An Ultra-Linear Amplifier
David Hafler* and Herbert I Keroes*

   The old standards for evaluating amplifier quality have fallen into disrepute. It can be audibly demonstrated that a wide pass band and low harmonic content do not necessarily mean that the amplifier satisfies the critical listener. Newer criteria have been developed such as intermodulation distortion analysis and square wave testing, both of which simulate dynamic conditions to some extent and take into consideration that music and speech are not of a static nature. These new tests produce higher correlation between experimental data and listener preference. Therefore, modern amplifiers sound better than the ones of a few years ago as a general rule. However, these tests do not always separate the wheat from the chaff. Amplifiers which measure well do not necessarily sound well although an amplifier which shows up as poor on measurements will not sound well. Excellent measurements are a necessary but not a sufficient condition for quality of sound. This means that the listening test is the one of most importance--it is the most stringent test of all.
   On the basis of listening tests (definitely not on the basis of measurements) the audio school has been divided into two camps--triodes versus tetrodes. There has been shifting between the popularity of the two, hut there has always been a distinct cleavage. When the triode-without-feedback was judged superior to the tetrode-without-feedback, the tetrode school added feedback and reaffirmed the merits of this tube type. This was again superseded by the triode-with-feedback, but the beam tetrode still has its followers, presently in the category of a defensive minority among the audio elite.
   The very fact that each tube type has ardent supporters is evidence that each has definite points of merit. Possibly the devotees of each type listen for different qualities of reproduction, and this causes divergence of opinion. The triode fan usually emphasizes "smoothness" or "sweetness" of sound. The beam power advocates seek "crispness" or

  Presenting a new output-stage connection in an otherwise conventional amplifier which provides a degree of listenability which is well above the average.

  IT HAS BEEN CLAIMED that there is no more room for improvement of power output stages since other elements of a complete sound system--particularly the electro-mechanical ones--are far inferior. There is a prevalent belief that "one good amplifier is only marginally different from another." The proponents of this line of thought imply that significant improvement in power amplifiers is extremely difficult to achieve, and with this idea the authors agree, but the authors disagree as to the need for further improvement. Obviously, the weaker links do need improvement, but this alone is no reason for abandoning the further development of stronger links in the chain of audio reproduction--the power amplifier and primarily the power output stage which is the prime generator of distortion in the purely electronic part of the audio system.
   Present thinking is very parallel to the views of the 1935 era when it was felt that the principle need was for better program sources and that the transducers and audio amplifiers had reached a stage of near perfection which could hardly be improved. Now, what audiophile would be satisfied with the reproduction standards of sixteen years ago when playing the new LP's or high grade tape? By analogy, therefore, as well as for the never-ending search for a never-attainable perfection, we must continue to seek improvement in every link of the audio chain.

* Arco Products Company, 369 Shurs Lane, Philadelphia 28, Pa.   Copyright © 2000 GlassWare. All Rights Reserved