VACUUM TUBE AMPLIFIER SYSTEMS
the closest approach to the original sound.
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The Quad
II-forty
ThE QUAd II-FoRTY valve amplifier is not only
one of the finest sounding valve power amplifiers
available in the market place today, it is an aesthetic
and engineering masterpiece.
An understated elegance lies beneath the smooth
curves and lustrous finish, for the power amplifiers
use a circuit dating back over fifty years to the time
when vacuum tubes ruled the electronics industry
and Peter Walker considered by many to be the
master of audiophile design.
he often shunned traditional thinking, preferring
to go his own way, relying on excellence in circuit
design. Circuits that are still admired today by
the finest minds in electronics-for their stunning
performance and, by today’s standards, simplicity.
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“Factor out nostalgia. Forget the miracle of Quad
making valve amps in the 21st century. Look
at the Quads in terms of a current pre/power
tube combination. Then look at the opposition.
Now factor in gorgeous styling, superb build
quality, perfect ergonomics, ample grunt, genuine
heritage and the kind of sound which allows valve
enthusiasts to show two fingers to transistor users.”
– Ken Kessler, hi-Fi News
Quad products have always
been amongst the finest au-
dio products in the world. When
Peter Walker started making com-
mercial PA systems in 1936, few could
have predicted the revolution in hi-fi that
many of his products were to cause, the lon-
gevity they would have and the esteem with
which they would be held.
In the early 1950s, he and Gilbert Briggs, founder
of Wharfedale, embarked upon a series of now fa-
mous concerts demonstrating how real high-fidelity
music should be virtually indistinguishable from a
live performance. In order to prove this they toured
such venues as the Royal Festival hall in London and
Carnegie hall in NewYork. Audiences were amazed
at the similarity the Quad amplifiers were able to
reproduce relative to the live performance.
The Quad II-forty power amplifiers are much more are visually stunning, sitting safely within a cage
than a tribute to one of the most famous designs in
audio - they are both refined and powerful in the
modern sense of the terms.
to protect them. Gold-plated phono connectors
replace the ‘Jones’ plugs of the original - with a
choice of 4ohm or 8ohm taps, allowing for the
widest possible choice of loudspeakers.
Although the circuit topology has changed little
“Musicians and recording
artists all over the world
have long recognised the
outstanding quality of
Quad products – exquisite
engineering, build-quality,
finesse and refinement.”
from the original, significantly better components
Every product we make is hand-built by craftsmen
Since these early demonstrations of true high-fidel-
ity, our product philosophy has remained constant,
to produce a sound which as closely as possible rep-
licates the experience of the live performance. For
many other manufacturers, the term hi-fi may mean
‘exciting’ or ‘dramatic’ and this can be easily pro-
duced without regard to what the music sounded
like in the concert hall for the listener and may sound
‘impressive’ to the uneducated ear, but in reality is
no substitute for the sound produced by ‘real’ hi-fi.
and a brand new transformer design - still for a lifetime of duty. Quad products rarely, if
delicately balanced -means that they are fast, crisp ever, need replacing and we regularly service
and detailed, whilst producing an output current, and maintain products over fifty years old.
more than capable of driving modern loudspeaker
loads, even our very own ESL loudspeakers are
well within their capabilities.
This means many of the original Quad II
amplifiers have been passed down, into the
hands of the children - and even grandchildren -
of the original owners, something that is uniquely
Quad in essence. A product that is relevant today
The champagne finished chassis is strong and
heavy, internally wired with solder tags - not PCBs.
The KT-88 output valves in push-pull configuration and will remain so for generations to come.
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The Latest addition to the Quad Valve Power amplifier
series is the long-awaited Quad II-eighty amplifier. The
perfect partner to our latest ESL speakers, the II-eighty is
not merely a nod in the direction of retro styling, it’s a
modern high-performance amplifier designed by one of
the most famous tube-amplifier designers in the world
(Tim de Paravicini) using more than a little inspiration
from the original Quad II amplifier.
This method reduces distortion when compared to the
usual Ultra Linear or triode methods, however, it does
require a greater grid driving voltage. This is achieved,
again in true Quad fashion, using a balanced pair
input stage.
The Quad
The output transformer has been designed to improve
high frequency performance and accurately balance
the two push pull halves. Also the transformer has been
designed to make sure that full output is realised at the
bass frequencies down to 20 hz.
II-eighty
The Quad ll-80 is an 80 watt mono power amplifier
following in the tradition and philosophies of PeterWalker.
The output section consists of 4 KT88 valves running as
two pairs in push pull with the hallmark Quad topology of
partial cathode loading in the output transformer.
