panels to sound this good! There was absolutely no sense of them being in an enclosure, perhaps because of the efforts taken to damp the enclosure. They could be placed against the back wall, and there was no perceivable loss of depth of the sound field or other effects which usually seems to happen to speakers with dynamic drivers when up against a back wall. They really do sound superb. Sound staging, transparency and naturalness are all right up there. They leave dynamic drivers for dead, I'm not sure if I will ever go back to standard speakers in a serious way. I will need to tweak the XO once measurements are done, but for a first listen, I am EXTREMELY happy. I can definitely state it is worth while trying these in a suitable enclosure, if anyone else wishes to try it. I also think that the damping inside the enclosure should be done with the aim of soaking up the entire back wave. I have some pics below, but none with the panels installed sorry.
Interestingly, the very quick measurement (Dayton Omnimic) that I have done showed an extremely flat response, at least in the mid and upper frequencies, it is almost ruler flat. The measurement was done in-room and gated to 5ms, so I have no objective info on the lower frequencies as yet. My graphs certainly look different to @NADMAD ’s graphs above, not sure why. Maybe Rob @ESLman (ERAudio) could offer some insight?
The panels are awesome. I have also found the Rythmik subwoofers to integrate seamlessly with electrostatic panels, albeit enclosed ones. I can highly recommend the combination.