Stacked Quads

Stacked Quads 57

Table of Contents

Introduction

2011/03/01

Since the late seventies I own Quad 57 electrostatic loudspeakers in a stacked configuration. During all the years I heard a lot of wonderful speakers, but never a construction which became dangerous for my beloved Quads. When this loadspeakers are operated with the right power amplifier – and the XA30.5 is definitely one of them – then they are capable to do astonishing things.

Below is a statement of Peter Walker about the Stacked Quads which I have borrowed from the The Quad ESL Homepage :

Another thing people like to do is to use two of our panels, one above the other. This is quite reasonable because it is really a strip source, you can extend the strip source without deteriorating anything. All you do is add 6dB at the bottom and 3 dB everywhere else. It gives you a louder sound, a more impressive sound. That’s all right. Adding woofers has never been very good. [Audio Amateur, 1978]

With the exception of the last sentence – a RiPol had disabused him – I only can confirm his conclusions.

Impedance Curve of the Quad 57

To choose the right power amplifier you can find the following advice in the operational manual of the 57th Quad:

AMPLIFIER REQUIREMENTS
Other amplifiers if they are properly regulated (not more than 33 peak volts with any load) are unlikely to cause damage to the loadspeaker, although they may not necessarily provide the correct performance because of the very strict stability requirements.

The maximum voltage per speaker should not exceed 33Vp. In my setup each individual ESL per channel is driven from one channel of an ES4 switched in Dual-Mono mode. Thus every Quad has 100W at 8Ω output power of it’s own and the maximum output voltage is about 44Vp. If you look at the impedance curve – see above – a power amplifier must be able to drive a load of about 2Ω at 15kHz – very strict stability requirements. Without a doubt the XA30.5 is able to do that, the amp is also able to operate two in parallel connected Quads without any problem – I used the prototypes of the amp in such a setup at the beginning.

A good overview of the advantages of the stacked configuration you find on the The Quad ESL homepage.

Construction of the Frames

2011/03/03

A Stacked Quad consists of two 57th Quad electrostatic speakers per channel (see above) arranged on top of each other in a frame. The lower system is installed upside down. My frames are home made with inputs from different projects and informations from the internet.


Construction Details of the Frames

Construction Details of the Frames


Many thanks to my father, who made the construction of this wonderful frames and who build them !!!

 

Refurbishment

2011/03/03

After my Quads have played for more than 30 years without any problems, I thought it becomes time to do some refurbishment and replacement work. Hereby the most important thing was the replacement of the diodes in the high voltage cascades, because they loose their performance over the time and as a result the output voltages of the cascades is below their nominal values (1.5kV for the high frequency and 6kV for the low frequency panels). Additionally I replaced the poor original speaker connectors. As for the time being last modification I included a high voltage protection for the tweeter panels.

First you have to disassemble the rearward grill to be able to reach the high voltage cascade and the input transformer. As a preparation for this the power of the speaker should be switched off and to be on the save side you have to wait a couple of hours. Afterwards you can remove the screws and get access to the back inside of the electrostatic speaker.

!!! I have switched off the Speaker at the previous Day !!!
Therefore it was secured that the foils are completely discharged.

High Voltage Cascade

The high voltage cascade is a four stages Villard circuit with overall 8 diodes. In this circuit the reverse voltage of the used diodes must be higher than twice the peak voltage of the transformers secundary output voltage. My speakers have the newer design and therefore I have 8 high voltage diodes in the cascade – in difference with the older design with 2 diodes in series and an overall amount of 16 diodes.

For the dismounting of the power supply first of all the earth wiring for the front and back grill have to removed. Afterwards the 3 high voltage cables of the low and high frequency panels and the ground can be desoldered. Please take care, the wires are wrapped and you have to work with the soldering iron near the dust cover. After all cables are removed the fixing screws at the bottom can be disassembled and you can take out the complete power supply.


Power Supply with High Voltage Cascade

Power Supply with High Voltage Cascade

The last thing you have to do before you can disassemble the high voltage cascade from the power supply module is to desolder the two cables of the transformer output voltage at the input of the cascade. Thereafter you can remove the two plastic screws and take out the cascade. The last work step before the exchange of the diodes is the melting of the wax the cascade box is filled with. This you can do with an adequate vessel at around 150°C in a normal baking oven.

Quad ESL High Voltage Cascade, old Diodes     Quad ESL High Voltage Cascade, new Diodes

Quad ESL High Voltage Cascade, old Diodes at the left and new at the right Picture

I replaced the original diodes (left picture above) with two 1N4007 standard diodes connected in series (right picture above) and therefore I got a maximum reverse voltage of 2kV in every stage of the Villard circuit.

Now you have to repeat every work step in a reverse order to assemble the refurbished cascade. Please don’t forget to put the plastic screws inside their holes before you pour the melted wax inside the cascade box. The srew thread is build from the wax during the cool down.


I only can advice every owner of an old Quad to do this exchange,
you have problems to recognize your speaker afterwards again.

Input Transformer

Because my speakers are disassembled I used this possibility to make also small changes in the audio section. All the years I was annoyed from the poor original speaker connectors which I changed during the refurbishment action. I also follow a proposal on Sheldon’s World and integrate an overvoltage protection for the high frequency foil.


Quad ESL Audio Transformer

Quad ESL Audio Transformer

At the picture above you clearly can see the new speaker connectors. To assemble them I have to enlarge the original holes. Also an adaptation of the rearward grill was necessary because of the bigger dimension of the new connectors. Furthermore the overvoltage protection at the input of the high frequency foil is easy to identify. In contrast to the Zener diode chain Quad implements in their later models which shall influence the audio performance negative, the gas discharge surge suppressor CP Clare CG3-1.5L with its extremly high resistance (GΩ) and very low capacitance (pF) is highly neutral.