As a product post World War II era English frugality and
common sense, did the paraphrase circuit withstood the test of time as a
brilliant audio engineering solution?
By: Ringo Bones
Maybe I’m just paraphrasing from Ken Kessler’s book Quad:
The Closest Approach whenever I hear a fully restored Quad II amplifier and
praising its sound quality made possible by an unseemly circuit design of the
famed amplifier’s input section called the paraphrase circuit, or am I just
looking this venerable vacuum tube based design that’s now over 70 years old
with rose-tinted glasses. Anyway, what is it about Peter J. Walker’s famed
paraphrase circuit that had pundits calling it a brilliant audio engineering
solution cobbled up in the austere fiscal climes of post-World War II Britain
that later become an iconic fixture in the Golden Age of Stereo?
A typical Peter J. Walker designed Quad II amplifier,
whether original, a 1996 era Diamond Jubilee Edition or a 2004 era Classic, is
a class-A push-pull monophonic amp with only two active stages. The input stage
contains two EF86 miniature low noise pentode tubes, each of which is
capacitor-coupled to its own output tetrode tube, but only one which –
designated as tube No.1 – gets the original input signal. From there, in
addition to driving its corresponding output tube, input tube No.1 drives input
tube No.2 with a portion of its reverse-phase output, attenuated by some 6dB.
Thus the output of the second EF86 is equal to but out of phase with the output
of the first EF86 tube, and this balance is maintained through the output
transformer’s primary winding, where the full music wave is put back together.
This is the working description of a paraphrase circuit, used to be quite
popular, arguable because it accomplished phase inversion and voltage gain in
one stroke – and so allowed the designer to keep both distortion and parts cost
to a minimum. By the way, the KT66 output tubes of the Quad II amplifier was
later used by the late, great Dr. Jim Marshall in designing his first Marshall
electric guitar amplifier – the JTM 45 Blues Breaker.
But to “audio purists” - what they see as the greatest
weakness of the paraphrase circuit is that the signals at the grids of the two
output tubes – and thus the two halves of the music waveform at the output
transformer – are not perfectly in time and in phase with one another, given
that the signal on the grid of input tube No. 2 has already been amplified,
whereas the signal on the grid of input tube No.1 has not. Of course, one has
to remain open to the possibility that this very technical anomaly of the
paraphrase circuit resulted in some form of another a “musically consonant form
of distortion” that most converts to Peter J. Walker’s Quad II amplifier just
happens to find pleasing. When paired with another of Peter J. Walker’s famed
designs – the Quad ESL-63 electrostatic loudspeaker – almost all hi-fi
enthusiasts on the planet claim it to be the most perfect hi-fi rig money can
buy.
Believe it or not, the only other power amplifier utilizing
the benefits of the paraphrase circuit was designed by Kevin Hayes in his VAC
PA160 mono amp which was marketed around the mid 1990s – a time when vacuum
tube circuit revival in the hi-fi world was steadily gathering steam. For all
intents and purposes, the VAC PA160 can be seen as a more powerful version of
the 12-watt Quad II amp that uses KT88 tubes that can provide 160 watts in
pentode mode, 152 watts in Ultra-Linear mode. Though quite different from the
paraphrase circuit used in the Quad II, Kevin Hayes’ version uses a 6SN7
octal-based dual triode configured in a floating paraphrase phase inverter. In
this arrangement, the input signal is applied to the grid of a triode acting as
a common cathode amplifier, which provides one phase of the stage’s output
signal. This output is resistively coupled to a common point – which for
convenience, I shall call “point x”. The other triode’s plate, also resistively
coupled to point x, provides the other phase of the stage’s output. A coupling
capacitor from point x drives the grid of the second triode. In actuality, this
amounts to a stage that provides one output phase directly and couples that
output to another stage, which has 100% inverting feedback, to provide the
other output phase. From a circuit purist’s standpoint – i.e. fans of
single-ended triode amps – this scheme has the glaring disadvantage that the
second generated phase goes through one more tube than the first generated
phase. Further, the output impedance of the two phases is likely to be quite
different. But given they resulted in a good sounding design, does this mean
that the paraphrase circuit – born out of postwar English frugality and common
sense truly had survived the test of time?