Each output valve has its own cathode resistors to
provide reliability and self-matching. The screen grids of
the KT88 valves are run from a lower voltage, again for
good reliability. The whole amplifier has a total of only
16 dB of feed-back providing good stability into virtually
any known loudspeaker and especially the famed Quad
electrostatic loudspeakers.
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“It is so much better in terms of high fidelity, in its
purest sense. It reminded me how Quad retain an
almost unique grip on the concept.”
– Noel Keywood, hi-Fi World
The Quad
QC-twentyfour
Unlike the power amplifiers, the Quad Control
unit utilises a completely new circuit design.
once again, the circuit is simple in concept yet
remarkable in execution. our designers scoured the
globe for the very finest components, with possibly
the best example of this being the new ‘old stock’
6111WA double triode.
designed as a special military valve, it is hard wired
to the circuit and has an estimated life of nearly
100,000 hours. Together with transformers designed
by one of the World’s leading authorities and relay
input switching, the musical signal is preserved in
its purest form - pursuant to the philosophy of the
theoretical perfect amplifier which, as Peter Walker
himself stated, is ‘a straight wire with gain’.
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“Quad has succeeded where few of hi-fi’s
gamblers dare: They’ve followed the ESL-989
with not just a worthy amplifier, but with a silent
The standard QC-twenty-four
is line level only. There is an
option of a solid-state phono
stage which can be added
when new, or at a later stage.
smooth and precise control unit as well.”
– Ken Kessler, hi-Fi News
Peter Walker always believed that an amplifier
should be as simple as possible - engineer with the
fewestpossiblecomponentsandwithaperformance
largely independent of those components.
The total line gain is 15dB, with a high quality
potentiometer controlling the output levels.
Reminiscent of the original QC-22 control
module, there is a simple balance control
allowing for variation from left to right of up
to 12dB.
The QC-twenty-four, a low-noise RIAA curve
phono stage has an understated elegant simplicity,
although weusehigh qualitycomponentswherever
these appear in the signal path.
Amplification is provided courtesy of a twin
triode 6111, new old-stock valve - originally
designed for use in the guidance systems of
missiles and manufactured to exceptionally high
tolerances. The remarkably long expected life-
A completely new design, the QC-twenty-four pre
the relay switching to the gold-plated terminals, each
amplifier is both neutral and detailed. Using a military
component has been selected specifically for its
Bifurcated contact relays (crossing each other at
right angles to make a better contact) are used on
specification 6111 valve, which is so reliable, it’s audio performance.
hard-wired to the circuit board.
each input to maintain the best possible signal span of this valve means we can comfortably
conduction.Gold-platedconnectorsandacarefully build it into the PCB without the compromises
screened enclosure are further examples of the usually experienced when using valve seats. Two
attention to detail throughout the construction. buffered tape loops are provided as standard.
The chassis is painted to match both the Quad II-
Classic and the Quad II-forty power amplifiers, making
it the perfect partner to both systems.
Everything has on this pre amplifier has been
designed to maintain optimum signal purity. From
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The Quad
QC-phono stages
For the analogue purist, the combination of a good valve
amplifierandagoodturntableistheclosestpossibleapproach
to the original recording.
For the connoisseur however, there is the Quad twenty-
four P dedicated valve phono stage. This has been
designed for Quad by valve supremo Tim deParavicini
- probably the finest valve amplifier designer in the
There are two options for adding such functionality to our world today. It has been voiced by the finest ears in the
valve pre-amplifiers.
industry and will add an entirely new dimension to your
enjoyment of vinyl recordings.
A solid-state module is available for the standard QC-twenty-
four pre-amplifier (known as the QC-24 phono stage) which
allows direct connection of a turntable with an internal
This unit features a moving magnet/ moving coil toggle
switch on the rear, and a choice of direct full level output,
switch to change between moving coil and moving magnet or a variable level output if you wish to connect directly
cartridges.This can be fitted as standard or added later. to a power amplifier.
designed by valve supremo, Tim deParavicini,
the Quad twenty-four P is a stunning addition
to the range. designed holistically with the
power and pre-amplifiers, it brings an entirely
new dimension to vinyl listening
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The Quad
II-classic
This is a faithful reproduction of the original,
Peter’s circuit, notably the design of the output
the new Quad II Classic offers audiophiles the transformer, has become the stuff of legend amongst
chance to recreate the spirit and atmosphere
produced from one of the most famous amplifiers
in audio history.
electronics engineers. Studied and analysed by
the finest minds in the industry, the elegance
and coherence of the design is still regarded as a
reference today.
The original Quad II was first launched in 1953,
designed by Peter Walker. At the time, many of Given the quite modest components of the day, he
his contemporaries were also producing good
was able to engineer something which was quite
quality valve amplifiers. The Quad II looked, in literally greater than the sum of its parts.
comparison, very simple and unassuming, but
its performance was far beyond that which its
diminutive dimensions might suggest.
