Tubes or Solid State

[Leporello]

Since the current setup uses a step-up transformer to boost the voltages, don't we already have very high voltages on the stators?
We see this when a stator seal cracks or peels away and arcs to the diaphragm.

The main difference vs direct drive is the risk of having that high voltage in the cables between the amp and the speaker; that's where the real problem lies.
I dealt with this conundrum in my design for the Monoray line by having a custom Kevlar-wrapped 'umbilical' cord to carry the signals (and any LV power) needed by the speakers. Using sealed locking connectors, an amp output relay that only closes once the connections are validated as safe. My direct-drive amp is a power-DAC like some of the NAD DDFA amps but with beefy modern power FET devices.

See the brochure PDF I wrote for more details: https://cdn.imagearchive.com/martinloganowners/data/attach/15/15199-MonorayBrochureFlat.pdf

The stepped up audio signal from a normal ESL driver transformer system could be harmful or lethal under unlucky circumstances, and you should always use caution when working inside the electronics, especially if feeding an audio signal. All the direct coupled tube amps I have seen use plate voltage for the DC bias, which is very dangerous. That could be mitigated somewhat by using a grounded anode arrangement with the cathode floating at -2000V or so, but I have not seen this. The Hermeyer 'stats, referenced in the Sanders white paper, had bare metal rods as stators, at transmitting tube plate voltage. No infants or pets should ever be in the room with them!

The Acoustat panels, when they went with direct coupled tubes, used insulated wires as stators inside plastic supports (fluorescent light diffusors were the material of choice for home made versions). So quite a bit of isolation between the user and high voltage. ML panels use perforated metal with propitiatory insulation--probably safe but I'd be nervous about touching both sators at once with a signal applied, let alone with both floating at transmitting tube plate voltages.

I have long believed the ideal setup for driving ESL's with tubes would be a single, low turns ratio transformer between the tube plates and ESL stators. Low turns ratio transformers are easier to build with less flux loss. Stepping the signal down to an 8 ohm nominal impedance then back up to high voltage to drive the 'stats just doesn't make engineering sense to me.

 

[JonFo]

ML panels use perforated metal with propitiatory insulation--probably safe

I saw the process at the Kansas factory during our MLO tour in 2007. It starts with flat perforated metal sheets, then runs through a curving press to give them the 30-degree arc, and then hangs them on a charged line that goes through a powder coating booth. Then they go into a baking oven to set it. I think it was a fairly standard silica-based powder coat material. It is able to insulate the high voltages as long as it's not cracked or peeled away.
From my efforts at separating the old panels for my sidewall installation, the coating is pretty resilient and can handle rough treatment without (visual) harm.