Hi Rmarcus - I've been going low budget with my ESL57's. I have a Bedini, and like it alot, but I like even more the VTL Tiny Triode. Other ones I considered were a Musical Reference EL84 amp (I like EL84's), Rogue (I forget the model). I also tried AtmaSphere M60's, which sounded good, I wasn't looking to invest that much in the system.
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An Acoustat TNT 200 will spank the Bedini. I've owned several Bedinis and like them. Still have a couple. But the Acoustats are more than a notch above. Extremely fast with extreme extension. They are simply stunning. Guaranteed to blow you away! Mind you the ones I have are rebuilt by Roy Esposito in Florida. He was one of the design engineers at Acoustat. He rebuilt 3 TNT200s for me and added balanced inputs and mono switch. They are 200wpch. in stereo, but in mono they produce over 1100 watts.
Sorry, but it makes no sense to drive ESL57s with 200W, much less 1kW. Either the recommended VTL Tiny Triode (used market, no longer made), the inexpensive Quicksilver Mini-Mite or the Music Reference RM-10 Mk II for a bit more money would be more than enough and will bring the best in your classic ESLs.
Of course he doesn't need the power. A TNT120 would suffice. It's just that the Acoustats were designed to adequately drive elecrtostats. Once Roy is done with them, they are SOA. A TNT 120 could be had for under $500. Roy would rebuild it for $400 which includes return shipping within the lower 48. Now you have a world class amp for about a grand.
The TNT amps were designed to mate with Acoustat's own line of big panel speakers which required much more power than any of the early Quads. I'd be afraid of frying the panels with that much power.
I'm with Syntax, Atma-Sphere would be a much better match and would be a huge step up from the Bedini.
The fact is electrostats prefer voltage amps (tube) not current amps (transistor). That speaker needs only 40-50Wrms maximum. Anything higher than that, and you're looking for trouble. I would also suggest having your Quads updated to include the clamp circuit. I do believe they are available. An ARC D70 MKII would work perfectly. Even an updated Dynaco ST70 would be sufficient. BTW the reason the Quad 405 needed the clamping circuit was because it would routinely damage Quad 57's and 63's because it delivered too much current. Even an early Quad tube amp or the commemorative version would work well and I believe that's only 25-30Wrms.
The 25/25 is considered the best match for the 57s by a lot of people. Why, I don't know. There are a lot better amps than the Bedini, even vintage.I can't speak to the voltage preference thing, but not only is he using a high current amp but he's using two that are bridged! Take a wild guess what kind of current is running through those 1megs now. I suppose the key to survival is to throttle your desire for SPLs.
I 've owned both the Bedini 25/25 and currently the Atmasphere S30 though I have not heard either with Quad 57. I can say that the Bedini 25/25 is highly regarded by Quad 57 owners as I was deluged with offers when I sold it.
That being said, I would second the recommendation for an Atmasphere OTL. Would recommend the M60 but you could use the S30 if the stacked Quads were wired in series (increasing the overall impedance).
The classic combination with ESLs since the 1950s has been ESLs and OTLs.
The impedance curve of the speaker is benign. The advantage that OTLs offer is that there is one less transformer in the signal chain so its more transparent. The other thing is that OTLs don't have any problem making bass on the speaker. In the bass region, the impedance of the speaker is so high that this is a problem for a lot of amps. This is why transistors don't do so well with them.
A minor correction: The Quad should not be driven by amps that make a lot of current, they prefer amps that make **power** (not just voltage). This means a tube amp, generally speaking. For more information about this see