The Quad II classic has been recreated accurately
with output valves and input valves the equivalent
of those used in the original design. The same
‘the closest approach to the original sound...’ has been a
Quad philosophy since 1936. More than just a slogan, it
underlies the company’s commitment to producing the
finest audio equipment, capable of reproducing all the
elements and emotion of a live performance.
The property which set the Quad II amplifier apart balancingacthasbeenemployedtoensurethesame
from other amplifiers of the time was its output
stage. It was arranged such that the cathode
coupled circuit emulated the performance of
triode operation, but with the size and efficiency
characteristics of pentode operation. It was this
breakthrough that ensured the Quad II was to
remain in continuous production until 1970.
wealth of detail and gentle tone of the original.
Theoutputvalvesproducearound 15wperchannel,
operating in Class A which, although slight by
modern standards, allows
a
gloriously sweet
and refined sound with appropriately matched
contemporary loudspeakers.
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Quad sound systems have always provided
high levels of listening pleasure to their owners.
our philosophy has always been one of repro-
ducing the ‘live’ experience in the comfort of
your own home.
“The Quad II Classic
uses the finest quality
components, each
hard wired onto a
robust chassis with
solder tags..”
Whilst many modern amplifiers are stunningly
accurate and offer breathtaking resolution and
distortion figures in tiny fractions of a percent,
many also loose sight of the overall aim - which
for Quad, is to bring pleasure to the listener by
recreating the entire range of sensory experi-
ences felt at a live performance.
For many audiophiles, just as vinyl will never
be truly replaced by digital music, the vacuum
tube communicates on an emotional level that
can never really be surpassed by a transistor
based design.
specifications
Model
II-forty
II-classic*
II-eighty
40W RMS
15W RMS
0.3% @ 12W
80W RMS
Power output
The Quad II classic is one of the finest of these
vacuum tube amplifiers. Although small in di-
mension and modest in design, the purity with
which it conveys, not just the musical score, but
the sense of space; the passion of the vocal and
the range of textures from the instruments in any
performance, is without peer.
0.3% @ 40W
0.03% @ 1W
0.3% @ 40W
0.5% @ 80W
Thd @ 700hz
Better than -80dB
Better than -90dB
Better than -94dB
10hz–30khz(+0dB/-0.5dB)
16 @ 1khz
Residual hum & noise
Frequency Response
damping Factor
Power Consumption
Loudspeaker Taps
Valves
10hz–30khz(+0dB/-0.5dB) 10hz–20khz(+0.2dB)
20 @ 700hz
11.75 @ 700hz
90Va maximum
8 ohm
As a piece of audio history, the Quad II circuit
was pivotal in creating what we know of as genu-
ine ‘hi-fi’ today. As a design statement, the Quad
II is not only as relevant today is it was over fifty
years ago, it is remarkable in its absolute longev-
ity.Yet the only important factor is that the Quad
II Classic can bring the same pleasure to today’s
discerning customer that has been experienced
by generations of Quad enthusiasts.
180W maximum
4 ohm and 8 ohm
180W maximum
4 ohm and 8 ohm
2 x 6Sh7 Input
2 x KT88 output
1 x 5U4G Rectifier
2 x EF86 input
2 x KT66 output
1 x GZ34 Rectifier
2 x 6Sh7 input
4 x KT88 output
1 x 5U4G Rectifier
1V RMS
1.4V RMS
1V RMS
Sensitivity
*The Quad II Classic power amplifiers are supplied in matched pairs.
Model
Valve Complement
Thd @ 1V RMS
QC-twentyfour
QC-twentyfour P
Valve Complement
Thd
4 x 6111
1 x 6111 twin triode
‹0.08% (A-weighted)
20hz–20khz (+0dB/-0.5dB)
Better than 60dB
‹0.2%
0.02% @ 1W
Frequency Response
hum and Noise
Input Impedance
Moving magnet
Moving coil
47Ω
10Ω
5hz–50khz (+0dB/-0.5dB)
Better than -80dB
15dB
Frequency Response
hum and Noise
Stage Gain
high sensitivity
Medium sensitivity
Low sensitivity
1.3mV (MM)/130uV (MC)
2.6mV (MM)/260uV (MC)
5.2mV (MM)/520uV (MC)
Line Stage Gain
50k ohms
Recommended Min. Load
Inputs
Nominal output level
(ref 5.2mV, MM, Low)
Inputs
1.0
5 x Line level
2 x Buffered tape loops
Selectable MC/MM cartridge
1 x fixed line level
1 x variable line level
15 VA maximum
outputs
Power consumption
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