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 of  an effective solution to  the 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 evidence  the 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 content  with 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 the  amplifier 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 products  are interesting since the achievement of circuit optimization (reduction of THD, increase of Bandwidth and so on) often is obtained through non conventional  remedies.                 

    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 can  strongly 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 linear  complementary interactions  inevitably produced in each amplification process for minimize the THD.

 

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