VALVE Magazine Online!
Acoustical Quad
Article By Dan Schmalle From VALVE Issue 4, April 1994

  This month's demo is a very special one. Through the generosity of Eric and Dave we will audition what is to many vintage buffs the ne plus ultra of vintage high fidelity systems, an entire QUAD system. It will be composed of the QUAD FM Tuner, QUAD Multiplex Decoder, QUAD 22 Control Unit, two Quad II Power Amplifiers and two QUAD ESLs. All of these components are in very nice shape and will be set up as originally intended right down to the KT-66 output tubes.

Acoustical Manufacturing Company, makers of the Quality Amplifier for Domestic use, was started by Peter J. Walker in London in the late 30's.

The original QUAD components were introduced around 1954. At the time the loudspeakers, while revolutionary in their construction and accuracy, were rather poorly received in an era of horn speakers with giant sized bass response. A very interesting interview of Peter Walker can be found in The Audio Amateur, 3/1978. I won't rehash the entire article here (we do have it in the library), but a few select plums from the article help to explain the design philosophy behind Peter Walker's products on differences in amplifiers: 

"If people test two different amplifiers and say "These sound different," there's no magic in it. Spend two days, maybe a whole week in the lab, and you find out  exactly why they're different and you can write the whole thing down in purely practical, physical terms."

On listening tests: "We designed our valve amplifier, put it on the market, and never actually listened to it."

"We never sit down and listen to a music record through an amplifier in the design stage. We listen to funny noises, funny distortions, and see whether these things are going to matter, to get a subjective assessment." On his satisfaction with the ESL: "We think our loudspeaker is very poor, but we think that the others are even poorer!"

"In the early days - most people didn't like it." 

"It couldn't shake the windows, which was the criterion in those days."

On adding subwoofers for improved response below 100 Hz: "People -- use two of our panels, one above the other. This is quite reasonable because it really is a strip source, you can extend the strip source without deteriorating anything. All you do is add 6dB at the bottom end and 3dB everywhere else. -- Adding woofers has never been very good."

On the ease of designing and building tube amps vs. transistor amps: "Much more forgiving, yes -- much easier"

You have to admire the conservative, thoughtful approach of the great British audio designers of the early days. QUAL ESL have gone through only one major redesign in 40 years, and still get high praise from the audiophile community.

Moving from history to componentry, here is a rundown on the equipment we'll be hearing.

The tuner uses a 6BH6 pentode in the RF stage, coupled to a 12AT7 mixer/oscillator, which is permeability tuned. 1st IF/AGC is a 6W6. 2nd IF is a 6BH6, as is the limiter. The phase discriminator uses a 6AL5. AFC and tuning indication at the discriminator are accomplished through the use of a 12AX7 used as a cathode coupled phase inverter feeding a neon bulb from each plate.

Mistuning turns off the neon in the direction of mistune, and one side of the triode acts as a reactance valve across the local oscillator coil for AFC action.

The multiplex decoder is a solid state unit which attaches to the back of the tuner. It consists of an impedance matching stage feeding a diode ring demodulaton. The demodulator is switched by a 38 kHz oscillator. The oscillator is synched to rectified 38 kHz pulses which are derived from the pilot tone. Power is supplied from the tuner.

The control unit is in a beautiful cast aluminum cabinet which matches the tuner. All power switching is performed through the unit which has sockets, cables and jacks on the back for the connection of amps, tuner, phono, tape, and adapters for various phono and tape loads. The input stage uses EF8616267's and the high level stage uses a pair of 12AX7's.

One interesting feature of the unit is a switching system which allows various EQ curves to be switched in when playing older phonograph recordings in the MONO mode. A list of settings for various labels is included in the owner's manual.

The power amps are as attractive looking as the rest of the components.

The input driver consists of two EF86's, each feeding one output tube. One tube is fed a signal 6dB below balance from the other. The imbalance is compensated by connection of the output tube grid returns at a point where the phase difference creates a small signal difference which compensates for the initial imbalance, as well as mismatch between tubes. The output stage uses KT-66's in push-pull and a unique transformer with five windings divided into fourteen subsections. It employs a cathode feedback winding, enabling greater power output than a standard "tetrode operated as triode" circuit.

The owner's manual makes reference to an article in the September 1952 issue of Wireless World which apparently gives the design rationale for the amp (does anyone have a collection of Wireless World issues? - dan).

Full chapters have been devoted to description of the electrostatic loudspeakers. The reference considered a must read on the subject is Peter Walker's original series in Wireless World, May, June and August, 1955. Another excellent article by Reg Williamson can be found in Speaker Builder, 1/82. I have capsulated his description of the speaker here:

"The basic design consist(s) of a  light, thin diaphragm (lighter than the air layers on either side) of plastic which was made weakly conductive and suspended between two outer electrodes."

"The diaphragm is also given a constant charge from a high voltage polarizing source."

"The driving signal is fed a crass the two fixed electrodes, a genuine push-pull system and fully symmetrical."

"The higher frequencies (are) handled by a narrow strip in the center and the bass by two rather wide strips on either side. In the first production models, the crossover point, around 1kHz, was mechanical."

"Post-1966 production models incorporated a simple high pass filter in the signal path to the center unit. Finally, to improve the waveshape, the entire unit was curved gently in the vertical plane."

Put it all together and you have one neat demo for our April 10th, 1994
meeting!