THD
Cancellation Techniques for Vacuum
Tube Amplifiers
"In
1921, the major task at the Western Electric Company's old West Street
laboratories in New York City was to improve the Bell System's new open-wire
telephone system. I discovered that the system's push-pull repeater amplifiers
were a major source of trouble. No one knew how to make amplifiers linear or
stable enough in those days, and consequently they were subject to an
intolerable amount of distortion. Starting from 1921 and during the next two
years, I spent many weekends and evenings reading all I could about the unwanted
generation of products. At 2 a.m. of March 17, 1923 I restated my own problems
as follows: Remove all distortion products from the amplifier output. In doing
this, I was accepting an imperfect amplifier and regarding its output as
composed of what was wanted plus what was not wanted. I immediately observed
that by reducing the output to the same amplitude as the input, and subtracting
one from the other, only the distortion would remain. The distortion could then
be amplified in a separate amplifier and used to cancel
out the distortion in the original amplifier output. This isolation and
elimination could be accomplished with two biconjugate networks as a
three-winding transformers or....." H.S. Black [1].
1
H.S.
Black
Inventing
the Negative Feed-Back Amplifier
IEEE
Spectrum, Vol. 14, pp. 55-60, 1977 Dec.
The foundation principle of the modern Control Theory, the negative feedback (NFB),
was invented in the search ofan effective solution tothe problem of non linear distortion in the amplifiers. The same
challenge in a historical period antecedent to FBN genesis was faced with a more
intuitive technique but whose application under the engineering profile
presented, to the epoch, not few problems: the Error FeedForward (EFF).
The common denominator of both the NFB and EFF is the inventor: H.R. Black,a Western Electric System Engineer. The simplicity (also in his more
exotic appearances) of NFB has made it the standard “de facto” for
amplifiers designed for audio use; nevertheless in the course of the years
alternatives were presented based on the FFE design. Starting from ’70
inspired engineers and scientists have begun to put in evidencethe theoretical and practical limits of such technique. Further, the
revival of the thermionic technology applied to the world of HI-Fi reproduction
has emphasized the “dark side” of the NFB since the excellent intrinsic
linearity that vacuum tubes possess (mainly between the “triodic” varieties)
has allowed the construction of great amplifiers Zero-Overall-FeedBack (ZOF)
with excellent sonic qualities and the diffusion of the cousciounsness that
elevated Feed-Back Factor (FF) can
resolve the main problems of technical nature but it leaves opened others of
more tightly musical concern. In fact musicians, sound engineers and Hi-End
criticals reveal, in amplifiers with medium to strong FF, a sound less vivid and
poor in the informative contentwith
respect to ZOF type counterparts. Others,recognize
this intrinsic peculiarity of FeedBack Amplifiers but observe that also ZOF
Amplifiers present serious problems as high levels of Total Harmonic Distortion
(THD), narrower bandwidth, low
damping factor and so on. In the western world, since in Japan it’s
consolidated from time, the new beginning designing philosophy is based on
realizations that trust less and less to the NFB as care for theamplifier problems; this imposes to designers a greater effort because to
build performing amplifiers without NFB is more difficult, further a changing in
the “forma mentis” is necessary
because it’s very difficult to renounce both the technical advantages and
flatteries of NFB. On the other hand some recent products of western high-end
market in the field of vacuum tube amplifications seem to have derived from
techniques and circuits of pre-feedback era and therefore dating back to the
dawns of electronic technology. This productsare interesting since the achievement of circuit optimization (reduction
of THD, increase of Bandwidth and so on) often is obtained through non
conventionalremedies.
The problem of an elevated THD that ZOF amplifiers presents in
overload conditions must be attentively valued because if it’s true that 5% in
THD can be not-audible “in se”, the intermodulation products associated with
this THD levels and with signals of very complex nature canstrongly degrade the original informative content. This problem could be
faced accepting, for example, small FF in the economy of the project, by mixing
sonic deterioration and instrumental performances or, when the circuit is simple,
by applying alternative techniques based on the harmonic cancellation.
The Harmonic Cancellation
Technique (HCT), uses the non
linearcomplementary interactionsinevitably produced in each amplification process for minimize the THD.
What did you
think of this article? Click here
to send us your comments, feedback and suggestions