Quad EHT Unit

RStAudio 57er Quad ESL EHT Unit

Table of Contents



An electrostatic loudspeaker consists of two stators and a swinging foil which is mechanically clamped between the stators. For this simple principle to work, the foil must be conductive and biased with an electrical voltage. The stators are then driven in antiphase by the music signal and the foil begins to swing in time with the music. Unfortunately, this does not work at low voltages, the foil must have enough space to move to transmit a noticeable loudness and air is a good insulator.

Consequently, the Quad ESL 57 applies approx. 1500V/DC (high frequency) and approx. 6000V/DC (bass) to the foils. Also the music signal needs much higher amplitudes than normal power amplifiers provide, so this signal is transformed by a transducer.

A mains transformer with connected voltage cascades in Greinacher circuitry is used to generate the high voltages required for the foils. The lower high voltage of the tweeter is tapped after the first cascade.

Over the years, the diodes in the Greinacher circuit become “tired” and the DC voltages on the foils decrease noticeably. The deflections of the foils decrease. Therefore it is absolutely necessary to replace at least the diodes in the cascade during a revision of the Quads. I have done something like this at the Renovation of my Quads 2011.

At the Refurbishment of my Quads 2020 I decided to build up and install a completely new EHT unit.

Description of the Circuitry


The aim of the development was to avoid potting the cascade with wax as in the original and to guarantee functional safety in spite of everything. Furthermore I didn’t want to take the high voltage of the tweeter from the cascade of the bass, instead the tweeter got its own cascade. At the output of the cascades there are also networks of resistor, capacitor and neon lamp separately for each individual foil.

These things are not new and have been described elsewhere for quite some time, but I think they are very good ideas and so I wanted to include them in my implementation of the Quad EHT Unit.

The output network should go back to Anders Enquist. Unfortunately I didn’t find a direct link to it, but I found a description of the network’s advantages in my huge documentation on Quads collected over the years:

The idea of it is that the neon lamp isolates the panel from the EHT supply until there is a voltage across the capacitor which is high enough to flash the neon lamp. This takes some 20 volts or so. When the lamp flashes a fraction of a second the panel is charged. Apart from improving the sound, it’s a health inspection as well. If everything is going well you should notice a short flash from the lamp now and then. If it flashes all the time there is a short circuit. If, on the other hand, it doesn’t flash at all, there is a component break down in which case the panel connected will be dead. Soundwise, this tweak will make the bass leaner but with better impact. Sounstage depth will increase too.
… This bring a considerably tighter bass. You may feel the mid-range opens too, but I believe that it is a subjective side-effect of better bass definition. …

Installation of a RStAudio Quad ESL 57 EHT Unit

Installation of a RStAudio Quad ESL 57 EHT Unit

As you can see in the picture above the EHT Unit is still connected to the 610V/AC winding of the transformer. For better adaptation to the existing voltage situation all my Quads are now connected to the 590V/AC tap. Together with an upstream RStAudio Quad ESL Voltage Regenerator this results in stable and optimal voltages at the foils.